Is there a set in which division of 0 by 0 is defined?











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The reason I ask this is that I've discovered that, even though they don't satisfy all field axioms, there are sets called projectively extended real number line and Riemann sphere, which are ℝ∪{∞} and ℂ∪{∞} where division of every nonzero number of the set by 0 is defined as ∞. However, the two sets' arithmetic operations aren't total and some operations are left undefined. These include: ∞+∞, ∞-∞, ∞·0, 0·∞, ∞/∞, and 0/0. My question is that if there is, or could be a field-like set that can also define the results of these operations. And could there be a logical definition for those operations, especially 0/0?










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    The reason I ask this is that I've discovered that, even though they don't satisfy all field axioms, there are sets called projectively extended real number line and Riemann sphere, which are ℝ∪{∞} and ℂ∪{∞} where division of every nonzero number of the set by 0 is defined as ∞. However, the two sets' arithmetic operations aren't total and some operations are left undefined. These include: ∞+∞, ∞-∞, ∞·0, 0·∞, ∞/∞, and 0/0. My question is that if there is, or could be a field-like set that can also define the results of these operations. And could there be a logical definition for those operations, especially 0/0?










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      up vote
      4
      down vote

      favorite
      1









      up vote
      4
      down vote

      favorite
      1






      1





      The reason I ask this is that I've discovered that, even though they don't satisfy all field axioms, there are sets called projectively extended real number line and Riemann sphere, which are ℝ∪{∞} and ℂ∪{∞} where division of every nonzero number of the set by 0 is defined as ∞. However, the two sets' arithmetic operations aren't total and some operations are left undefined. These include: ∞+∞, ∞-∞, ∞·0, 0·∞, ∞/∞, and 0/0. My question is that if there is, or could be a field-like set that can also define the results of these operations. And could there be a logical definition for those operations, especially 0/0?










      share|cite|improve this question













      The reason I ask this is that I've discovered that, even though they don't satisfy all field axioms, there are sets called projectively extended real number line and Riemann sphere, which are ℝ∪{∞} and ℂ∪{∞} where division of every nonzero number of the set by 0 is defined as ∞. However, the two sets' arithmetic operations aren't total and some operations are left undefined. These include: ∞+∞, ∞-∞, ∞·0, 0·∞, ∞/∞, and 0/0. My question is that if there is, or could be a field-like set that can also define the results of these operations. And could there be a logical definition for those operations, especially 0/0?







      analysis field-theory infinity binary-operations






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      asked Nov 17 at 13:09









      ozigzagor

      794




      794






















          2 Answers
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          up vote
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          accepted










          A different approach than that of the wheel structure(s) in E. Joseph's answer is given by considering the linear/vector subspaces of the plane (e.g. $mathbb R^2$), regarded as binary relations and with appropriate operations.



          Motivation



          One way to look at numbers is as linear operators: The number $r$ corresponds to the function given by $f(x)=rx$. This gives us a new lens through which to examine things, especially multiplication and multiplicative inverses.



          Given $f(x)=rx$ and $g(x)=sx$, then $fcirc g$ sends $x$ to $(rs)x$, so multiplication arises as composition of functions. Similarly, for $rne0$, the inverse function of $f(x)=rx$ would be $f^{-1}(x)=left(frac{1}{r}right)x$.



          Both of these ideas can be understood in more generality as composition of relations and inverse relations (sometimes called converse relations). So if we select a slightly broader class of relations than "1-d linear operators", we can embed the numbers in a larger structure. By thinking of the graphs of these operators/relations, one "broader class" would be all of the subspaces of the plane.



          Setup



          Old and New Elements



          For each number $r$, there is a corresponding subspace $[r]={(x,y)mid y=rx}$. So the numbers can be viewed as non-vertical lines through the origin with the number as its slope.



          There are three other subspaces of the plane:




          1. The vertical line ${(x,y)mid x=0}$, which we will denote by $infty$.

          2. The origin ${(0,0)}$, which we will denote by $bot$.

          3. The whole plane, which we will denote by $top$.


          The operations



          There are four particularly important operations on numbers: the unary operations for negation (aka minus) and reciprocals, and the binary operations of addition and multiplication.



          All of these can be understood in terms of the corresponding subspaces, without appealing to the nonlinear structure directly:




          1. The negation of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(-r)x}$, which is ${(x,y)mid (x,-y)in [r]}$.

          2. If $rne0$, the reciprocal of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(r^{-1})x}$, which is ${(x,y)mid (y,x)in [r]}$.

