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 A380509 #33 Jan 31 2025 04:23:45
%S A380509 6,11,12,16,19,20,21,26,29,30,31,33,36,38,40,41,42,46,47,51,52,54,55,
%T A380509 56,61,63,65,66,68,71,72,74,75,76,81,82,83,85,86,89,90,91,92,94,96,
%U A380509 101,103,106,107,109,110,111,116,117,118,119,120,121,123,124,126,128,129,131,132,133
%N A380509 Numbers of the form i+j+4ij for i,j >= 1 together with numbers of the form -i-j+4ij for i,j >= 2.
%e A380509 1+1+4*1*1 = 6; -2-2+4*2*2 = 12.
%p A380509 L := 150: S := {}:
%p A380509 for i to L do for j to L do
%p A380509 if 4*i*j + i + j <= L then S := `union`(S, {4*i*j+i+j}, {4*i*j+3*i+3*j+2}) fi;
%p A380509 od; od;
%p A380509 S; # _Peter Bala_, Jan 30 2025
%Y A380509 Cf. A054520, A380140, A380572 (complement), A380549, A380550.
%K A380509 nonn
%O A380509 1,1
%A A380509 _Davide Rotondo_, Jan 26 2025
%E A380509 More terms from _Stefano Spezia_, Jan 26 2025
%E A380509 Missing terms 111, 121 and 126 added by _Peter Bala_, Jan 30 2025