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A135980
Numbers k such that the Mersenne number 2^prime(k)-1 is composite.
3
5, 9, 10, 12, 13, 14, 15, 16, 17, 19, 20, 21, 22, 23, 25, 26, 27, 29, 30, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78
OFFSET
1,1
COMMENTS
A135979 is a subsequence of this sequence.
FORMULA
prime(a(n)) = A054723(n).
MATHEMATICA
k = {}; Do[If[ ! PrimeQ[2^Prime[n] - 1], AppendTo[k, n]], {n, 1, 40}]; k
m = PrimePi @ MersennePrimeExponent @ Range[13]; Complement[Range[m[[-1]]], m] (* Amiram Eldar, Mar 12 2020 *)
PROG
(PARI) isok(k) = !isprime(2^prime(k)-1); \\ Michel Marcus, Mar 12 2020
KEYWORD
nonn
AUTHOR
Artur Jasinski, Dec 09 2007
EXTENSIONS
More terms from Amiram Eldar, Mar 12 2020
STATUS
approved