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A135976 Mersenne composites A065341 with exactly 2 prime factors. 6
2047, 8388607, 137438953471, 2199023255551, 576460752303423487, 147573952589676412927, 9671406556917033397649407, 158456325028528675187087900671, 2535301200456458802993406410751 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..9.

Wikipedia, Semiprime

MAPLE

A135976 := proc(n) local i;

i := 2^(ithprime(n))-1:

if (nops(numtheory[factorset](i)) = 2) then

   RETURN (i)

fi: end: [ seq(A135976(n), n=1..26) ]; # Jani Melik, Feb 09 2011

MATHEMATICA

k = {}; Do[If[ ! PrimeQ[2^Prime[n] - 1], c = FactorInteger[2^Prime[n] - 1]; d = Length[c]; If[d == 2, AppendTo[k, 2^Prime[n] - 1]]], {n, 1, 40}]; k

PROG

(PARI) forprime(p=1, 1e2, if(bigomega(2^p-1)==2, print1(2^p-1, ", "))) \\ Felix Fröhlich, Aug 12 2014

CROSSREFS

Subsequence of A065341.

Cf. A000225, A054723, A134852, A135975, A135977, A135978, A135979.

Sequence in context: A069438 A131952 A065341 * A236373 A289476 A222526

Adjacent sequences:  A135973 A135974 A135975 * A135977 A135978 A135979

KEYWORD

nonn

AUTHOR

Artur Jasinski, Dec 09 2007

STATUS

approved

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Last modified September 28 17:43 EDT 2020. Contains 337393 sequences. (Running on oeis4.)