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 A207339 #11 Sep 18 2019 20:43:00 %S A207339 6,21,36,66,91,136,171,351,496,561,741,946,1176,1326,1596,2016,2346, %T A207339 2701,2926,3321,3486,4851,6216,6441,7626,8646,8911,9591,10011,10296, %U A207339 11026,11476,13041,15051,16471,16836,17391,18336,19701,21736 %N A207339 Triangular numbers T from A000217 such that (4*T+1)/5 is prime. %C A207339 The corresponding primes are gven in A207337, where also equivalent formulations are found. %C A207339 The indices of these triangular numbers are given by (A002733(n)-1)/2. %H A207339 Harvey P. Dale, <a href="/A207339/b207339.txt">Table of n, a(n) for n = 1..1000</a> %F A207339 a(n) = T(K(n)):= A000217(K(n)) with K(n)=(m(n)-1)/2, and m(n) given in A002733(n). %e A207339 a(3) = 36 = T((17-1)/2) = T(8)=A000217(8). (4*36+1)/5 = 29 = A207337(3). %t A207339 Select[Accumulate[Range[300]],PrimeQ[(4#+1)/5]&] (* _Harvey P. Dale_, Sep 18 2019 *) %Y A207339 Cf. A207337, A129307, A027862, A002731. %K A207339 nonn %O A207339 1,1 %A A207339 _Wolfdieter Lang_, Feb 27 2012