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 A034955 #23 Feb 16 2025 08:32:37
%S A034955 6,28,66,190,276,496,946,1128,1770,2278,2556,3160,3486,5356,5778,8128,
%T A034955 8646,9730,11476,13366,14028,16110,18336,19900,22366,24976,25878,
%U A034955 28680,31626,34716,36856,40186,47278,48516,54946,60378,64620,67528,72010,73536,87990
%N A034955 Even triangular numbers with prime indices.
%C A034955 For prime indices p such that p(p+1)/2 is even, see A002145 (Primes of form 4n+3).
%H A034955 G. C. Greubel, <a href="/A034955/b034955.txt">Table of n, a(n) for n = 1..5000</a>
%H A034955 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a>
%F A034955 a(n) = A000217(A002145(n)). - _Michel Marcus_, Nov 04 2017
%t A034955 Select[Table[Prime[n] (Prime[n] + 1)/2, {n, 500}], EvenQ[#] &] (* _G. C. Greubel_, Nov 03 2017 *)
%o A034955 (Python)
%o A034955 from sympy import primerange
%o A034955 def aupto(lim):
%o A034955 pitri = (p*(p+1)//2 for p in primerange(2, int((2*lim)**.5)+1))
%o A034955 return [t for t in pitri if t%2 == 0]
%o A034955 print(aupto(90000)) # _Michael S. Branicky_, Jun 28 2021
%Y A034955 Cf. A000217, A002145.
%Y A034955 Cf. A014493, A034953, A034954.
%K A034955 nonn
%O A034955 1,1
%A A034955 _Patrick De Geest_, Oct 15 1998