          3. The sum of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(r+s)x}$, which is ${(x,z)mid exists y_1,y_2: (x,y_1)in [r] land (x,y_2)in [s]land y_1+y_2=z}$.

          4. The product of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(rs)x}$, which is ${(x,z)mid exists y: (x,y)in [s] land (y,z)in [r]}$ (the order is inspired by composing the linear functions).


          But those latter characterizations work just as well for other relations, so we will take those as general definitions:




          1. $-R={(x,y)mid (x,-y)in R}$


          2. $R^+={(x,y)mid (y,x)in R}$ (we might call this pseudoinverse)

          3. $R+S={(x,z)mid exists y_1,y_2: (x,y_1)in R land (x,y_2)in Sland y_1+y_2=z}$

          4. $R*S={(x,z)mid exists y: (x,y)in S land (y,z)in R}$


          In particular, we can look at $[0]^+$ (even though $frac{1}{0}$ does not define a number) and evaluate any of these operations on the three subspaces that do not correspond to numbers.



          0/0?



          We have not defined division, but for numbers $r,s$ with $rne0$, $[s/r]=[s]*[r]^+=[r]^+*[s]$. Therefore, $0/0$ could be interpreted as either $[0]*[0]^+$ or $[0]^+*[0]$.



          Note that $[0]^+=infty$ (if you reflect a horizontal line about $y=x$ you get a vertical line), so the question reduces to the value(s) of $[0]*infty$ and $infty*[0]$.



          For $[0]*infty$, the only input $infty$ allows is $0$ and the only output of $[0]$ is $0$, so this is just the origin: $bot$.



          For $infty*[0]$, $[0]$ sends all inputs to $0$, and $infty$ sends $0$ to all outputs, so this is the entire plane: $top$.



          Operation Tables



          For completeness, we can show all of the results of these operations. Below, $r$ and $s$ represent any nonzero number.



          Negation



          begin{matrix}X: & [r] & [0] & infty & bot & top\
          -X: & [-r] & [0] & infty & bot & top
          end{matrix}



          Pseudoinverse



          begin{matrix}X: & [r] & [0] & infty & bot & top\
          X^{+}: & [frac{1}{r}] & infty & 0 & bot & top
          end{matrix}



          Addition



          Note that addition of these subspaces is commutative since addition of numbers is:
          begin{array}{r|ccccc}+ & [s] & [0] & infty & bot & top\hline{}[r] & [r+s] & [r] & infty & bot & top\{}[0] & [s] & [0] & infty & bot & top\infty & infty & infty & infty & infty & infty\bot & bot & bot & infty & bot & infty\top & top & top & infty & infty & topend{array}



          Multiplication



          Since Multiplication is not commutative, $R*S$ will be the entry with the row $R$ and column $S$:
          begin{array}{r|ccccc}
          * & [s] & [0] & infty & bot & top\hline
          [r] & [rs] & [0] & infty & bot & top\{}
          [0] & [0] & [0] & bot & bot & [0]\
          infty & infty & top & infty & infty & top\
          bot & bot & [0] & bot & bot & [0]\
          top & top & top & infty & infty & top
          end{array}



          Source



          None of these ideas are my own. I first saw this in the Graphical Linear Algebra blog (though there it's wrapped up with a discussion of, well, graphical linear algebra). The most relevant entry is Keep Calm and Divide by Zero, but the following two entries contain interesting context as well. Given his work on graphical linear algebra, this approach may have been discovered by Paweł Sobociński.






          share|cite|improve this answer




























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            2
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            There is a structure called a wheel, whose purpose is to define division by $0$. More specifically, there is $x/0$ for $xne 0$ and $0/0$ in a wheel, and those two elements are not the same.



            It is used for formel computations in computers.



            You can find more informations there (and how it is formally constructed).






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            • Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
              – ozigzagor
              Nov 17 at 13:53










            • As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
              – E. Joseph
              Nov 17 at 14:09










            • But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
              – ozigzagor
              Nov 17 at 14:39






            • 1




              As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
              – ozigzagor
              Nov 17 at 15:20






            • 1




              @ozigzagor I understand that too.
              – E. Joseph
              Nov 17 at 15:46











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            up vote
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            down vote



            accepted










            A different approach than that of the wheel structure(s) in E. Joseph's answer is given by considering the linear/vector subspaces of the plane (e.g. $mathbb R^2$), regarded as binary relations and with appropriate operations.



