A159935 Least integer such that a(n)^2 - n is the sum of two nonzero squares.

5, 3, 2, 4, 3, 5, 4, 3, 4, 7, 6, 4, 5, 9, 4, 5, 6, 5, 6, 6, 5, 11, 12, 5, 7, 15, 6, 8, 6, 7, 8, 6, 7, 13, 6, 8, 7, 21, 8, 7, 9, 7, 10, 12, 7, 15, 8, 7, 10, 9, 10, 8, 9, 11, 8, 9, 8, 17, 30, 8, 10, 9, 8, 9, 9, 13, 10, 18, 9, 11, 12, 9, 12, 9, 10, 10, 9, 15, 16, 9, 10, 11, 10, 10, 11, 21, 12, 10, 15

Offset: 0

Author

Franklin T. Adams-Watters, Apr 26 2009

Keywords

• nonn

Crossrefs

Cf. A008846, A050795.

Programs

PARI

issum2sq(n) = local(fm, hf); hf=0;fm=factor(n);for(i=1,matsize(fm)[1],if(fm[i,1]==2,if(fm[i,2]%2,hf=1),if(fm[i,1]%4==1,hf=1,if(fm[i, 2]%2,return(0)))));hf
minsum2sq(n) = local(k); k=1;while(!issum2sq(k^2-n),k++);k
/* Note: the issum2sq function depends on PARI returning -1 as a factor for negative n. */