This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A247881 #16 Jun 20 2025 10:47:10
%S A247881 0,1,4,9,13,14,16,17,22,25,29,36,38,49,52,53,56,61,62,64,68,77,81,88,
%T A247881 94,100,101,113,116,117,118,121,126,133,134,142,144,152,153,157,166,
%U A247881 169,173,181,182,196,198,208,209,212,217,221,224,225,233,238,244,248,256,257,261,269
%N A247881 Numbers of the form x^2 + 13*y^2.
%C A247881 Norms of numbers in Z[sqrt(-13)].
%H A247881 Robert Israel, <a href="/A247881/b247881.txt">Table of n, a(n) for n = 1..10000</a>
%p A247881 N:= 1000: # for terms <= N
%p A247881 S:= {seq(seq(x^2 + 13*y^2, x=0 .. floor(sqrt(N-13*y^2))),y=0..floor(sqrt(N/13)))}:
%p A247881 sort(convert(S,list)); # _Robert Israel_, Jun 19 2025
%Y A247881 Cf. A033210 (subsequence of primes).
%K A247881 easy,nonn
%O A247881 1,3
%A A247881 _Alonso del Arte_, Sep 25 2014