            Motivation



            One way to look at numbers is as linear operators: The number $r$ corresponds to the function given by $f(x)=rx$. This gives us a new lens through which to examine things, especially multiplication and multiplicative inverses.



            Given $f(x)=rx$ and $g(x)=sx$, then $fcirc g$ sends $x$ to $(rs)x$, so multiplication arises as composition of functions. Similarly, for $rne0$, the inverse function of $f(x)=rx$ would be $f^{-1}(x)=left(frac{1}{r}right)x$.



            Both of these ideas can be understood in more generality as composition of relations and inverse relations (sometimes called converse relations). So if we select a slightly broader class of relations than "1-d linear operators", we can embed the numbers in a larger structure. By thinking of the graphs of these operators/relations, one "broader class" would be all of the subspaces of the plane.



            Setup



            Old and New Elements



            For each number $r$, there is a corresponding subspace $[r]={(x,y)mid y=rx}$. So the numbers can be viewed as non-vertical lines through the origin with the number as its slope.



            There are three other subspaces of the plane:




            1. The vertical line ${(x,y)mid x=0}$, which we will denote by $infty$.

            2. The origin ${(0,0)}$, which we will denote by $bot$.

            3. The whole plane, which we will denote by $top$.


            The operations



            There are four particularly important operations on numbers: the unary operations for negation (aka minus) and reciprocals, and the binary operations of addition and multiplication.



            All of these can be understood in terms of the corresponding subspaces, without appealing to the nonlinear structure directly:




            1. The negation of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(-r)x}$, which is ${(x,y)mid (x,-y)in [r]}$.

            2. If $rne0$, the reciprocal of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(r^{-1})x}$, which is ${(x,y)mid (y,x)in [r]}$.

            3. The sum of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(r+s)x}$, which is ${(x,z)mid exists y_1,y_2: (x,y_1)in [r] land (x,y_2)in [s]land y_1+y_2=z}$.

            4. The product of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(rs)x}$, which is ${(x,z)mid exists y: (x,y)in [s] land (y,z)in [r]}$ (the order is inspired by composing the linear functions).


            But those latter characterizations work just as well for other relations, so we will take those as general definitions:




            1. $-R={(x,y)mid (x,-y)in R}$


            2. $R^+={(x,y)mid (y,x)in R}$ (we might call this pseudoinverse)

            3. $R+S={(x,z)mid exists y_1,y_2: (x,y_1)in R land (x,y_2)in Sland y_1+y_2=z}$

            4. $R*S={(x,z)mid exists y: (x,y)in S land (y,z)in R}$


            In particular, we can look at $[0]^+$ (even though $frac{1}{0}$ does not define a number) and evaluate any of these operations on the three subspaces that do not correspond to numbers.



            0/0?



            We have not defined division, but for numbers $r,s$ with $rne0$, $[s/r]=[s]*[r]^+=[r]^+*[s]$. Therefore, $0/0$ could be interpreted as either $[0]*[0]^+$ or $[0]^+*[0]$.



            Note that $[0]^+=infty$ (if you reflect a horizontal line about $y=x$ you get a vertical line), so the question reduces to the value(s) of $[0]*infty$ and $infty*[0]$.



            For $[0]*infty$, the only input $infty$ allows is $0$ and the only output of $[0]$ is $0$, so this is just the origin: $bot$.



            For $infty*[0]$, $[0]$ sends all inputs to $0$, and $infty$ sends $0$ to all outputs, so this is the entire plane: $top$.



            Operation Tables



            For completeness, we can show all of the results of these operations. Below, $r$ and $s$ represent any nonzero number.



            Negation



            begin{matrix}X: & [r] & [0] & infty & bot & top\
            -X: & [-r] & [0] & infty & bot & top
            end{matrix}



            Pseudoinverse



            begin{matrix}X: & [r] & [0] & infty & bot & top\
            X^{+}: & [frac{1}{r}] & infty & 0 & bot & top
            end{matrix}



            Addition



            Note that addition of these subspaces is commutative since addition of numbers is:
            begin{array}{r|ccccc}+ & [s] & [0] & infty & bot & top\hline{}[r] & [r+s] & [r] & infty & bot & top\{}[0] & [s] & [0] & infty & bot & top\infty & infty & infty & infty & infty & infty\bot & bot & bot & infty & bot & infty\top & top & top & infty & infty & topend{array}



