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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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified September 21 08:28 EDT 2023. Contains 365499 sequences. (Running on oeis4.)