6

Given six distinct integers $(a_1, a_2, a_3,\dots,a_6)$, after observing patterns in the computer data of this post, why is it possible to solve a system of six cyclic equations, call it $C_6$,

$$a_1^3+a_2^3+z_1^3 = (z_1+1)^3\\ a_2^3+a_3^3+z_2^3 = (z_2+1)^3\\ a_3^3+a_4^3+z_3^3 = (z_3+1)^3\\ \vdots\\ a_6^3+a_1^3+z_6^3 = (z_6+1)^3$$

for infinitely many sets of 6 $a_k$? For example, let $a_k = (-9, 16, 51, 82, 57, 22)$. Then,

$$ 22^3 + (\color{blue}{-9})^3 + 57^3 = 58^3\\ (\color{blue}{-9})^3 + \color{blue}{16}^3 + 33^3 = \color{red}{34}^3\\ \color{blue}{16}^3 + \color{blue}{51}^3 + 213^3 = 214^3\\ \color{blue}{51}^3 + \color{blue}{82}^3 + 477^3 = \color{red}{478}^3\\ \color{blue}{82}^3 + \color{blue}{57}^3 + 495^3 = 496^3\\ \color{blue}{57}^3 + \color{blue}{22}^3 + 255^3 = \color{red}{256}^3\\ \color{blue}{22}^3 + (-9)^3 + 57^3 = 58^3 $$

and the cycle repeats, akin to the closed-chain of a cycle graph with 6 vertices. Or a second set $a_k = (15, -2, 9, 58, 75, 64)$. Then,

$$ 64^3 + \color{blue}{15}^3 + 297^3 = 298^3\\ \color{blue}{15}^3 + (\color{blue}{-2})^3 + 33^3 = \color{red}{34}^3\\ (\color{blue}{-2})^3 \,+ \color{blue}{9}^3 + 15^3 \,=\, 16^3\\ \;\color{blue}{9}^3 + \color{blue}{58}^3 + 255^3 = \color{red}{256}^3\\ \color{blue}{58}^3 + \color{blue}{75}^3 + 453^3 = 454^3\\ \color{blue}{75}^3 + \color{blue}{64}^3 + 477^3 = \color{red}{478}^3\\ \color{blue}{64}^3 + 15^3 + 297^3 = 298^3 $$

The fact that three pairs of sums (in red) are the same suggests these numbers are not random. They are members of an infinite family given by,

\begin{align}\qquad a_1 &= 3n + 3n^2 + 9n^3\\ a_2 &= 1 + 6n^2 - 9n^3\\ a_3 &= -3n + 3n^2 - 18n^3 + 27n^4\\ a_4 &= 1 - 3n + 15n^2 - 9n^3 + 54n^4\\ a_5 &= 12n^2 + 9n^3 + 54n^4\\ a_6 &= 1 + 3n + 15n^2 + 18n^3 + 27n^4 \end{align}

where the first set was just $n =-1,$ and the second set was $n=1.$ Note that,

$$a_1^3+a_2^3+(6n^2 + 27n^4)^3 = (1 + 6n^2 + 27n^4)^3$$

which explains why (for this pair) two terms are immune to sign changes. The polynomials for the other $b_k$ can be found by substituting into the equations above and factoring.

Question: Why is it possible to solve the cyclic system $C_6$ in the first place? (It is hard enough to solve a Diophantine system of deg-$2$ equations. One would expect a system of deg-$3$ equations would be harder, unless there was a simplifying principle at work -- which I'm looking for.)

  • I'm not sure whether this leads anywhere useful but noticed that each of your numerical examples has $b_i=4x+3y-4$ where $x,y$ are respectively the even and odd terms of the $a_i$'s, eg $4(22)+3(-9)-4=57$. So it may be of interest to consider the equation: $$x^3+y^3+(4x+3y-4)^3=(4x+3y-3)^3$$ which simplifies to: $$a^3+b^3-48a^2+84a-27b^2+63b-72ab-37-0$$. – Adam Bailey Dec 06 '23 at 12:36
  • @AdamBailey Thanks, I will look more deeply at it. – Tito Piezas III Dec 06 '23 at 12:41
  • Sorry, wasn't quick enough to edit my earlier comment, the last equation should be: $$x^3+y^3−48x^2+84x−27y^2+63y−72xy−37=0$$ – Adam Bailey Dec 06 '23 at 13:05
  • @AdamBailey Here is another post on computational mathematics you may be interested in. I have a feeling it can be reduced to an elliptic curve. – Tito Piezas III Jan 06 '24 at 13:11