            Multiplication



            Since Multiplication is not commutative, $R*S$ will be the entry with the row $R$ and column $S$:
            begin{array}{r|ccccc}
            * & [s] & [0] & infty & bot & top\hline
            [r] & [rs] & [0] & infty & bot & top\{}
            [0] & [0] & [0] & bot & bot & [0]\
            infty & infty & top & infty & infty & top\
            bot & bot & [0] & bot & bot & [0]\
            top & top & top & infty & infty & top
            end{array}



            Source



            None of these ideas are my own. I first saw this in the Graphical Linear Algebra blog (though there it's wrapped up with a discussion of, well, graphical linear algebra). The most relevant entry is Keep Calm and Divide by Zero, but the following two entries contain interesting context as well. Given his work on graphical linear algebra, this approach may have been discovered by Paweł Sobociński.






            share|cite|improve this answer

























              up vote
              3
              down vote



              accepted










              A different approach than that of the wheel structure(s) in E. Joseph's answer is given by considering the linear/vector subspaces of the plane (e.g. $mathbb R^2$), regarded as binary relations and with appropriate operations.



              Motivation



              One way to look at numbers is as linear operators: The number $r$ corresponds to the function given by $f(x)=rx$. This gives us a new lens through which to examine things, especially multiplication and multiplicative inverses.



              Given $f(x)=rx$ and $g(x)=sx$, then $fcirc g$ sends $x$ to $(rs)x$, so multiplication arises as composition of functions. Similarly, for $rne0$, the inverse function of $f(x)=rx$ would be $f^{-1}(x)=left(frac{1}{r}right)x$.



              Both of these ideas can be understood in more generality as composition of relations and inverse relations (sometimes called converse relations). So if we select a slightly broader class of relations than "1-d linear operators", we can embed the numbers in a larger structure. By thinking of the graphs of these operators/relations, one "broader class" would be all of the subspaces of the plane.



              Setup



              Old and New Elements



              For each number $r$, there is a corresponding subspace $[r]={(x,y)mid y=rx}$. So the numbers can be viewed as non-vertical lines through the origin with the number as its slope.



              There are three other subspaces of the plane:




              1. The vertical line ${(x,y)mid x=0}$, which we will denote by $infty$.

              2. The origin ${(0,0)}$, which we will denote by $bot$.

              3. The whole plane, which we will denote by $top$.


              The operations



              There are four particularly important operations on numbers: the unary operations for negation (aka minus) and reciprocals, and the binary operations of addition and multiplication.



              All of these can be understood in terms of the corresponding subspaces, without appealing to the nonlinear structure directly:




              1. The negation of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(-r)x}$, which is ${(x,y)mid (x,-y)in [r]}$.

              2. If $rne0$, the reciprocal of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(r^{-1})x}$, which is ${(x,y)mid (y,x)in [r]}$.

              3. The sum of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(r+s)x}$, which is ${(x,z)mid exists y_1,y_2: (x,y_1)in [r] land (x,y_2)in [s]land y_1+y_2=z}$.

              4. The product of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(rs)x}$, which is ${(x,z)mid exists y: (x,y)in [s] land (y,z)in [r]}$ (the order is inspired by composing the linear functions).


              But those latter characterizations work just as well for other relations, so we will take those as general definitions:




              1. $-R={(x,y)mid (x,-y)in R}$


              2. $R^+={(x,y)mid (y,x)in R}$ (we might call this pseudoinverse)

              3. $R+S={(x,z)mid exists y_1,y_2: (x,y_1)in R land (x,y_2)in Sland y_1+y_2=z}$

              4. $R*S={(x,z)mid exists y: (x,y)in S land (y,z)in R}$


              In particular, we can look at $[0]^+$ (even though $frac{1}{0}$ does not define a number) and evaluate any of these operations on the three subspaces that do not correspond to numbers.



              0/0?



              We have not defined division, but for numbers $r,s$ with $rne0$, $[s/r]=[s]*[r]^+=[r]^+*[s]$. Therefore, $0/0$ could be interpreted as either $[0]*[0]^+$ or $[0]^+*[0]$.



              Note that $[0]^+=infty$ (if you reflect a horizontal line about $y=x$ you get a vertical line), so the question reduces to the value(s) of $[0]*infty$ and $infty*[0]$.



