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 A328791 #12 Sep 15 2021 11:15:57 %S A328791 3,28,903,30628,1040403,35343028,1200622503,40785822028,1385517326403, %T A328791 47066803275628,1598885794044903,54315050194251028, %U A328791 1845112820810490003,62679520857362409028,2129258596329511416903,72332112754346025765628,2457162575051435364614403 %N A328791 Triangular numbers of the form k^2 + 3. %C A328791 There exist triangular numbers of the form k^2 + j for j=0 (A001110), j=1 (A164055), j=2 (A214838), and j=3 (this sequence), but not for j=4,7,8,13,16,18,... (A328792). %F A328791 a(1) = 3, a(2) = 28; for n > 2, a(n) = 34*a(n-1) - a(n-2) - 46. %o A328791 (Python) %o A328791 limit = 10**7 # rough limit for k %o A328791 A000217 = set(k*(k+1)//2 for k in range(14*limit//10)) %o A328791 A117950 = set(k**2 + 3 for k in range(limit)) %o A328791 print(sorted(A000217 & A117950)) # _Michael S. Branicky_, Mar 28 2021 %Y A328791 Cf. A001110, A164055, A214838, A328792. %Y A328791 Intersection of A000217 and A117950. %Y A328791 Cf. A276598 (the k's). %K A328791 nonn %O A328791 1,1 %A A328791 _Jon E. Schoenfield_, Oct 27 2019