3-colourings of a 3×3 table with one of 3 colors up to symmetries

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Color each cell of a $3×3$ table with $3$ colors. What is the number of ways to do so if adjacent cells have different colors?




Of course we consider two paintings the same (equivalent) if exist reflection or rotation which take one to another. So
beginarray r
hline
colorblueB& coloryellowY &colorredR \
hline
colorredR& colorredR&colorredR\
hline
colorredR& colorredR& colorredR \
hline
endarray



and
beginarray r
hline
colorredR& colorredR& colorblueB \
hline
colorredR& colorredR& coloryellowY \
hline
colorredR& colorredR& colorredR \
hline
endarray

are the same paintings (by the way, how can I aline them?).



Since marked cells are ''independent'' we can color them at random but not with all 3 colors.



beginarray r
hline
& X & \
hline
X & &X \
hline
& X& \
hline
endarray



Case 1: If all $X$ are colored with the same color, then for each unmarked cell we have 2 posibilites. So in this case we have $3cdot 2^5$ possibile colorings. But clearly some of them are equivalent. What should I do? Divide this with 4? Or 16? Something else?



Case 2: $Y$ is of different color then $X$. Now we have $3$ colors for $Y$ and $2$ for $X$. Rest of the places we can color $1^3cdot 2^2$ so we have $6cdot 2^2$ possibile colorings. But again reflections across midlle colum give us equivalent colorings so we should divide this by $2$?
beginarray r
hline
& Y & \
hline
X & &X \
hline
& X& \
hline
endarray



Case 3: ...
beginarray r
hline
& Y & \
hline
Y & &X \
hline
& X& \
hline
endarray



Is there more elegant aproach?










share|cite|improve this question



























    up vote
    4
    down vote

    favorite












    Color each cell of a $3×3$ table with $3$ colors. What is the number of ways to do so if adjacent cells have different colors?




    Of course we consider two paintings the same (equivalent) if exist reflection or rotation which take one to another. So
    beginarray r
    hline
    colorblueB& coloryellowY &colorredR \
    hline
    colorredR& colorredR&colorredR\
    hline
    colorredR& colorredR& colorredR \
    hline
    endarray



    and
    beginarray r
    hline
    colorredR& colorredR& colorblueB \
    hline
    colorredR& colorredR& coloryellowY \
    hline
    colorredR& colorredR& colorredR \
    hline
    endarray

    are the same paintings (by the way, how can I aline them?).



    Since marked cells are ''independent'' we can color them at random but not with all 3 colors.



    beginarray r
    hline
    & X & \
    hline
    X & &X \
    hline
    & X& \
    hline
    endarray



    Case 1: If all $X$ are colored with the same color, then for each unmarked cell we have 2 posibilites. So in this case we have $3cdot 2^5$ possibile colorings. But clearly some of them are equivalent. What should I do? Divide this with 4? Or 16? Something else?



    Case 2: $Y$ is of different color then $X$. Now we have $3$ colors for $Y$ and $2$ for $X$. Rest of the places we can color $1^3cdot 2^2$ so we have $6cdot 2^2$ possibile colorings. But again reflections across midlle colum give us equivalent colorings so we should divide this by $2$?
    beginarray r
    hline
    & Y & \
    hline
    X & &X \
    hline
    & X& \
    hline
    endarray



    Case 3: ...
    beginarray r
    hline
    & Y & \
    hline
    Y & &X \
    hline
    & X& \
    hline
    endarray



    Is there more elegant aproach?










    share|cite|improve this question

























      up vote
      4
      down vote

      favorite









      up vote
      4
      down vote

      favorite











      Color each cell of a $3×3$ table with $3$ colors. What is the number of ways to do so if adjacent cells have different colors?




      Of course we consider two paintings the same (equivalent) if exist reflection or rotation which take one to another. So
      beginarray r
      hline
      colorblueB& coloryellowY &colorredR \
      hline
      colorredR& colorredR&colorredR\
      hline
      colorredR& colorredR& colorredR \
      hline
      endarray



      and
      beginarray r
      hline
      colorredR& colorredR& colorblueB \
      hline
      colorredR& colorredR& coloryellowY \
      hline
      colorredR& colorredR& colorredR \
      hline
      endarray

      are the same paintings (by the way, how can I aline them?).