              For $[0]*infty$, the only input $infty$ allows is $0$ and the only output of $[0]$ is $0$, so this is just the origin: $bot$.



              For $infty*[0]$, $[0]$ sends all inputs to $0$, and $infty$ sends $0$ to all outputs, so this is the entire plane: $top$.



              Operation Tables



              For completeness, we can show all of the results of these operations. Below, $r$ and $s$ represent any nonzero number.



              Negation



              begin{matrix}X: & [r] & [0] & infty & bot & top\
              -X: & [-r] & [0] & infty & bot & top
              end{matrix}



              Pseudoinverse



              begin{matrix}X: & [r] & [0] & infty & bot & top\
              X^{+}: & [frac{1}{r}] & infty & 0 & bot & top
              end{matrix}



              Addition



              Note that addition of these subspaces is commutative since addition of numbers is:
              begin{array}{r|ccccc}+ & [s] & [0] & infty & bot & top\hline{}[r] & [r+s] & [r] & infty & bot & top\{}[0] & [s] & [0] & infty & bot & top\infty & infty & infty & infty & infty & infty\bot & bot & bot & infty & bot & infty\top & top & top & infty & infty & topend{array}



              Multiplication



              Since Multiplication is not commutative, $R*S$ will be the entry with the row $R$ and column $S$:
              begin{array}{r|ccccc}
              * & [s] & [0] & infty & bot & top\hline
              [r] & [rs] & [0] & infty & bot & top\{}
              [0] & [0] & [0] & bot & bot & [0]\
              infty & infty & top & infty & infty & top\
              bot & bot & [0] & bot & bot & [0]\
              top & top & top & infty & infty & top
              end{array}



              Source



              None of these ideas are my own. I first saw this in the Graphical Linear Algebra blog (though there it's wrapped up with a discussion of, well, graphical linear algebra). The most relevant entry is Keep Calm and Divide by Zero, but the following two entries contain interesting context as well. Given his work on graphical linear algebra, this approach may have been discovered by Paweł Sobociński.






              share|cite|improve this answer























                up vote
                3
                down vote



                accepted







                up vote
                3
                down vote



                accepted






                A different approach than that of the wheel structure(s) in E. Joseph's answer is given by considering the linear/vector subspaces of the plane (e.g. $mathbb R^2$), regarded as binary relations and with appropriate operations.



                Motivation



                One way to look at numbers is as linear operators: The number $r$ corresponds to the function given by $f(x)=rx$. This gives us a new lens through which to examine things, especially multiplication and multiplicative inverses.



                Given $f(x)=rx$ and $g(x)=sx$, then $fcirc g$ sends $x$ to $(rs)x$, so multiplication arises as composition of functions. Similarly, for $rne0$, the inverse function of $f(x)=rx$ would be $f^{-1}(x)=left(frac{1}{r}right)x$.



                Both of these ideas can be understood in more generality as composition of relations and inverse relations (sometimes called converse relations). So if we select a slightly broader class of relations than "1-d linear operators", we can embed the numbers in a larger structure. By thinking of the graphs of these operators/relations, one "broader class" would be all of the subspaces of the plane.



                Setup



                Old and New Elements



                For each number $r$, there is a corresponding subspace $[r]={(x,y)mid y=rx}$. So the numbers can be viewed as non-vertical lines through the origin with the number as its slope.



                There are three other subspaces of the plane:




                1. The vertical line ${(x,y)mid x=0}$, which we will denote by $infty$.

                2. The origin ${(0,0)}$, which we will denote by $bot$.

                3. The whole plane, which we will denote by $top$.


                The operations



                There are four particularly important operations on numbers: the unary operations for negation (aka minus) and reciprocals, and the binary operations of addition and multiplication.



                All of these can be understood in terms of the corresponding subspaces, without appealing to the nonlinear structure directly:




                1. The negation of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(-r)x}$, which is ${(x,y)mid (x,-y)in [r]}$.

                2. If $rne0$, the reciprocal of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(r^{-1})x}$, which is ${(x,y)mid (y,x)in [r]}$.

                3. The sum of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(r+s)x}$, which is ${(x,z)mid exists y_1,y_2: (x,y_1)in [r] land (x,y_2)in [s]land y_1+y_2=z}$.

                4. The product of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(rs)x}$, which is ${(x,z)mid exists y: (x,y)in [s] land (y,z)in [r]}$ (the order is inspired by composing the linear functions).