3 Answers3

6

There are many functions $f(x,y)$ such that the equation:

$$x^3 + y^3 + (f(x,y))^3 = (f(x,y)+1)^3\tag1$$

has isolated solutions in integers, not forming cycles except in the trivial sense that reversing $x$ and $y$ forms a 2-cycle. For example, if $f(x,y)=x+y-2$, then the only solutions of (1) are $(3,4), (4,3), (0,1), (1,0)$. Note that any solution must have $x$ and $y$ of opposite odd-even parity.

Underlying the cycles described in the question is the case in which, for some integer $k$:

$$f(x,y) = 3k^2(x+y-1)+y-1$$

In that case, (1) is equivalent to:

$$x^3 + y^3 – 27k^4x^2 – (27k^4+18k^2+3)y^2 – (54k^4+18k^2)xy + (54k^4+9k^2)x + (54k^4+27k^2+3)y – (27k^4+9k^2+1)=0\tag2$$


I. A first simplication is that this factorises as:

$$(x+y-1)\big[x^2 - xy + y^2 - (27k^4-1)x - (27k^4+18k^2+2)y +(27k^4+9k^2+1)\big] = 0\tag3$$

Equating the first factor to zero leads only to trivial solutions, but equating the second to zero yields the equation of an ellipse. Using notation from Wikipedia, this can be written as:

$$Ax^2 + Bxy + Cy^2 + Dx + Ey + F = 0\tag4$$

where:

$\qquad A = 1,\; B = -1,\; C = 1$

$\qquad D = -(27k^4-1)$

$\qquad E = -(27k^4+18k^2+2)$

$\qquad F = 27k^4+9k^2+1$

and $D+E+2F = 1.\,$ Since $x,y$ must be of opposite parity, then $Ax^2 + Bxy + Cy^2$ is odd, also $F$ is odd regardless of the parity of $k$. So a solution of (4) must have $Dx+Ey$ even, which requires either odd $(k,x)$ and even $y$, OR even $(k,x)$ and odd $y$.


II. A second simplification is that from any one integer solution of (4) we can obtain another. To see this, suppose we have a solution in integers $k$ (odd), $x_0$ (odd) and $y_0$ (even). Substituting $y_0$ only into (4) we have a quadratic equation in $x$:

$$Ax^2 + (By_0+D)x + (Cy_0^2+Ey_0+F) = 0\tag5$$

This will normally have two roots, one of which must be the known $x_0$. To obtain the other – call it $x_1$ – we can use the fact that the sum of the roots of an equation $ax^2+bx+c=0$ is $-b/a$, so:

$$x_1 = -\dfrac{By_0+D}{A}-x_0\tag6$$

Since $B,y_0,D$ are integers while the denominator $A=1$, $x_1$ must also be an integer, so (barring degenerate cases where $x_1=x_0$), then $(x_1,y_0)$ is a new solution in integers of (4). This reasoning remains valid for any integer value of $k$.

As an illustration, if $k=1$ and $(x_0,y_0)=(-9,16)$, equation (5) becomes:

$$x^2 + \big(-1(16) –(27-1) \big)x + \big(16^2 – (27+18+2)16 + (27 + 9 +1 \big) = 0\tag7$$

which simplifies to:

$$x^2 -42x - 459 = 0\tag8$$

yielding $(x_1,y_1)=(51,16)$ as a new solution of (4). The two solutions are at the intersections of the ellipse and the horizontal line $y=16$ as shown in Chart 1.

Chart 1

This procedure can be repeated by substituting $x_1$ into (4) and solving in the same way the resulting quadratic equation in $y$ to obtain a further integer solution of (4) which will lie on the intersection of the ellipse with a vertical line through $(x_1,y_0)$. And there is no need to stop there: we can proceed indefinitely, at each iteration substituting into (4) the new value of $x$ or $y$ obtained at the previous iteration. Starting from $(-9,16)$, this yields the sequence of solutions $(-9,16),(16,51),(51,82),(82,57),(57,22),(22,-9),\dots$.

