Given a,b solve ax^4+by^4=z^2. With a,b>0?

Rewrite to: ak^2+bl^2=m with k=x^2, l=y^2, m=z^2.

Forgetting the conditions on k,l,m it's easy to see that k,l ->m.

A fast routine would not test all possible (k,l) but only those who can be a square and testing m=z^2 at the last step.

For instance k=x^2 ,gives k=0,1 mod 4, regardless of x mod 4, the same for l so we have depending on the combinations (k,l) mod 4:

(0,0) m=0 mod 4 no further restriction on k,l

(1,0) m=a mod 4 a=0,1 mod 4 possible solution, else no solution

(0,1) m=b mod 4 b=0,1 mod 4 possible solution, else no solution

(1,1) m=(a+b) mod 4 a+b=0,1 mod 4 possible solution, else no solution

Other moduli will/could further restrict the choices for k,l.

If (x,y,z) is a solution so is (cx,cy,c^2z). So make sure x and y are relative prime.

Doing this 'fast' is not a matter of using UBASIC (can help) but thinking beforehand about possible solutions.

Jan van Delden

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