                But those latter characterizations work just as well for other relations, so we will take those as general definitions:




                1. $-R={(x,y)mid (x,-y)in R}$


                2. $R^+={(x,y)mid (y,x)in R}$ (we might call this pseudoinverse)

                3. $R+S={(x,z)mid exists y_1,y_2: (x,y_1)in R land (x,y_2)in Sland y_1+y_2=z}$

                4. $R*S={(x,z)mid exists y: (x,y)in S land (y,z)in R}$


                In particular, we can look at $[0]^+$ (even though $frac{1}{0}$ does not define a number) and evaluate any of these operations on the three subspaces that do not correspond to numbers.



                0/0?



                We have not defined division, but for numbers $r,s$ with $rne0$, $[s/r]=[s]*[r]^+=[r]^+*[s]$. Therefore, $0/0$ could be interpreted as either $[0]*[0]^+$ or $[0]^+*[0]$.



                Note that $[0]^+=infty$ (if you reflect a horizontal line about $y=x$ you get a vertical line), so the question reduces to the value(s) of $[0]*infty$ and $infty*[0]$.



                For $[0]*infty$, the only input $infty$ allows is $0$ and the only output of $[0]$ is $0$, so this is just the origin: $bot$.



                For $infty*[0]$, $[0]$ sends all inputs to $0$, and $infty$ sends $0$ to all outputs, so this is the entire plane: $top$.



                Operation Tables



                For completeness, we can show all of the results of these operations. Below, $r$ and $s$ represent any nonzero number.



                Negation



                begin{matrix}X: & [r] & [0] & infty & bot & top\
                -X: & [-r] & [0] & infty & bot & top
                end{matrix}



                Pseudoinverse



                begin{matrix}X: & [r] & [0] & infty & bot & top\
                X^{+}: & [frac{1}{r}] & infty & 0 & bot & top
                end{matrix}



                Addition



                Note that addition of these subspaces is commutative since addition of numbers is:
                begin{array}{r|ccccc}+ & [s] & [0] & infty & bot & top\hline{}[r] & [r+s] & [r] & infty & bot & top\{}[0] & [s] & [0] & infty & bot & top\infty & infty & infty & infty & infty & infty\bot & bot & bot & infty & bot & infty\top & top & top & infty & infty & topend{array}



                Multiplication



                Since Multiplication is not commutative, $R*S$ will be the entry with the row $R$ and column $S$:
                begin{array}{r|ccccc}
                * & [s] & [0] & infty & bot & top\hline
                [r] & [rs] & [0] & infty & bot & top\{}
                [0] & [0] & [0] & bot & bot & [0]\
                infty & infty & top & infty & infty & top\
                bot & bot & [0] & bot & bot & [0]\
                top & top & top & infty & infty & top
                end{array}



                Source



                None of these ideas are my own. I first saw this in the Graphical Linear Algebra blog (though there it's wrapped up with a discussion of, well, graphical linear algebra). The most relevant entry is Keep Calm and Divide by Zero, but the following two entries contain interesting context as well. Given his work on graphical linear algebra, this approach may have been discovered by Paweł Sobociński.






                share|cite|improve this answer












                A different approach than that of the wheel structure(s) in E. Joseph's answer is given by considering the linear/vector subspaces of the plane (e.g. $mathbb R^2$), regarded as binary relations and with appropriate operations.



                Motivation



                One way to look at numbers is as linear operators: The number $r$ corresponds to the function given by $f(x)=rx$. This gives us a new lens through which to examine things, especially multiplication and multiplicative inverses.



                Given $f(x)=rx$ and $g(x)=sx$, then $fcirc g$ sends $x$ to $(rs)x$, so multiplication arises as composition of functions. Similarly, for $rne0$, the inverse function of $f(x)=rx$ would be $f^{-1}(x)=left(frac{1}{r}right)x$.



                Both of these ideas can be understood in more generality as composition of relations and inverse relations (sometimes called converse relations). So if we select a slightly broader class of relations than "1-d linear operators", we can embed the numbers in a larger structure. By thinking of the graphs of these operators/relations, one "broader class" would be all of the subspaces of the plane.



                Setup



                Old and New Elements



                For each number $r$, there is a corresponding subspace $[r]={(x,y)mid y=rx}$. So the numbers can be viewed as non-vertical lines through the origin with the number as its slope.