      Since marked cells are ''independent'' we can color them at random but not with all 3 colors.



      beginarray r
      hline
      & X & \
      hline
      X & &X \
      hline
      & X& \
      hline
      endarray



      Case 1: If all $X$ are colored with the same color, then for each unmarked cell we have 2 posibilites. So in this case we have $3cdot 2^5$ possibile colorings. But clearly some of them are equivalent. What should I do? Divide this with 4? Or 16? Something else?



      Case 2: $Y$ is of different color then $X$. Now we have $3$ colors for $Y$ and $2$ for $X$. Rest of the places we can color $1^3cdot 2^2$ so we have $6cdot 2^2$ possibile colorings. But again reflections across midlle colum give us equivalent colorings so we should divide this by $2$?
      beginarray r
      hline
      & Y & \
      hline
      X & &X \
      hline
      & X& \
      hline
      endarray



      Case 3: ...
      beginarray r
      hline
      & Y & \
      hline
      Y & &X \
      hline
      & X& \
      hline
      endarray



      Is there more elegant aproach?










      share|cite|improve this question















      Color each cell of a $3×3$ table with $3$ colors. What is the number of ways to do so if adjacent cells have different colors?




      Of course we consider two paintings the same (equivalent) if exist reflection or rotation which take one to another. So
      beginarray r
      hline
      colorblueB& coloryellowY &colorredR \
      hline
      colorredR& colorredR&colorredR\
      hline
      colorredR& colorredR& colorredR \
      hline
      endarray



      and
      beginarray r
      hline
      colorredR& colorredR& colorblueB \
      hline
      colorredR& colorredR& coloryellowY \
      hline
      colorredR& colorredR& colorredR \
      hline
      endarray

      are the same paintings (by the way, how can I aline them?).



      Since marked cells are ''independent'' we can color them at random but not with all 3 colors.



      beginarray r
      hline
      & X & \
      hline
      X & &X \
      hline
      & X& \
      hline
      endarray



      Case 1: If all $X$ are colored with the same color, then for each unmarked cell we have 2 posibilites. So in this case we have $3cdot 2^5$ possibile colorings. But clearly some of them are equivalent. What should I do? Divide this with 4? Or 16? Something else?



      Case 2: $Y$ is of different color then $X$. Now we have $3$ colors for $Y$ and $2$ for $X$. Rest of the places we can color $1^3cdot 2^2$ so we have $6cdot 2^2$ possibile colorings. But again reflections across midlle colum give us equivalent colorings so we should divide this by $2$?
      beginarray r
      hline
      & Y & \
      hline
      X & &X \
      hline
      & X& \
      hline
      endarray



      Case 3: ...
      beginarray r
      hline
      & Y & \
      hline
      Y & &X \
      hline
      & X& \
      hline
      endarray



      Is there more elegant aproach?







      combinatorics graph-theory coloring






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      edited 29 mins ago

























      asked 2 hours ago









      greedoid

      32.6k114388




      32.6k114388




















          2 Answers
          2






          active

          oldest

          votes

















          up vote
          3
          down vote













          We can use Burnside's lemma to account for symmetries. By the OEIS, there are 246 different 3-colourings of a labelled 3×3 grid graph (i.e. before accounting for symmetry).



          This graph's non-identity symmetries are as follows. The general form of a colouring invariant under this symmetry is shown, then a calculation of the number of such colourings.




          • 90° left/right rotations. A colouring invariant under this transformation looks like this:
            $$beginarrayr
            hline a&b&a\
            hline b&c&b\
            hline a&b&a\hlineendarray$$

            After $b$ is chosen, we have two possibilities each for $a,c$. Thus there are $3×2×2=12$ such colourings invariant under each of these symmetries.


