A376205 Prime indices of triangular numbers k such that k + 1 and k - 2 are not prime.

2, 31, 59, 67, 79, 83, 101, 103, 109, 113, 127, 137, 139, 151, 173, 181, 191, 199, 227, 229, 239, 241, 263, 283, 293, 311, 317, 331, 347, 349, 353, 367, 373, 379, 383, 397, 421, 433, 449, 461, 463, 479, 487, 491, 499, 523, 547, 557, 563, 569, 577, 587, 593, 599

Offset: 1

Author

Stefano Spezia, Sep 15 2024

Keywords

• nonn
• easy

Examples

			31 is a term since it is prime and both A000217(31) + 1 = 497 and A000217(31) - 2 = 494 are not 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.

Crossrefs

Cf. A000040 (supersequence), A000124, A000217, A034856.

Complement of A376206 in A000040.

Programs

Mathematica

Select[Range[600], PrimeQ[#]&&!PrimeQ[t=#(#+1)/2+1]&&!PrimeQ[t-3] &]