A155565 Intersection of A001481 and A020669: N = a^2 + b^2 = c^2 + 5d^2 for some integers a,b,c,d.

0, 1, 4, 5, 9, 16, 20, 25, 29, 36, 41, 45, 49, 61, 64, 80, 81, 89, 100, 101, 109, 116, 121, 125, 144, 145, 149, 164, 169, 180, 181, 196, 205, 225, 229, 241, 244, 245, 256, 261, 269, 281, 289, 305, 320, 324, 349, 356, 361, 369, 389, 400, 401, 404, 405, 409, 421

Offset

1,3

Comments

Contains A155575 as a subsequence (obtained by restricting a,b,c,d to be nonzero). Also contains A000290 (squares) as subsequence.

Links

Table of n, a(n) for n=1..57.

Prog

(PARI) isA155565(n, /* use optional 2nd arg to get other analogous sequences */c=[5, 1]) = { for(i=1, #c, for(b=0, sqrtint(n\c[i]), issquare(n-c[i]*b^2) & next(2)); return); 1}
for( n=1, 500, isA155565(n) & print1(n", "))

Crossrefs

Cf. A001481, A002479, A003136, A002481, A020668 ff.

Sequence in context: A010437 A020682 A243702 * A278336 A051216 A255978

Adjacent sequences: A155562 A155563 A155564 * A155566 A155567 A155568

Keyword

easy,nonn

Author

M. F. Hasler, Jan 25 2009

Status

approved