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%I #19 Oct 31 2013 12:17:48
%S 55,120,1485,1953,2278,2850,3240,6555,9870,10585,16110,17020,18528,
%T 34453,36585,37950,47895,49455,52003,56280,57970,70125,72010,80200,
%U 96580,110685,127260,129795,133903,165025,169653,172578,244650
%N Triangular numbers T from A000217 such that (4*T+1)/13 is prime.
%C The corresponding primes are gven in A208292, where equivalent formulations are found.
%C The indices of these triangular numbers are given by (A208293(n)-1)/2.
%H Vincenzo Librandi, <a href="/A208294/b208294.txt">Table of n, a(n) for n = 1..5000</a>
%F a(n) = T(K(n)):= A000217(K(n)) with K(n)=(A208293(n)-1)/2.
%e a(2) = 120. m(2)= 31: 120 = T((31-1)/2) = T(15)=A000217(15). (4*120+1)/13 = 37 = A208292(2).
%t tri = # (# + 1)/2 & /@ Range@ 1000; Select[ tri, PrimeQ[(4 # + 1)/13] &] (* _Robert G. Wilson v_, Feb 28 2012 *)
%Y Cf. A208292. A208293, A207337, A207339, A129307, A027862, A002731.
%K nonn
%O 1,1
%A _Wolfdieter Lang_, Feb 27 2012
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