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 A092561 "Mersenne" semiprimes, semiprimes of the form 2^k-1. 8
 15, 511, 2047, 8388607, 137438953471, 2199023255551, 562949953421311, 576460752303423487, 147573952589676412927, 9671406556917033397649407, 158456325028528675187087900671, 2535301200456458802993406410751 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Amiram Eldar, Table of n, a(n) for n = 1..42 FORMULA a(n) = A000225(A085724(n)). - Amiram Eldar, Jun 18 2022 EXAMPLE 2047 is a member because 2047 = 2^11 - 1 = 23*89. MATHEMATICA a := Select[Range[2, 120], ! PrimeQ[2^# - 1] && Length[Divisors[2^# - 1]] <= 4 &]; 2^a - 1 (* Stefan Steinerberger, Apr 12 2006 *) Select[2^Range[0, 110]-1, PrimeOmega[#] == 2&] (* Harvey P. Dale, Feb 22 2013 *) CROSSREFS Corresponding k: A085724. Cf. A000225, A092558, A092559, A092562. Sequence in context: A218188 A218365 A203326 * A218312 A218280 A218632 Adjacent sequences: A092558 A092559 A092560 * A092562 A092563 A092564 KEYWORD nonn AUTHOR Zak Seidov, Feb 27 2004 EXTENSIONS One more term from Stefan Steinerberger, Apr 12 2006 STATUS approved

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Last modified March 25 18:33 EDT 2023. Contains 361528 sequences. (Running on oeis4.)