Yesterday I found the following:
x2 + 2y2 = 3n+2
must have integer non-zero solutions for x and y, for n = 0 and any positive integer n.
The iterator here is:
u2 + 2v2 = F
then it must also be true that
(u-2v)2 + 2(u+v)2 = 3F
One starting result for x and y, for n=0, is u = 1 and u = 2:
12 + 2*22 = 9
For n = 1, iterator gives next solution is x = -3, y = 3:
32 + 2*32 = 27
For n = 2, next is x = 3, y = 0. So that stops. Interesting.
Here's another option.
Starting for n=0, with x = 1, and y=-2:
12 + 2*(-2)2 = 9
For n = 1, iterator says next is x = 5, y = -1:
52 + 2*(-1)2 = 27
For n = 2, next is x = 7, y = 4:
72 + 2*42 = 81
For n = 3, next is x = 1, y = 11:
12 + 2*112 = 243
That's more interesting for a bit, but next iteration with positives will pull in factors of 3 again. Which can be avoided by using a negative value again.
I'll do that just to see, using -11 for y.
For n = 4, with that change iterator says next is x = 23, y = -10:
232 + 2*(-10)2 = 729
So you can just switch to a negative whenever to avoid having 3 as a factor of x or y.
Interesting but tedious.
So it can be a little tricky using the iterator.
James Harris
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