Elliptic curves for $a^4+b^4+c^4 = d^4+e^4$ with $d\neq \pm e$?
问题内容
(Moved from previous post since the answers were not elliptic curves.)
I. Question
We seek to find infinitely many primitive solutions to,
$$a^4+b^4+c^4 = d^4+e^4$$
where $d \color{red}{\ne} e$ using polynomial solutions or elliptic curves. The most well-known case when $d = e$ is,
$$a^4+b^4+(a+b)^4 = 2(a^2+ab+b^2)^2$$
where one then solves $a^2+ab+b^2 = z^k$ for $k=2$. (In fact, it can be solved for any positive integer $k$.) But there are infinitely many solutions where $d\pm e \neq 0$,
$$(6n)^4 + (4 - 3 n^4 - n^8)^4 + (4 n - n^9)^4 =\\ (4 + 3 n^4 - n^8)^4 + (2 n + n^9)^4$$
$$(1 - 2 n^4 - n^8)^4 + (2 n - 2 n^9)^4 + (3 n^3 - n^{11})^4 =\\ (1 + 2 n^4 - n^8)^4 + (n^3 + n^{11})^4$$
by Gerardin and Norrie, of deg-$9$ and deg-$11$, respectively. (Are there any of smaller degree?)
II. Numerical solutions
They seem quite rare compared to the case $d=e$. A quick Mathematica search yields only $14$ primitive solutions with all terms $x_i<100$, the smallest $10$ being,
3^4 + 20^4 + 26^4 = 7^4 + 28^4
3^4 + 26^4 + 35^4 = 17^4 + 37^4
1^4 + 25^4 + 42^4 = 17^4 + 43^4
25^4 + 26^4 + 42^4 = 37^4 + 38^4
7^4 + 29^4 + 50^4 = 21^4 + 51^4
5^4 + 42^4 + 78^4 = 51^4 + 76^4
25^4 + 76^4 + 60^4 = 35^4 + 82^4
17^4 + 52^4 + 80^4 = 48^4 + 81^4
39^4 + 60^4 + 79^4 = 51^4 + 83^4
4^4 + 57^4 + 85^4 = 11^4 + 89^4
III. Elliptic curve 1
The $3$rd solution, or $1^4 + 25^4 + 42^4 = 17^4 + 43^4$ also discussed in the previous post, has symmetries we can exploit, namely,
$$1^4 + (a + b)^4 + (2 b)^4 = (-a + b)^4 + (2 b + 1)^4$$
Expanding, this reduces to just a quadratic in $b$,
$$a^3 - 3 b + (a - 4) b^2 = 1$$
where its discriminant $D_1$ must be made a square $D_1 = 4 (a^3 - 1)(4 - a) + 9 = y^2$. From initial rational point $a=4$, we can find an infinite more rational $a$.
IV. Elliptic curve 2
The $4$th solution, or $25^4 + 26^4 + 42^4 = 37^4 + 38^4$, also has symmetries we can exploit,
$$(a - 2d)^4 + (a + 2d)^4 + (4c)^4 = (b - 2d)^4 + (b + 2d)^4$$
Expanding, this reduces to a quadratic in $d$,
$$a^4 - b^4 + 128 c^4 - 24 (-a^2 + b^2) d^2=0$$
Its discriminant $D_2$ must be made a square,
$$D_2 = 24 (-a^2 + b^2)(a^4 - b^4 + 128 c^4) = y^2$$
which is only a quartic in $c$. From the rational point $(a,b)=(17,\,25)$, we find initial rational point $c = 7$, then an infinite more rational (but not integral) $c$.
Note 1: A computer search for small integer $(a,b,c)$ with $\text{GCD}(a,b,c)=1$ yields,
(a,b,c) = (7, 31, 19)
(a,b,c) = (8, 16, 11)
(a,b,c) = (8, 20, 7)
(a,b,c) = (17, 25, 7)
though each of these integer pairs $(a,b)<200$ will yield infinitely many rational $c$.
