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 A034954 #18 Feb 16 2025 08:32:37
%S A034954 3,15,91,153,435,703,861,1431,1891,2701,4005,4753,5151,5995,6441,9453,
%T A034954 11175,12403,15051,16471,18721,19503,26335,27261,29161,33153,36315,
%U A034954 38503,39621,43071,49141,50403,56953,61075,62481,69751,75855,79003
%N A034954 Odd triangular numbers with prime indices.
%C A034954 For prime indices p such that p(p+1)/2 is 'odd' see A002313 (primes of form 4n+1).
%H A034954 G. C. Greubel, <a href="/A034954/b034954.txt">Table of n, a(n) for n = 1..5000</a>
%H A034954 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/TriangularNumber.html">Triangular Number</a>
%F A034954 a(n) = A000217(A002313(n)). - _Michel Marcus_, Nov 04 2017
%t A034954 Select[Table[Prime[n] (Prime[n] + 1)/2, {n, 500}], OddQ[#] &] (* _G. C. Greubel_, Nov 03 2017 *)
%t A034954 With[{nn=400},Select[Thread[{Accumulate[Range[nn]],Range[nn]}],OddQ[ #[[1]]] && PrimeQ[#[[2]]]&]][[All,1]] (* _Harvey P. Dale_, Mar 06 2019 *)
%Y A034954 Cf. A000217, A002313.
%Y A034954 Cf. A014493, A034953, A034955.
%K A034954 nonn
%O A034954 1,1
%A A034954 _Patrick De Geest_, Oct 15 1998