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A081944 Numbers k such that both k*(k+1) - 1 and k*(k-1) - 1 are primes. 2

%I

%S 3,4,5,6,9,10,11,16,20,21,31,39,45,46,54,55,56,60,65,66,86,94,101,115,

%T 126,131,141,145,149,154,155,159,160,164,181,196,219,220,231,241,259,

%U 264,265,306,331,350,351,361,374,386,396,419,420,449,450,456,474,475

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

%H Daniel Starodubtsev, <a href="/A081944/b081944.txt">Table of n, a(n) for n = 1..10000</a>

%e 10 belongs to this sequence as both 9*10 - 1 = 89 and 10* 11 - 1 = 109 are primes.

%o (PARI) isok(k) = isprime(k^2+k-1) && isprime(k^2-k-1); \\ _Michel Marcus_, Nov 19 2019

%Y Cf. A081945.

%K nonn

%O 1,1

%A _Amarnath Murthy_, Apr 02 2003

%E More terms from _Don Reble_, Apr 08 2003

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Last modified July 24 04:25 EDT 2021. Contains 346273 sequences. (Running on oeis4.)