A020670 Numbers of form x^2 + 7y^2.

0, 1, 4, 7, 8, 9, 11, 16, 23, 25, 28, 29, 32, 36, 37, 43, 44, 49, 53, 56, 63, 64, 67, 71, 72, 77, 79, 81, 88, 92, 99, 100, 107, 109, 112, 113, 116, 121, 127, 128, 137, 144, 148, 149, 151, 161, 163, 169, 172, 175, 176, 179, 184, 191, 193, 196, 197, 200, 203, 207, 211, 212, 224

Offset

1,3

Links

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references)

Mathematica

lim=250; k=7; Union@Flatten@Table[x^2 + k y^2, {y, 0, Sqrt[lim/k]}, {x, 0, Sqrt[lim-k y^2]}] (* Vincenzo Librandi, Aug 31 2016 *)

Prog

(PARI) is(n)=my(f=factor(n)); for(i=1, #f[, 1], if(kronecker(f[i, 1], 7)<0 && f[i, 2]%2, return(0))); n%4!=2 \\ Charles R Greathouse IV, Nov 18 2012
(Magma) [n: n in [0..230] | NormEquation(7, n) eq true]; // Vincenzo Librandi, Aug 31 2016

Crossrefs

Cf. A033207.

Sequence in context: A309264 A161986 A324940 * A253472 A255060 A047538

Adjacent sequences: A020667 A020668 A020669 * A020671 A020672 A020673

Keyword

easy,nonn

Author

David W. Wilson

Status

approved