                There are three other subspaces of the plane:




                1. The vertical line ${(x,y)mid x=0}$, which we will denote by $infty$.

                2. The origin ${(0,0)}$, which we will denote by $bot$.

                3. The whole plane, which we will denote by $top$.


                The operations



                There are four particularly important operations on numbers: the unary operations for negation (aka minus) and reciprocals, and the binary operations of addition and multiplication.



                All of these can be understood in terms of the corresponding subspaces, without appealing to the nonlinear structure directly:




                1. The negation of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(-r)x}$, which is ${(x,y)mid (x,-y)in [r]}$.

                2. If $rne0$, the reciprocal of $[r]={(x,y)mid y=rx}$ should be ${(x,y)mid y=(r^{-1})x}$, which is ${(x,y)mid (y,x)in [r]}$.

                3. The sum of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(r+s)x}$, which is ${(x,z)mid exists y_1,y_2: (x,y_1)in [r] land (x,y_2)in [s]land y_1+y_2=z}$.

                4. The product of $[r]={(x,y)mid y=rx}$ and $[s]={(x,y)mid y=sx}$ should be ${(x,y)mid y=(rs)x}$, which is ${(x,z)mid exists y: (x,y)in [s] land (y,z)in [r]}$ (the order is inspired by composing the linear functions).


                But those latter characterizations work just as well for other relations, so we will take those as general definitions:




                1. $-R={(x,y)mid (x,-y)in R}$


                2. $R^+={(x,y)mid (y,x)in R}$ (we might call this pseudoinverse)

                3. $R+S={(x,z)mid exists y_1,y_2: (x,y_1)in R land (x,y_2)in Sland y_1+y_2=z}$

                4. $R*S={(x,z)mid exists y: (x,y)in S land (y,z)in R}$


                In particular, we can look at $[0]^+$ (even though $frac{1}{0}$ does not define a number) and evaluate any of these operations on the three subspaces that do not correspond to numbers.



                0/0?



                We have not defined division, but for numbers $r,s$ with $rne0$, $[s/r]=[s]*[r]^+=[r]^+*[s]$. Therefore, $0/0$ could be interpreted as either $[0]*[0]^+$ or $[0]^+*[0]$.



                Note that $[0]^+=infty$ (if you reflect a horizontal line about $y=x$ you get a vertical line), so the question reduces to the value(s) of $[0]*infty$ and $infty*[0]$.



                For $[0]*infty$, the only input $infty$ allows is $0$ and the only output of $[0]$ is $0$, so this is just the origin: $bot$.



                For $infty*[0]$, $[0]$ sends all inputs to $0$, and $infty$ sends $0$ to all outputs, so this is the entire plane: $top$.



                Operation Tables



                For completeness, we can show all of the results of these operations. Below, $r$ and $s$ represent any nonzero number.



                Negation



                begin{matrix}X: & [r] & [0] & infty & bot & top\
                -X: & [-r] & [0] & infty & bot & top
                end{matrix}



                Pseudoinverse



                begin{matrix}X: & [r] & [0] & infty & bot & top\
                X^{+}: & [frac{1}{r}] & infty & 0 & bot & top
                end{matrix}



                Addition



                Note that addition of these subspaces is commutative since addition of numbers is:
                begin{array}{r|ccccc}+ & [s] & [0] & infty & bot & top\hline{}[r] & [r+s] & [r] & infty & bot & top\{}[0] & [s] & [0] & infty & bot & top\infty & infty & infty & infty & infty & infty\bot & bot & bot & infty & bot & infty\top & top & top & infty & infty & topend{array}



                Multiplication



                Since Multiplication is not commutative, $R*S$ will be the entry with the row $R$ and column $S$:
                begin{array}{r|ccccc}
                * & [s] & [0] & infty & bot & top\hline
                [r] & [rs] & [0] & infty & bot & top\{}
                [0] & [0] & [0] & bot & bot & [0]\
                infty & infty & top & infty & infty & top\
                bot & bot & [0] & bot & bot & [0]\
                top & top & top & infty & infty & top
                end{array}



                Source



                None of these ideas are my own. I first saw this in the Graphical Linear Algebra blog (though there it's wrapped up with a discussion of, well, graphical linear algebra). The most relevant entry is Keep Calm and Divide by Zero, but the following two entries contain interesting context as well. Given his work on graphical linear algebra, this approach may have been discovered by Paweł Sobociński.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Nov 21 at 12:32









                Mark S.