A simple argument shows that this procedure must eventually result in a complete cycle. There are only a finite number of lattice points (points with integer coordinates) in the vicinity of an ellipse defined by equation (4), for example the whole of Chart 1 contains 121 such points. The infinite series of integer points generated as above, all lying on the ellipse (and, a fortiori, within that vicinity) must therefore eventually revisit at least one point.


III. A third consideration is that the configuration of the ellipses defined by equation (4) is conducive to cycles of 6 points, rather than some other number of points. Intuitively, the number of points that can form a cycle linked by alternate horizontal and vertical lines within an ellipse is related to a) the direction of the axes, and b) its eccentricity, one indicator of which is the ratio of the major to the minor axis.

If the axes are horizontal and vertical, a 4-cycle is always possible as in Chart 2 below. Note that it is not claimed that any ellipse must pass through at least one integer point, nor that if it does then another such point must lie on the intersection of the ellipse with a horizontal or vertical line throught the first point. The purpose of Charts 2 and 3 is to show that, if those conditions are met, the number of points in the resulting cycles will depend on the configuration of the ellipse.

Chart 2

If the axes are at $45^{\circ}$ to the horizontal, then a high degree of eccentricity will require a cycle with a large number of points, as in Chart 3 below in which the ratio of the axes is about $6$:

Chart 3

The ellipses defined by equation (4) are moderately eccentric as shown in Chart 1. This is true whatever the value of $k$ since the ratio of major to minor axis is independent of $k$, being given by (see Wikipedia, formula for a,b under General Ellipse):

$$\sqrt{\Bigg(\dfrac{A+C+\sqrt{(A-C)^2+B^2}}{A+C-\sqrt{(A-C)^2+B^2}}\Bigg)}=\sqrt{\Bigg(\dfrac{1+1+\sqrt{(1-1)^2+(-1)^2}}{1+1-\sqrt{(1-1)^2+(-1)^2}}\Bigg)}=\sqrt{3}$$

I am not aware of a precise formula relating the cycle size to the degree of eccentricity of an ellipse with axes at $45^{\circ}$ to the horizontal, but having seen the example of the ellipse in Chart 3 it seems unsurprising that ellipses with an axis ratio of $\sqrt{3}$, ie moderately eccentric, should generate 6-cycles, two of which from the question are illustrated below:

Chart 4

Each of the two cycles is the mirror image of the other, the axis of symmetry being the line $y=x+7$ which coincides with the major axis of the ellipse.

Adam Bailey
  • 4,197
  • +1 Beautiful! I knew there had to be a underlying reason behind the cycles. Chart 4 is especially nice, to see those numbers in a geometric context. – Tito Piezas III Dec 20 '23 at 06:37
  • In fact, I found four families with a 6-cycle, though i only put the first family in the post to prevent clutter. These families have a common feature which might explain why they exist. I will post them shortly. – Tito Piezas III Dec 20 '23 at 08:02
  • I made some minor changes to improve flow. A favor though. Chart 4 is the first set (−9,16,51,82,57,22). Can you make a Chart 5 showing the second set (15,−2,9,58,75,64)? It would be nice to see how similar they look. – Tito Piezas III Dec 22 '23 at 04:00
  • 1
    @TitoPiezasIII No problem when I have time. It may be best for me to add the second cycle to Chart 4, in a different colour, as I think the two cycles will show a certain symmetry. – Adam Bailey Dec 22 '23 at 15:11
  • I didn't know you updated Chart 4. Anyway, I also know how to do it now using the online Desmos calculator. You may be interested in this new post about ellipses. – Tito Piezas III Jan 15 '24 at 05:20
  • @TitoPiezasIII Sorry, perhaps I should at the time have added a comment to draw your attention to the update. I wasn't aware of Desmos - it looks as if it would have saved me time as compared with what I did using the graphics in Excel. – Adam Bailey Jan 15 '24 at 13:54
  • Since Jan-Magnus answered my question which turned out to be the ellipse's center, I made a new question about pairs of lattice point triplets that are not mirror images. Might be interesting. – Tito Piezas III Jan 16 '24 at 08:39
1

In this answer I reverse engineer the answer given by Adam Bailey, given three points with centroid the center of the resulting ellipse with the factor $(x+y-1),$ from other questions by OP.