Note 2: To find useful symmetries on the other solutions was a bit more elusive.
V. Summary
So can we find more polynomial parameterizations or elliptic curves for $a^4+b^4+c^4 = d^4+e^4$ where $d\neq e$?
回答 (1)
$$a^4+b^4+c^4=d^4+e^4 \tag{1}$$
Let $a=x, b=q, c=p-mx, d=p+x, e=q+mx.$ We have $$(-4pm^3-4p-4qm^3)x^2+(-6p^2-6q^2m^2+6p^2m^2)x-4q^3m-4p^3m-4p^3=0 \tag{2}$$
Taking $m = -\dfrac{p^3}{q^3+p^3}$, then we have $$x = \dfrac{-3pq(q^6-q^5p+q^4p^2+2q^3p^3-q^2p^4+qp^5+p^6)}{2(q^7-q^6p+q^5p^2+2q^4p^3-2q^3p^4+2q^2p^5+qp^6-p^7)}.$$
Hence, we get a parametric solution. \begin{align*} a &=3pq(p+q)(q^2-pq+p^2)(p^3+p^2q-pq^2+q^3),\\ b &=2q(p^3+p^2q+q^3)(-p^4+2p^3q-pq^3+q^4),\\ c &=p(-p+q)(p^2+q^2)(2p^4+3p^3q+2q^4),\\ d &=p(2p^3+p^2q-pq^2+q^3)(q^4+p^4),\\ e &=q(2q^7-2q^6p+2q^5p^2+4q^4p^3-q^3p^4+q^2p^5+5qp^6+p^7). \end{align*} $p,q$ are any integers.
Elliptic curve method.
Since the discriminant of the equation $(2)$ must be square number, we get
$$V^2 = (-7m^4-18m^2-7-16m^3-16m)U^4+(-16m^4-16m^3)U^3+(-18m^4+18m^2)U^2+(-16m-16m^4)U-7m^4$$ where $U=\dfrac{p}{q}, V=v/q^2.$ We searched the rational points $(U,V)$ where $m<100$ and height$(U)<100000$ as follows.
m, a,b,c,d,e
[1, 29, 50, 7, 51, 21]
[2, 1093, 2145, 1068, 2211, 41]
[3, 167, 3430, 3672, 3004, 3931]
[4, 28677, 114010, 29140, 114245, 698]
[6, 557, 25970, 21114, 25013, 22628]
[8, 317686, 32944968, 34322405, 31463231, 35486456]
[12, 16402, 79625, 121623, 91603, 117199]
[27, 6, 91, 75, 93, 71]
[28, 611, 14085, 3626, 14093, 3023]
In the case of $m = 1.$
$$V^2 = -64U^4-32U^3-32U-7$$
This equation can be transformed to Weierstrass form :
$$E: Y^2+Y = X^3-3X -18$$
$E$ has has rank $1$ and generator is $P(X,Y)=(3,0).$
We have a new point $(U,V)=\Bigl(-\dfrac{14743}{26084},\ \dfrac{273385569}{85046882} \Bigr)$ using the group law.
Thus, we get a new solution
\begin{align*}
a &=6081641,\\
b &=14789628,\\
c &=2277640,\\
d &=14440922,\\
e &=8707987.
\end{align*}
Similarly, we get
\begin{align*}
a &=65813545798939,\\
b &=11079710231186,\\
c &=61533732099735,\\
d &=70093359498143,\\
e &=54733835567753.
\end{align*}
\begin{align*}
a &=6271992105169230121345,\\
b &=20245820450875964633884,\\
c &=6563170894078724213113,\\
d &=19107155104417184455803,\\
e &=13973828345706734512539.
\end{align*}
Since an elliptic curve $E$ has infinitely many rational points, an equation $(1)$ has nfinitely many rational solutions.