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 A055131 #9 Jul 02 2025 16:01:59
%S A055131 15,39,51,87,111,123,159,183,219,267,291,303,327,339,411,447,471,519,
%T A055131 543,579,591,687,699,723,771,807,831,843,879,939,951,1011,1047,1059,
%U A055131 1119,1167,1191,1203,1227,1263,1299,1347,1371,1383,1527,1563,1623,1671
%N A055131 Those composite s for which A055095[s] = 2.
%F A055131 a(n) = 3*((4*A005098[n])+1) = 3*A002144[n] ??? (Conjecture, not yet proved)
%p A055131 find_A055095_is_2_composites := proc(upto_n) local j,a; a := []; for j from 1 to upto_n do if(-1 = (j - wt(GrayCode(qrs2bincode((2*j)+1))))) then if(not isprime((2*j)+1)) then a := [op(a),((2*j)+1)]; fi; fi; od; RETURN(a); end;
%t A055131 A005811[n_] := Length[Length /@ Split[IntegerDigits[n, 2]]];
%t A055131 A055094[n_] := With[{rr = Table[Mod[k^2, n], {k, 1, n-1}] // Union}, Boole[MemberQ[rr, #]] & /@ Range[n-1]] // FromDigits[#, 2]&;
%t A055131 A055095[1] = 0; A055095[n_] := 2*A005811[A055094[n]] - (n-1);
%t A055131 A055131 = Position[Array[A055095, 2000], 2] // Flatten // Select[#, CompositeQ]& (* _Jean-François Alcover_, Mar 06 2016 *)
%K A055131 nonn
%O A055131 0,1
%A A055131 _Antti Karttunen_, Apr 04 2000
%E A055131 More terms from _James Sellers_, Apr 21 2000