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 A365849 #14 Nov 18 2023 13:11:19
%S A365849 21,253,1081,13861,34453,64261,73153,114481,126253,258121,351541,
%T A365849 371953,392941,482653,869221,933661,1034641,1104841,1660753,2077741,
%U A365849 2126953,2434321,2992681,4142881,4212253,4495501,4567753,4862521,5013361,6561253,7459453,8026021
%N A365849 Triangular numbers that are the product of two distinct prime numbers of the form 4*k + 3.
%C A365849 Intersection of A068443 and A016105.
%C A365849 Subsequence of A156592.
%C A365849 Apparently, a(n) == 1 (mod 36) for n >= 2. - _Hugo Pfoertner_, Nov 03 2023
%e A365849 21 = A068443(4) and 21 = A016105(1), so 21 is a term.
%e A365849 253 = A068443(7) and 253 = A016105(18), so 253 is a term.
%t A365849 Select[Accumulate[Range[4500]], (f = FactorInteger[#])[[;; , 2]] == {1, 1} && Mod[f[[;; , 1]], 4] == {3, 3} &] (* _Amiram Eldar_, Oct 11 2023 *)
%o A365849 (Magma) pd:=PrimeDivisors; blum:=func<n|#Divisors(n) eq 4 and #pd(n) eq 2 and pd(n)[1] mod 4 eq 3 and pd(n)[2] mod 4 eq 3>; [n:n in [1..9000000]|IsSquare(8*n+1) and blum(n)];
%Y A365849 Cf. A000217, A001358, A068443, A156592, A016105.
%K A365849 nonn
%O A365849 1,1
%A A365849 _Marius A. Burtea_, Oct 09 2023