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A376205
Prime indices of triangular numbers k such that k + 1 and k - 2 are not prime.
1
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.
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
KEYWORD
nonn,easy
AUTHOR
Stefano Spezia, Sep 15 2024
STATUS
approved