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A136033 a(n) = smallest number k such that number of prime factors of 2^k-1 is exactly n (counted with multiplicity). 1

%I

%S 2,4,6,16,12,18,24,40,54,36,102,110,60,72,108,140,120,156,144,200,216,

%T 210,240,180,456,288,336,300,396,480,882,360,468,700

%N a(n) = smallest number k such that number of prime factors of 2^k-1 is exactly n (counted with multiplicity).

%p N:= 24: # to get a(1) to a(N)

%p unknown:= N:

%p for k from 2 while unknown > 0 do

%p q:= numtheory:-bigomega(2^k-1);

%p if q <= N and not assigned(A[q]) then

%p A[q]:= k;

%p unknown:= unknown - 1;

%p fi

%p od:

%p seq(A[i],i=1..N); # _Robert Israel_, Oct 24 2014

%o (PARI) a(n) = {k = 1; while(bigomega(2^k-1) != n, k++); k;} \\ _Michel Marcus_, Nov 04 2013

%Y Cf. A000225, A003260, A016047, A046051, A049479, A088863, A136030, A136031.

%K nonn,more,hard

%O 1,1

%A _Artur Jasinski_, Dec 11 2007

%E a(15)-a(20) from _Michel Marcus_, Nov 04 2013

%E a(21)-a(24) from _Derek Orr_, Oct 23 2014

%E a(25)-a(34) from _Jinyuan Wang_, Jun 07 2019

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Last modified April 13 19:26 EDT 2021. Contains 342941 sequences. (Running on oeis4.)