The linear factor is likely given the line we focus on on the cubic surface $x^3+y^3+z^3−(z+w)^3=0$ which is $$(x:y:z:w)=(s:−t:−s:s−t), \\s^3+(−t)^3+(−s)^3−(−s+(s−t))^3=0$$ or $$x+y−w=y−z−w=0.$$ This way we get the factorization with the ellipse from a special pencil of planes through this line: $$λ⋅(x+y−w)+μ⋅(y−z−w)=0.$$

The following is a mix of maxima CAS and M2 code for the case $w=1$ for the point $(x_A,y_A)=(9n^3+3n^2+3n,-9n^3+6n^2+1)$

R=QQ[n][x,y,z]
f=3*n^2*x+(3*n^2+1)*y-3*n^2-1
factor(x^3+y^3+f^3-(f+1)^3)
(mons,cofs)=coefficients(x^2-x*y+y^2+(-27*n^4+1)*x+(-27*n^4-18*n^2-2)*y+27*n^4+9*n^2+1)
A=1;B=-1;C=1;D=-27*n^4+1;E=-27*n^4-18*n^2-2;F=27*n^4+9*n^2+1;
xA=9*n^3+3*n^2+3*n
factor(A*xA^2 +B* xA*y + C*y^2 +D*xA +E*y +F)
yA=(-9*n^3)+6*n^2+1
factor(A*x^2 +B* x*yA + C*yA^2 +D*x +E*yA +F)
xB=27*n^4-18*n^3+3*n^2-3*n
factor(A*xB^2 +B* xB*y + C*y^2 +D*xB +E*y +F)
yB=54*n^4-9*n^3+15*n^2-3*n+1
factor(A*x^2 +B* x*yB + C*yB^2 +D*x +E*yB +F)
xC=54*n^4+9*n^3+12*n^2
factor(A*xC^2 +B* xC*y + C*y^2 +D*xC +E*y +F)
yC=27*n^4+18*n^3+15*n^2+3*n+1

xA:9n^3+3n^2+3n;yA:(-9n^3)+6n^2+1;xB:-3n+3n^2-18n^3+27n^4;yB:1-3n+15n^2-9n^3+54n^4;xC:12n^2+9n^3+54n^4;yC:1+3n+15n^2+18n^3+27n^4; xO:(xA+xB+xC)/3;yO:(yA+yB+yC)/3; solve([a(xA-xO)^2+2b(xA-xO)(yA-yO)+c(yA-yO)^2-1,a(xB-xO)^2+2b(xB-xO)(yB-yO)+c(yB-yO)^2-1,a(xC-xO)^2+2b(xC-xO)(yC-yO)+c*(yC-yO)^2-1],[a,b,c]); # [[a = 1/(729n^8+486n^6+108n^4+9n^2), b = -1/(1458n^8+972n^6+216n^4+18n^2), c = 1/(729n^8+486n^6+108n^4+9n^2)]] factor(a(x-xO)^2+2b(x-xO)(y-yO)+c(y-yO)^2-1); # (y^2-xy-27n^4y-18n^2y-2y+x^2-27n^4x+x+27n^4+9n^2+1)/(9n^2(3n^2+1)(27n^4+9*n^2+1))

R=QQ[n][a,b,c][x,y,z] (mons, cofs)=coefficients(x^3+y^3+(ax+by+c)^3-(ax+by+c+1)^3-(x+y-1)(y^2-xy-27n^4y-18n^2y-2y+x^2-27n^4x+x+27n^4+9n^2+1)) primaryDecomposition ideal cofs -- {ideal(b+c+1,a+c,c-3n^2), ideal(b+c,a+c+1,c+3n^2+1)} solve([b+c+1,a+c,c-3n^2],[a,b,c]); # [[a = -3n^2,b = (-3n^2)-1,c = 3*n^2]] solve([b+c,a+c+1,c+3*n^2+1],[a,b,c]); # [[a = 3n^2,b = 3n^2+1,c = (-3*n^2)-1]]

So, from the first, we recover $z=3n^2x+(3n^2+1)y-3n^2-1$ or $-3n^2(x+y-1)-(y-z-1)=0.$

Similarly we get for

$$(x_A,y_A)=(6n^2 - 36n^3 + 36n^4 + 648n^6 - 1296n^7, 1 + 12n^2 + 72n^4 + 648n^6 + 1296n^7)$$

