A081944 Numbers k such that both k*(k+1) - 1 and k*(k-1) - 1 are primes.

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

Daniel Starodubtsev, Table of n, a(n) for n = 1..10000

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

Adjacent sequences: A081941 A081942 A081943 * A081945 A081946 A081947

Keyword

nonn

Author

Amarnath Murthy, Apr 02 2003

Extensions

More terms from Don Reble, Apr 08 2003

Status

approved