          • 180° rotation.
            $$beginarrayr
            hline a&b&c\
            hline d&e&d\
            hline c&b&a\hlineendarray$$

            Either $b,d$ are different colours (6 ways), in which case $a,c,e$ can only assume the third colour, or $b,d$ are the same (3 ways) and $a,c,e$ can be one of two colours. There are $6×1^3+3×2^3=30$ invariant colourings.


          • Horizontal/vertical reflections.
            $$beginarrayr
            hline a&b&c\
            hline d&e&f\
            hline a&b&c\hlineendarray$$

            This is equivalent to the number of 3-colourings of a 2×3 grid graph. Appealing to the OEIS again, we see that there are 54 such colourings for each symmetry.


          • Diagonal reflections.
            $$beginarrayr
            hline a&b&c\
            hline d&e&b\
            hline f&d&a\hlineendarray$$

            This is equivalent to the number of colourings of a square, 18 according to the OEIS, multiplied by 4, yielding 72. The square is formed by $a,b,d,e$, and for each colouring of the square $c,f$ can be either of the two colours not used for $b,d$ respectively, hence the $2^2$ multiplier.

          Burnside's lemma then gives the number of colourings up to symmetries as
          $$frac246+2×12+30+2×54+2×728=colorred69$$






          share|cite|improve this answer





























            up vote
            1
            down vote













            My colors are $0$ and $pm1$. I put a $0$ in the center. Then up to rotations, reflections, and multiplication by $-1$ we only have the four following patterns to analyze further:
            $$pmleft[matrix&1&cr1&0&1cr&1&crright]>,quadpmleft[matrix&-1&cr1&0&1cr&1&crright]>,quadleft[matrix&-1&cr1&0&1cr&-1&crright]>,quadleft[matrix&1&cr-1&0&1cr&-1&crright] .tag1$$
            As an example consider the second pattern (apart from th $pm$). Here two $0$s in the upper corner are enforced. In the lower corner we can have zero, one,or two $0$s and the rest $-1$s:
            $$left[matrix0&-1&0cr1&0&1cr-1&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&0crright] .$$
            These three solutions have different numbers of z$0$s in total, hence are essentially different. And we don't have to be afraid that they are the same as solutions stemming from the other patterns in $(1)$.



            I leave the other patterns in $(1)$ to you. At the end multiply by $3!$ in order to assign your colors to mine.






            share|cite|improve this answer




















            • What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
              – greedoid
              37 mins ago










            • @greedoid: Putting $0$ at the center is no restriction of generality.
              – Christian Blatter
              15 mins ago










            Your Answer




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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes








            up vote
            3
            down vote













            We can use Burnside's lemma to account for symmetries. By the OEIS, there are 246 different 3-colourings of a labelled 3×3 grid graph (i.e. before accounting for symmetry).



            This graph's non-identity symmetries are as follows. The general form of a colouring invariant under this symmetry is shown, then a calculation of the number of such colourings.




            • 90° left/right rotations. A colouring invariant under this transformation looks like this:
              $$beginarrayr
              hline a&b&a\
              hline b&c&b\
              hline a&b&a\hlineendarray$$

              After $b$ is chosen, we have two possibilities each for $a,c$. Thus there are $3×2×2=12$ such colourings invariant under each of these symmetries.


            • 180° rotation.
              $$beginarrayr
              hline a&b&c\
              hline d&e&d\
              hline c&b&a\hlineendarray$$

              Either $b,d$ are different colours (6 ways), in which case $a,c,e$ can only assume the third colour, or $b,d$ are the same (3 ways) and $a,c,e$ can be one of two colours. There are $6×1^3+3×2^3=30$ invariant colourings.


            • Horizontal/vertical reflections.
              $$beginarrayr
              hline a&b&c\
              hline d&e&f\
              hline a&b&c\hlineendarray$$

              This is equivalent to the number of 3-colourings of a 2×3 grid graph. Appealing to the OEIS again, we see that there are 54 such colourings for each symmetry.


            • Diagonal reflections.
              $$beginarrayr
              hline a&b&c\
              hline d&e&b\
              hline f&d&a\hlineendarray$$

              This is equivalent to the number of colourings of a square, 18 according to the OEIS, multiplied by 4, yielding 72. The square is formed by $a,b,d,e$, and for each colouring of the square $c,f$ can be either of the two colours not used for $b,d$ respectively, hence the $2^2$ multiplier.

