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A081944
Numbers k such that both k*(k+1) - 1 and k*(k-1) - 1 are primes.
2
3, 4, 5, 6, 9, 10, 11, 16, 20, 21, 31, 39, 45, 46, 54, 55, 56, 60, 65, 66, 86, 94, 101, 115, 126, 131, 141, 145, 149, 154, 155, 159, 160, 164, 181, 196, 219, 220, 231, 241, 259, 264, 265, 306, 331, 350, 351, 361, 374, 386, 396, 419, 420, 449, 450, 456, 474, 475
OFFSET
1,1
LINKS
EXAMPLE
10 belongs to this sequence as both 9*10 - 1 = 89 and 10* 11 - 1 = 109 are primes.
PROG
(PARI) isok(k) = isprime(k^2+k-1) && isprime(k^2-k-1); \\ Michel Marcus, Nov 19 2019
CROSSREFS
Cf. A081945.
Sequence in context: A338321 A220844 A047250 * A129948 A050414 A342469
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Apr 02 2003
EXTENSIONS
More terms from Don Reble, Apr 08 2003
STATUS
approved