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A208294
Triangular numbers T from A000217 such that (4*T+1)/13 is prime.
3
55, 120, 1485, 1953, 2278, 2850, 3240, 6555, 9870, 10585, 16110, 17020, 18528, 34453, 36585, 37950, 47895, 49455, 52003, 56280, 57970, 70125, 72010, 80200, 96580, 110685, 127260, 129795, 133903, 165025, 169653, 172578, 244650
OFFSET
1,1
COMMENTS
The corresponding primes are gven in A208292, where equivalent formulations are found.
The indices of these triangular numbers are given by (A208293(n)-1)/2.
LINKS
FORMULA
a(n) = T(K(n)):= A000217(K(n)) with K(n)=(A208293(n)-1)/2.
EXAMPLE
a(2) = 120. m(2)= 31: 120 = T((31-1)/2) = T(15)=A000217(15). (4*120+1)/13 = 37 = A208292(2).
MATHEMATICA
tri = # (# + 1)/2 & /@ Range@ 1000; Select[ tri, PrimeQ[(4 # + 1)/13] &] (* Robert G. Wilson v, Feb 28 2012 *)
KEYWORD
nonn
AUTHOR
Wolfdieter Lang, Feb 27 2012
STATUS
approved