            Burnside's lemma then gives the number of colourings up to symmetries as
            $$frac246+2×12+30+2×54+2×728=colorred69$$






            share|cite|improve this answer


























              up vote
              3
              down vote













              We can use Burnside's lemma to account for symmetries. By the OEIS, there are 246 different 3-colourings of a labelled 3×3 grid graph (i.e. before accounting for symmetry).



              This graph's non-identity symmetries are as follows. The general form of a colouring invariant under this symmetry is shown, then a calculation of the number of such colourings.




              • 90° left/right rotations. A colouring invariant under this transformation looks like this:
                $$beginarrayr
                hline a&b&a\
                hline b&c&b\
                hline a&b&a\hlineendarray$$

                After $b$ is chosen, we have two possibilities each for $a,c$. Thus there are $3×2×2=12$ such colourings invariant under each of these symmetries.


              • 180° rotation.
                $$beginarrayr
                hline a&b&c\
                hline d&e&d\
                hline c&b&a\hlineendarray$$

                Either $b,d$ are different colours (6 ways), in which case $a,c,e$ can only assume the third colour, or $b,d$ are the same (3 ways) and $a,c,e$ can be one of two colours. There are $6×1^3+3×2^3=30$ invariant colourings.


              • Horizontal/vertical reflections.
                $$beginarrayr
                hline a&b&c\
                hline d&e&f\
                hline a&b&c\hlineendarray$$

                This is equivalent to the number of 3-colourings of a 2×3 grid graph. Appealing to the OEIS again, we see that there are 54 such colourings for each symmetry.


              • Diagonal reflections.
                $$beginarrayr
                hline a&b&c\
                hline d&e&b\
                hline f&d&a\hlineendarray$$

                This is equivalent to the number of colourings of a square, 18 according to the OEIS, multiplied by 4, yielding 72. The square is formed by $a,b,d,e$, and for each colouring of the square $c,f$ can be either of the two colours not used for $b,d$ respectively, hence the $2^2$ multiplier.

              Burnside's lemma then gives the number of colourings up to symmetries as
              $$frac246+2×12+30+2×54+2×728=colorred69$$






              share|cite|improve this answer
























                up vote
                3
                down vote










                up vote
                3
                down vote









                We can use Burnside's lemma to account for symmetries. By the OEIS, there are 246 different 3-colourings of a labelled 3×3 grid graph (i.e. before accounting for symmetry).



                This graph's non-identity symmetries are as follows. The general form of a colouring invariant under this symmetry is shown, then a calculation of the number of such colourings.




                • 90° left/right rotations. A colouring invariant under this transformation looks like this:
                  $$beginarrayr
                  hline a&b&a\
                  hline b&c&b\
                  hline a&b&a\hlineendarray$$

                  After $b$ is chosen, we have two possibilities each for $a,c$. Thus there are $3×2×2=12$ such colourings invariant under each of these symmetries.


                • 180° rotation.
                  $$beginarrayr
                  hline a&b&c\
                  hline d&e&d\
                  hline c&b&a\hlineendarray$$

                  Either $b,d$ are different colours (6 ways), in which case $a,c,e$ can only assume the third colour, or $b,d$ are the same (3 ways) and $a,c,e$ can be one of two colours. There are $6×1^3+3×2^3=30$ invariant colourings.


                • Horizontal/vertical reflections.
                  $$beginarrayr
                  hline a&b&c\
                  hline d&e&f\
                  hline a&b&c\hlineendarray$$

                  This is equivalent to the number of 3-colourings of a 2×3 grid graph. Appealing to the OEIS again, we see that there are 54 such colourings for each symmetry.


                • Diagonal reflections.
                  $$beginarrayr
                  hline a&b&c\
                  hline d&e&b\
                  hline f&d&a\hlineendarray$$

                  This is equivalent to the number of colourings of a square, 18 according to the OEIS, multiplied by 4, yielding 72. The square is formed by $a,b,d,e$, and for each colouring of the square $c,f$ can be either of the two colours not used for $b,d$ respectively, hence the $2^2$ multiplier.

