# show no solutions exists using table x^3mod9

• May 21st 2012, 04:01 AM
MattWT
show no solutions exists using table x^3mod9
Hi wondering if anyone can help with this question:

Make a table of values x^3, mod 9, and show that the equation x^2 + 2y^2 = 9z^2 has no non zero integer solutions.

My table looks like

n mod 9 - -4 -3 -2 -1 0 1 2 3 4
n^3mod 9 - -1 0 1 -1 0 1 -1 0 1
2n^3mod 9 --2 0 2 -2 0 2 -2 0 2

i can see that by adding the two rows i get 0's at 0 and +-3 but dont really know where to go from here.

Also if i am not given the modulo to use, how do i decide which modulo to use.
• May 23rd 2012, 01:02 AM
MathFan
Re: show no solutions exists using table x^3mod9
Quote:

Originally Posted by MattWT
Hi wondering if anyone can help with this question:

My table looks like

n mod 9 - -4 -3 -2 -1 0 1 2 3 4 ?
n^3mod 9 - -1 0 1 -1 0 1 -1 0 1 ?

 img.top {vertical-align:15%;} $n\ mod\ 9$ 0 1 2 3 4 5 6 7 8 img.top {vertical-align:15%;} $n^3\ mod\ 9$ 0 1 8 0 1 8 0 1 8

btw, are you sure you're not being asked to construct table for $x^2\ mod\ 9$ not $x^3\ mod\ 9$ ?
• Jun 12th 2012, 10:02 PM
richard1234
Re: show no solutions exists using table x^3mod9
You'll want to make a table for $x^2$ mod 9. For example,

$1^2 \equiv 1$
$2^2 \equiv 4$
$3^2 \equiv 0$
$4^2 \equiv 7$
$5^2 \equiv 7$
$6^2 \equiv 0$
$7^2 \equiv 4$
$8^2 \equiv 1$
$9^2 \equiv 0$

Assume that at least one of x,y,z is not a multiple of 3. Even then, you can't prove that using the table, for example, you could have

$1^2 + 2*7^2 \equiv 0 (mod 9)$