                11.4k22568




                11.4k22568






















                    up vote
                    2
                    down vote













                    There is a structure called a wheel, whose purpose is to define division by $0$. More specifically, there is $x/0$ for $xne 0$ and $0/0$ in a wheel, and those two elements are not the same.



                    It is used for formel computations in computers.



                    You can find more informations there (and how it is formally constructed).






                    share|cite|improve this answer





















                    • Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
                      – ozigzagor
                      Nov 17 at 13:53










                    • As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
                      – E. Joseph
                      Nov 17 at 14:09










                    • But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
                      – ozigzagor
                      Nov 17 at 14:39






                    • 1




                      As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
                      – ozigzagor
                      Nov 17 at 15:20






                    • 1




                      @ozigzagor I understand that too.
                      – E. Joseph
                      Nov 17 at 15:46















                    up vote
                    2
                    down vote













                    There is a structure called a wheel, whose purpose is to define division by $0$. More specifically, there is $x/0$ for $xne 0$ and $0/0$ in a wheel, and those two elements are not the same.



                    It is used for formel computations in computers.



                    You can find more informations there (and how it is formally constructed).






                    share|cite|improve this answer





















                    • Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
                      – ozigzagor
                      Nov 17 at 13:53










                    • As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
                      – E. Joseph
                      Nov 17 at 14:09










                    • But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
                      – ozigzagor
                      Nov 17 at 14:39






                    • 1




                      As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
                      – ozigzagor
                      Nov 17 at 15:20






                    • 1




                      @ozigzagor I understand that too.
                      – E. Joseph
                      Nov 17 at 15:46













                    up vote
                    2
                    down vote










                    up vote
                    2
                    down vote









                    There is a structure called a wheel, whose purpose is to define division by $0$. More specifically, there is $x/0$ for $xne 0$ and $0/0$ in a wheel, and those two elements are not the same.



                    It is used for formel computations in computers.



                    You can find more informations there (and how it is formally constructed).






                    share|cite|improve this answer












                    There is a structure called a wheel, whose purpose is to define division by $0$. More specifically, there is $x/0$ for $xne 0$ and $0/0$ in a wheel, and those two elements are not the same.



                    It is used for formel computations in computers.



                    You can find more informations there (and how it is formally constructed).







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Nov 17 at 13:14









                    E. Joseph

                    11.5k82856




                    11.5k82856












                    • Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
                      – ozigzagor
                      Nov 17 at 13:53










                    • As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
                      – E. Joseph
                      Nov 17 at 14:09










                    • But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
                      – ozigzagor
                      Nov 17 at 14:39






                    • 1




                      As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
                      – ozigzagor
                      Nov 17 at 15:20






                    • 1




                      @ozigzagor I understand that too.
                      – E. Joseph
                      Nov 17 at 15:46


















                    • Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
                      – ozigzagor
                      Nov 17 at 13:53










                    • As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
                      – E. Joseph
                      Nov 17 at 14:09










                    • But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
                      – ozigzagor
                      Nov 17 at 14:39






                    • 1




                      As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
                      – ozigzagor
                      Nov 17 at 15:20






                    • 1




                      @ozigzagor I understand that too.
                      – E. Joseph
                      Nov 17 at 15:46
















                    Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
                    – ozigzagor
                    Nov 17 at 13:53




                    Do wheels also have ∞+∞, ∞-∞, ∞·0, 0·∞, and ∞/∞ defined?
                    – ozigzagor
                    Nov 17 at 13:53












                    As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
                    – E. Joseph
                    Nov 17 at 14:09




                    As you can see in the document, $1/0$ is denoted by $infty$, but this is a formal construction. And $infty/infty=0/0$ which is denoted by $perp$.
                    – E. Joseph
                    Nov 17 at 14:09












                    But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
                    – ozigzagor
                    Nov 17 at 14:39




                    But is it possible to add 0/0 to itself or to subtract 0/0 from itself? The document is highly technical so it's very hard to understand.
                    – ozigzagor
                    Nov 17 at 14:39




                    1




                    1




                    As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
                    – ozigzagor
                    Nov 17 at 15:20




                    As far as I understand, the value of ⊥ is unchangeable no matter what you do with arithmetic operations?
                    – ozigzagor
                    Nov 17 at 15:20




                    1




                    1




                    @ozigzagor I understand that too.
                    – E. Joseph
                    Nov 17 at 15:46




                    @ozigzagor I understand that too.
                    – E. Joseph
                    Nov 17 at 15:46


















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