R=QQ[n][x,y,z]
f=36*n^4*x + (1 + 36*n^4)*(y - 1)
factor(x^3+y^3+f^3-(f+1)^3)
(mons,cofs)=coefficients(x^2-x*y+y^2+(-3888*n^8+1)*x+(-3888*n^8-216*n^4-2)*y+3888*n^8+108*n^4+1)
A=1;B=-1;C=1;D=-3888*n^8+1;E=-3888*n^8-216*n^4-2;F=3888*n^8+108*n^4+1;
xA=6*n^2 - 36*n^3 + 36*n^4 + 648*n^6 - 1296*n^7
factor(A*xA^2 +B* xA*y + C*y^2 +D*xA +E*y +F)
yA=3888*n^8-2592*n^7+180*n^4-36*n^3-6*n^2+1
factor(A*x^2 +B* x*yA + C*yA^2 +D*x +E*yA +F)
xB=7776*n^8-1296*n^7-648*n^6+144*n^4-12*n^2
factor(A*xB^2 +B* xB*y + C*y^2 +D*xB +E*y +F)
yB=7776*n^8+1296*n^7-648*n^6+180*n^4+36*n^3-6*n^2+1
factor(A*x^2 +B* x*yB + C*yB^2 +D*x +E*yB +F)
xC=3888*n^8+2592*n^7+36*n^4+36*n^3+6*n^2
factor(A*xC^2 +B* xC*y + C*y^2 +D*xC +E*y +F)
yC=1296*n^7+648*n^6+72*n^4+12*n^2+1

xA:6n^2 - 36n^3 + 36n^4 + 648n^6 - 1296n^7; yA:3888n^8-2592n^7+180n^4-36n^3-6n^2+1; xB:7776n^8-1296n^7-648n^6+144n^4-12n^2; yB:7776n^8+1296n^7-648n^6+180n^4+36n^3-6n^2+1; xC:3888n^8+2592n^7+36n^4+36n^3+6n^2; yC:1296n^7+648n^6+72n^4+12n^2+1; xO:(xA+xB+xC)/3;yO:(yA+yB+yC)/3; solve([a(xA-xO)^2+2b(xA-xO)(yA-yO)+c(yA-yO)^2-1,a(xB-xO)^2+2b(xB-xO)(yB-yO)+c(yB-yO)^2-1,a(xC-xO)^2+2b(xC-xO)(yC-yO)+c*(yC-yO)^2-1],[a,b,c]); # [[a = 1/(15116544n^16+839808n^12+15552n^8+108n^4), b = -1/(30233088n^16+1679616n^12+31104n^8+216n^4), c = 1/(15116544n^16+839808n^12+15552n^8+108n^4)]]

factor(a(x-xO)^2+2b(x-xO)(y-yO)+c(y-yO)^2-1); # (y^2-xy-3888n^8y-216n^4y-2y+x^2-3888n^8x+x+3888n^8+108n^4+1)/(108n^4(36n^4+1)(3888n^8+108*n^4+1))

R=QQ[n][a,b,c][x,y,z] (mons, cofs)=coefficients(x^3+y^3+(ax+by+c)^3-(ax+by+c+1)^3-(x+y-1)(y^2-xy-3888n^8y-216n^4y-2y+x^2-3888n^8x+x+3888n^8+108n^4+1)) primaryDecomposition ideal cofs -- {ideal(b+c+1,a+c,c-36n^4), ideal(b+c,a+c+1,c+36n^4+1)} solve([b+c+1,a+c,c-36n^4],[a,b,c]); # [[a = -36n^4,b = (-36n^4)-1,c = 36*n^4]] solve([4b+c,a+c+1,c+36n^4+1],[a,b,c]); # [[a = 36n^4,b = (36n^4+1)/4,c = (-36*n^4)-1]]

That is we recover $-36n^4(x+y-1)-(y+z-1)=0.$

And thirdly, $$(x_A,y_A)=(6n + 18n^2 + 180n^3 + 540n^4 + 3240n^5 + 5832n^6 + 34992n^7, \\1 + 36n^2 - 72n^3 + 864n^4 - 3240n^5 + 5832n^6 - 34992n^7)$$ gives

