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

1,1

References

David Wells, The Penguin Dictionary of Curious and Interesting Numbers. Penguin Books, NY, 1986, Revised edition 1987. See entry 496 at p. 155.

Links

Table of n, a(n) for n=1..54.

Example

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

Mathematica

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

Crossrefs

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

Complement of A376206 in A000040.

Sequence in context: A370157 A129900 A262834 * A229206 A105757 A091252

Adjacent sequences: A376202 A376203 A376204 * A376206 A376207 A376208

Keyword

nonn,easy

Author

Stefano Spezia, Sep 15 2024

Status

approved