How did Letac solve $x_1^k + x_2^k + \dots +x_9^k = 0$ for $k = 1, 3, 5, 7$ in 1942?

388 Views Asked by At

It's quite easy to find integer solutions to,

$$x_0^k + x_1^k + \dots +x_9^k = 0$$

for $k = 1, 3, 5, 7$. One I found is, if $x^2-10y^2 = 9$, then,

$$1 + 5^k + (3+2y)^k + (3-2y)^k + (-3+3y)^k + (-3-3y)^k = (-2+x)^k + (-2-x)^k + (5-y)^k + (5+y)^k$$

However, Letac found in 1942 two special cases with $x_0 = 0$ (the case $x_0 = x_1 = 0$ is impossible), namely,

$$\{\,0, 34, 58, 82, 98\,\} = \{\,13, 16, 69, 75, 99\,\}$$

$$\{\,0, 63, 119, 161, 169\,\} = \{\,8, 50, 132, 148, 174\,\}$$

How did he do it?

The reference is A. Letac, Gazeta Matematica, 48 (1942), p. 68-69, and I'm hoping someone with JSTOR can explicitly give the answer.

1

There are 1 best solutions below

1
On BEST ANSWER

Of the two solutions you mention, only the second appears in Letac’s note.

He used (among other things) the following two lemmas :

Fact 1. If $a_1^k+a_2^k+a_3^k=b_1^k+b_2^k+b_3^k$ for $k=2$ and $k=4$, then $$ a_1^p+(\pm a_2)^p+(\pm a_3)^p+(2b_1)^p+(2b_2)^p+(2b_3)^p= b_1^p+(\pm b_2)^p+(\pm b_3)^p+(2a_1)^p+(2a_2)^p+(2a_3)^p \ (p=2,4,6,8) $$

Fact 2. Under the hypotheses of fact 1, for $h=\frac{a_1+a_2-a_3}{4}$ we have

$$ (2a_1-3h)^k+(2a_2-h)^k+(2a_2-3h)^k+(2a_2-h)^k+(2b_3+h)^k= (2a_3+h)^k+(2a_3+h)^k+(2b_1-h)^k+(2b_1-h)^k+(2b_2-h)^k+(3h)^k \ (k=1,3,5) $$