A376206 Prime indices of triangular numbers k such that either k + 1 or k - 2 is prime.
3, 5, 7, 11, 13, 17, 19, 23, 29, 37, 41, 43, 47, 53, 61, 71, 73, 89, 97, 107, 131, 149, 157, 163, 167, 179, 193, 197, 211, 223, 233, 251, 257, 269, 271, 277, 281, 307, 313, 337, 359, 389, 401, 409, 419, 431, 439, 443, 457, 467, 503, 509, 521, 541, 571, 613, 641, 673, 691
Offset: 1
Examples
29 is a term since it is prime and A000217(29) - 2 = 433 is prime.
References
- David Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1987. See entry 496 at p. 155.
Programs
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Mathematica
Select[Range[700], PrimeQ[#]&&(PrimeQ[t=#(#+1)/2+1]||PrimeQ[t-3]) &]