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A297362
Numbers k such that (2^ord(2, k) - 1)/k is prime, where ord(2, k) is the multiplicative order of 2 (mod k).
2
5, 9, 21, 23, 33, 47, 51, 73, 85, 89, 93, 129, 167, 217, 223, 263, 315, 341, 381, 585, 819, 1057, 1365, 3591, 3855, 4681, 4871, 5461, 6141, 6223, 6719, 7487, 8193, 11447, 13107, 13367, 13797, 14329, 16513, 18631, 21845, 24573, 25575, 26431, 33825, 37449
OFFSET
1,1
COMMENTS
The corresponding primes are 3, 7, 3, 89, 31, 178481, 5, 7, 3, 23, 11, ...
LINKS
EXAMPLE
5 is in the sequence since ord(2, 5) = 4 and (2^4 - 1)/5 = 3 is prime.
MATHEMATICA
aQ[n_] := PrimeQ[(2^MultiplicativeOrder[2, n] - 1)/n]; Select[Range[10000], aQ]
PROG
(PARI) is(n) = n%2 && isprime((2^znorder(Mod(2, n))-1)/n); \\ Amiram Eldar, Aug 26 2023
CROSSREFS
Sequence in context: A273150 A273388 A273449 * A376288 A175364 A244642
KEYWORD
nonn
AUTHOR
Amiram Eldar, Dec 29 2017
STATUS
approved