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A155717 Numbers of the form N = a^2 + 7b^2 for some positive integers a,b. 11
8, 11, 16, 23, 29, 32, 37, 43, 44, 53, 56, 64, 67, 71, 72, 77, 79, 88, 92, 99, 107, 109, 112, 113, 116, 121, 127, 128, 137, 144, 148, 149, 151, 161, 163, 172, 176, 179, 184, 191, 193, 197, 200, 203, 207, 211, 212, 224, 232, 233, 239, 253, 256, 259, 261, 263, 268 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Subsequence of A020670 (which allows for a and b to be zero).

LINKS

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

MATHEMATICA

Select[Range[300], Reduce[a>0 && b>0 && # == a^2 + 7b^2, {a, b}, Integers] =!= False&] (* Jean-Fran├žois Alcover, Nov 17 2016 *)

PROG

(PARI) isA155717(n, /* optional 2nd arg allows us to get other sequences */c=7) = { for(b=1, sqrtint((n-1)\c), issquare(n-c*b^2) & return(1))}

for( n=1, 300, isA155717(n) & print1(n", "))

CROSSREFS

Cf. A000404, A154777, A092572, A097268, A154778, A155707-A155716, A155560-A155578.

Sequence in context: A154685 A068591 A065348 * A188197 A234096 A111254

Adjacent sequences:  A155714 A155715 A155716 * A155718 A155719 A155720

KEYWORD

easy,nonn

AUTHOR

M. F. Hasler, Jan 25 2009

STATUS

approved

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Last modified December 7 11:24 EST 2016. Contains 278874 sequences.