                Burnside's lemma then gives the number of colourings up to symmetries as
                $$frac246+2×12+30+2×54+2×728=colorred69$$






                share|cite|improve this answer














                We can use Burnside's lemma to account for symmetries. By the OEIS, there are 246 different 3-colourings of a labelled 3×3 grid graph (i.e. before accounting for symmetry).



                This graph's non-identity symmetries are as follows. The general form of a colouring invariant under this symmetry is shown, then a calculation of the number of such colourings.




                • 90° left/right rotations. A colouring invariant under this transformation looks like this:
                  $$beginarrayr
                  hline a&b&a\
                  hline b&c&b\
                  hline a&b&a\hlineendarray$$

                  After $b$ is chosen, we have two possibilities each for $a,c$. Thus there are $3×2×2=12$ such colourings invariant under each of these symmetries.


                • 180° rotation.
                  $$beginarrayr
                  hline a&b&c\
                  hline d&e&d\
                  hline c&b&a\hlineendarray$$

                  Either $b,d$ are different colours (6 ways), in which case $a,c,e$ can only assume the third colour, or $b,d$ are the same (3 ways) and $a,c,e$ can be one of two colours. There are $6×1^3+3×2^3=30$ invariant colourings.


                • Horizontal/vertical reflections.
                  $$beginarrayr
                  hline a&b&c\
                  hline d&e&f\
                  hline a&b&c\hlineendarray$$

                  This is equivalent to the number of 3-colourings of a 2×3 grid graph. Appealing to the OEIS again, we see that there are 54 such colourings for each symmetry.


                • Diagonal reflections.
                  $$beginarrayr
                  hline a&b&c\
                  hline d&e&b\
                  hline f&d&a\hlineendarray$$

                  This is equivalent to the number of colourings of a square, 18 according to the OEIS, multiplied by 4, yielding 72. The square is formed by $a,b,d,e$, and for each colouring of the square $c,f$ can be either of the two colours not used for $b,d$ respectively, hence the $2^2$ multiplier.

                Burnside's lemma then gives the number of colourings up to symmetries as
                $$frac246+2×12+30+2×54+2×728=colorred69$$







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited 1 hour ago

























                answered 1 hour ago









                Parcly Taxel

                38.5k137098




                38.5k137098




















                    up vote
                    1
                    down vote













                    My colors are $0$ and $pm1$. I put a $0$ in the center. Then up to rotations, reflections, and multiplication by $-1$ we only have the four following patterns to analyze further:
                    $$pmleft[matrix&1&cr1&0&1cr&1&crright]>,quadpmleft[matrix&-1&cr1&0&1cr&1&crright]>,quadleft[matrix&-1&cr1&0&1cr&-1&crright]>,quadleft[matrix&1&cr-1&0&1cr&-1&crright] .tag1$$
                    As an example consider the second pattern (apart from th $pm$). Here two $0$s in the upper corner are enforced. In the lower corner we can have zero, one,or two $0$s and the rest $-1$s:
                    $$left[matrix0&-1&0cr1&0&1cr-1&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&0crright] .$$
                    These three solutions have different numbers of z$0$s in total, hence are essentially different. And we don't have to be afraid that they are the same as solutions stemming from the other patterns in $(1)$.



                    I leave the other patterns in $(1)$ to you. At the end multiply by $3!$ in order to assign your colors to mine.






                    share|cite|improve this answer




















                    • What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
                      – greedoid
                      37 mins ago










                    • @greedoid: Putting $0$ at the center is no restriction of generality.
                      – Christian Blatter
                      15 mins ago














                    up vote
                    1
                    down vote













                    My colors are $0$ and $pm1$. I put a $0$ in the center. Then up to rotations, reflections, and multiplication by $-1$ we only have the four following patterns to analyze further:
                    $$pmleft[matrix&1&cr1&0&1cr&1&crright]>,quadpmleft[matrix&-1&cr1&0&1cr&1&crright]>,quadleft[matrix&-1&cr1&0&1cr&-1&crright]>,quadleft[matrix&1&cr-1&0&1cr&-1&crright] .tag1$$
                    As an example consider the second pattern (apart from th $pm$). Here two $0$s in the upper corner are enforced. In the lower corner we can have zero, one,or two $0$s and the rest $-1$s:
                    $$left[matrix0&-1&0cr1&0&1cr-1&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&0crright] .$$
                    These three solutions have different numbers of z$0$s in total, hence are essentially different. And we don't have to be afraid that they are the same as solutions stemming from the other patterns in $(1)$.