R=QQ[n][x,y,z]
f=(12*n^2 + 324*n^4)*x + (1 + 12*n^2 + 324*n^4)*(y - 1)
factor(x^3+y^3+f^3-(f+1)^3)
(mons,cofs)=coefficients(x^2-x*y+y^2+(-314928*n^8-23328*n^6-432*n^4+1)*x+(-314928*n^8-23328*n^6-2376*n^4-72*n^2-2)*y+314928*n^8+23328*n^6+1404*n^4+36*n^2+1)
A=1;B=-1;C=1;D=-314928*n^8-23328*n^6-432*n^4+1;E=-314928*n^8-23328*n^6-2376*n^4-72*n^2-2;F=314928*n^8+23328*n^6+1404*n^4+36*n^2+1;
xA=6*n + 18*n^2 + 180*n^3 + 540*n^4 + 3240*n^5 + 5832*n^6 + 34992*n^7
factor(A*xA^2 +B* xA*y + C*y^2 +D*xA +E*y +F)
yA=-34992*n^7+5832*n^6-3240*n^5+864*n^4-72*n^3+36*n^2+1
factor(A*x^2 +B* x*yA + C*yA^2 +D*x +E*yA +F)
xB=314928*n^8-69984*n^7+23328*n^6-6480*n^5+756*n^4-252*n^3+18*n^2-6*n
factor(A*xB^2 +B* xB*y + C*y^2 +D*xB +E*y +F)
yB=629856*n^8-34992*n^7+40824*n^6-3240*n^5+2268*n^4-180*n^3+54*n^2-6*n+1
factor(A*x^2 +B* x*yB + C*yB^2 +D*x +E*yB +F)
xC=629856*n^8+34992*n^7+40824*n^6+3240*n^5+1944*n^4+72*n^3+36*n^2
factor(A*xC^2 +B* xC*y + C*y^2 +D*xC +E*y +F)
yC=314928*n^8+69984*n^7+23328*n^6+6480*n^5+2052*n^4+252*n^3+54*n^2+6*n+1

xA:6n + 18n^2 + 180n^3 + 540n^4 + 3240n^5 + 5832n^6 + 34992n^7; yA:-34992n^7+5832n^6-3240n^5+864n^4-72n^3+36n^2+1; xB:314928n^8-69984n^7+23328n^6-6480n^5+756n^4-252n^3+18n^2-6n; yB:629856n^8-34992n^7+40824n^6-3240n^5+2268n^4-180n^3+54n^2-6n+1; xC:629856n^8+34992n^7+40824n^6+3240n^5+1944n^4+72n^3+36n^2; yC:314928n^8+69984n^7+23328n^6+6480n^5+2052n^4+252n^3+54n^2+6n+1; xO:(xA+xB+xC)/3;yO:(yA+yB+yC)/3; solve([a(xA-xO)^2+2b(xA-xO)(yA-yO)+c(yA-yO)^2-1,a(xB-xO)^2+2b(xB-xO)(yB-yO)+c(yB-yO)^2-1,a(xC-xO)^2+2b(xC-xO)(yC-yO)+c*(yC-yO)^2-1],[a,b,c]); # [[a = 1/(99179645184n^16+14693280768n^14+1428513408n^12+88179840n^10+3965760n^8+124416n^6+2700n^4+36n^2),b= -1/(198359290368n^16+29386561536n^14+2857026816n^12 +176359680n^10+7931520n^8+248832n^6+5400n^4+72n^2), c=1/(99179645184n^16+14693280768n^14+1428513408n^12+88179840n^10+3965760n^8+124416n^6+2700n^4+36n^2)]] factor(a(x-xO)^2+2b(x-xO)(y-yO)+c(y-yO)^2-1); # (y^2-xy-314928n^8y-23328n^6y-2376n^4y-72n^2y-2y+x^2-314928n^8x-23328n^6x-432n^4x+x+314928n^8+23328n^6+1404n^4+36n^2+1)/(36n^2(27n^2+1)(324n^4+12n^2+1)(314928n^8+23328n^6+1404n^4+36n^2+1))

