A195917 Numbers k such that both k+1 and 7k+1 are squares.

0, 24, 120, 6240, 30624, 1585080, 7778520, 402604224, 1975713600, 102259887960, 501823476024, 25973608937760, 127461187196640, 6597194410303224, 32374639724470680, 1675661406608081280, 8223031028828356224, 425611400084042342040, 2088617506682678010360

Offset

1,2

Links

Harvey P. Dale, Table of n, a(n) for n = 1..832

Index entries for linear recurrences with constant coefficients, signature (1,254,-254,-1,1).

Formula

a(n) = A161852(n)^2-1. - Bruno Berselli, Oct 28 2011

G.f.: -24*x^2*(1+4*x+x^2) / ( (x-1)*(x^2-16*x+1)*(x^2+16*x+1) ). - R. J. Mathar, Nov 04 2011

Mathematica

LinearRecurrence[{1, 254, -254, -1, 1}, {0, 24, 120, 6240, 30624}, 20] (* Harvey P. Dale, Apr 01 2020 *)

Crossrefs

Cf. A195878, A161852.

Sequence in context: A293050 A052581 A052605 * A334189 A042120 A002980

Adjacent sequences: A195914 A195915 A195916 * A195918 A195919 A195920

Keyword

nonn,easy

Author

Sture Sjöstedt, Oct 26 2011

Extensions

More terms from Bruno Berselli, Oct 28 2011

Status

approved