                    I leave the other patterns in $(1)$ to you. At the end multiply by $3!$ in order to assign your colors to mine.






                    share|cite|improve this answer




















                    • What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
                      – greedoid
                      37 mins ago










                    • @greedoid: Putting $0$ at the center is no restriction of generality.
                      – Christian Blatter
                      15 mins ago












                    up vote
                    1
                    down vote










                    up vote
                    1
                    down vote









                    My colors are $0$ and $pm1$. I put a $0$ in the center. Then up to rotations, reflections, and multiplication by $-1$ we only have the four following patterns to analyze further:
                    $$pmleft[matrix&1&cr1&0&1cr&1&crright]>,quadpmleft[matrix&-1&cr1&0&1cr&1&crright]>,quadleft[matrix&-1&cr1&0&1cr&-1&crright]>,quadleft[matrix&1&cr-1&0&1cr&-1&crright] .tag1$$
                    As an example consider the second pattern (apart from th $pm$). Here two $0$s in the upper corner are enforced. In the lower corner we can have zero, one,or two $0$s and the rest $-1$s:
                    $$left[matrix0&-1&0cr1&0&1cr-1&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&0crright] .$$
                    These three solutions have different numbers of z$0$s in total, hence are essentially different. And we don't have to be afraid that they are the same as solutions stemming from the other patterns in $(1)$.



                    I leave the other patterns in $(1)$ to you. At the end multiply by $3!$ in order to assign your colors to mine.






                    share|cite|improve this answer












                    My colors are $0$ and $pm1$. I put a $0$ in the center. Then up to rotations, reflections, and multiplication by $-1$ we only have the four following patterns to analyze further:
                    $$pmleft[matrix&1&cr1&0&1cr&1&crright]>,quadpmleft[matrix&-1&cr1&0&1cr&1&crright]>,quadleft[matrix&-1&cr1&0&1cr&-1&crright]>,quadleft[matrix&1&cr-1&0&1cr&-1&crright] .tag1$$
                    As an example consider the second pattern (apart from th $pm$). Here two $0$s in the upper corner are enforced. In the lower corner we can have zero, one,or two $0$s and the rest $-1$s:
                    $$left[matrix0&-1&0cr1&0&1cr-1&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&-1crright]>,quad left[matrix0&-1&0cr1&0&1cr0&1&0crright] .$$
                    These three solutions have different numbers of z$0$s in total, hence are essentially different. And we don't have to be afraid that they are the same as solutions stemming from the other patterns in $(1)$.



                    I leave the other patterns in $(1)$ to you. At the end multiply by $3!$ in order to assign your colors to mine.







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                    share|cite|improve this answer










                    answered 1 hour ago









                    Christian Blatter

                    168k7111319




                    168k7111319











                    • What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
                      – greedoid
                      37 mins ago










                    • @greedoid: Putting $0$ at the center is no restriction of generality.
                      – Christian Blatter
                      15 mins ago
















                    • What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
                      – greedoid
                      37 mins ago










                    • @greedoid: Putting $0$ at the center is no restriction of generality.
                      – Christian Blatter
                      15 mins ago















                    What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
                    – greedoid
                    37 mins ago




                    What about this one: $$pmleft[matrix&1&cr1&-1&1cr&1&crright]$$
                    – greedoid
                    37 mins ago












                    @greedoid: Putting $0$ at the center is no restriction of generality.
                    – Christian Blatter
                    15 mins ago




                    @greedoid: Putting $0$ at the center is no restriction of generality.
                    – Christian Blatter
                    15 mins ago

















                     

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