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A195917
Numbers k such that both k+1 and 7k+1 are squares.
2
0, 24, 120, 6240, 30624, 1585080, 7778520, 402604224, 1975713600, 102259887960, 501823476024, 25973608937760, 127461187196640, 6597194410303224, 32374639724470680, 1675661406608081280, 8223031028828356224, 425611400084042342040, 2088617506682678010360
OFFSET
1,2
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
Sequence in context: A293050 A052581 A052605 * A334189 A042120 A002980
KEYWORD
nonn,easy
AUTHOR
Sture Sjöstedt, Oct 26 2011
EXTENSIONS
More terms from Bruno Berselli, Oct 28 2011
STATUS
approved