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