R=QQ[n][a,b,c][x,y,z] (mons, cofs)=coefficients(x^3+y^3+(ax+by+c)^3-(ax+by+c+1)^3-(x+y-1)(y^2-xy-314928n^8y-23328n^6y-2376n^4y-72n^2y-2y+x^2-314928n^8x-23328n^6x-432n^4x+x+314928n^8+23328n^6+1404n^4+36n^2+1)) primaryDecomposition ideal cofs -- {ideal(b+c+1,a+c,c-324n^4-12n^2), ideal(b+c,a+c+1,c+324n^4+12n^2+1)} solve([b+c+1,a+c,c-324n^4-12n^2],[a,b,c]); # [[a = (-324n^4)-12n^2,b = (-324n^4)-12n^2-1,c = 324n^4+12n^2]] solve([b+c,a+c+1,c+324n^4+12n^2+1],[a,b,c]); # [[a = 324n^4+12n^2,b = 324n^4+12n^2+1,c = (-324n^4)-12*n^2-1]]

That is we recover $-12n^2(27n^2+1)(x+y-1)-(y+z-1)=0.$

Do you have other points, not necessarily in a family, that I could run through this machine?

  • Thanks for this detailed analysis. Regarding other points, assume Ellipse Family 1 with $n=\pm2$, then we get the same ellipse: $$x^2 - x y + y^2 - 431 x - 506 y + 469 = 0$$ This has 48 lattice points, with $4\times6= 24$ points given by the parameterizations. The other 24 points are the $2\times12$ vertices of the four "triangles" $T_k$ below (and their reflection along the minor axis):
    $$T_1 = (−78,139),(648,1015),(798,289)\ T_2 = (264,823),(990,673),(114,−53)\ T_3 = (−42,49),(522,979),(888,415)\ T_4 = (24,−17),(390,913),(954,547)$$
    – Tito Piezas III Jan 22 '24 at 12:13
  • Note that I've arranged these lattice points such that if seen as the vertices of triangle $T_k$ with $(x_1, y_1), (x_2, y_2), (x_3, y_3)$, then ALL four triangles have the same value $\alpha = x_1+x_2+x_3 = 3\times456$ and $\beta = y_1+y_2+y_3 = 3\times481$ with $\big(\alpha/3, \beta/3\big)$ being the centroid. – Tito Piezas III Jan 22 '24 at 16:37
  • $12(x-y+1)+(y-z-1)=0$ works. factor(x^3+y^3+(12*x+13*y-13)^3-(12*x+13*y-13+1)^3) -- (x+y-1)*(x^2-x*y+y^2-431*x-506*y+469) – Jan-Magnus Økland Jan 22 '24 at 17:01
  • These are the non-parametric lattice points of Ellipse family 1 with $n=2$, so $12 =3n^2$ should still work. I do know other options, but they may involve only 2 known points, 3rd one unknown, so will be tricky. I will go over my notes. – Tito Piezas III Jan 22 '24 at 17:19
  • Typo: It should say: $12(x+y-1)+(y-z-1)=0.$ – Jan-Magnus Økland Jan 22 '24 at 17:31
  • I found Ellipse family 4, but it has only 2 lattice points as polynomials. (The third point may only be rational, not a lattice, point.) See update of the original Sept 2023 post which started these series of posts. – Tito Piezas III Jan 23 '24 at 06:33
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Comment: May be I misunderstood your question, but for degree 2 we may use the following type of Pythagorean triple:

$x=2i+1$

$y=2i(i+1)$

$z=2i(i+1)+1$

For example we have:

$i=1\rightarrow 3^2+4^2=5^2$

$i=2\rightarrow 5^2+12^2=13^2\rightarrow 3^2+4^2+12^2=13^2$

$i=6\rightarrow 13^2+84^2=85^2\rightarrow 5^2+12^2+84^2=85^2$

$i=42\rightarrow 85^2+3612^2=3613^2\rightarrow 13^2+84^2+3612^2=2613^2$

$i=1806\rightarrow 3613^2+6526884^2=6526885^2\rightarrow 85^2+3612^2+6526884^2=6526885^2$

The I's have following recursive relations:

$i_k=(i_{k-1})(i_{k-1}+1)$:

$i= 2, 6, 42, 1806, 3563442\cdot\cdot\cdot $

$a_n$ for involved Pythagorean triples has the following relation in terms of n:

$a_1=3=2\times 1+1$

$a_2=5=2\times 2+1$

$a_3=13$

$a_4=85$

for $n>2$ we have:

$a_n=(n+1)(2^n+1)$

That means there can be infinite quadruples of this kind for degree 2 which only certain numbers are involved in.

sirous
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