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 A078723 #23 Jun 14 2025 00:21:48
%S A078723 3,11,23,43,71,103,149,193,251,313,389,463,569,653,761,881,1013,1129,
%T A078723 1289,1451,1601,1777,1979,2143,2357,2591,2789,3023,3301,3539,3793,
%U A078723 4057,4349,4651,4969,5297,5639,5927,6299,6673,7013,7459,7823,8237,8677,9067,9479
%N A078723 a(n) = prime(n*(n+1)/2 + n).
%C A078723 The sum of the reciprocals appears to converge.
%F A078723 a(n) = A000040(A000096(n)).
%t A078723 Table[Prime[(n(n+1))/2+n],{n,50}] (* _Harvey P. Dale_, Jan 04 2011 *)
%o A078723 (PARI) triprimes(n) = { sr = 0; for(j=1,n, x = j*(j+1)/2 +j; z = prime(x); sr+=1.0/z; print1(z" "); ); print(); print(sr); }
%Y A078723 Primes in second diagonal from right in triangle in A078721.
%Y A078723 Cf. A000040, A000096, A000217.
%K A078723 easy,nonn
%O A078723 1,1
%A A078723 _Cino Hilliard_, Dec 20 2002
%E A078723 Offset changed to 1 by _Alois P. Heinz_, May 18 2019