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%I #27 Apr 14 2018 11:06:34
%S 1,3,10,39,2466,39457,157827,646456993,694127911065419642,
%T 186328542329173367306815834,48844909400338823199277929902126,
%U 51217599719369681875006054625051616350
%N Number of digits in 2^A000668(n)-1.
%C The sequence is a subsequence of A227683.
%H Ivan Panchenko, <a href="/A276641/b276641.txt">Table of n, a(n) for n = 1..18</a>
%H MathWorld, <a href="http://mathworld.wolfram.com/DoubleMersenneNumber.html">Double Mersenne Number</a>.
%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Double_Mersenne_number">Double Mersenne number</a>.
%F a(n) = A055642(A000225(A000668(n))).
%o (PARI) next_a000668(n) = my(expo=nextprime(ceil(log(n+1)/log(2)))); while(!ispseudoprime(2^expo-1), expo=nextprime(expo+1)); 2^expo-1
%o terms(n) = my(p=3, i=0); while(i < n, print1(#Str(2^p-1), ", "); i++; p=next_a000668(p+1))
%o terms(7) \\ print initial seven terms
%Y Cf. A227683, A263686.
%K nonn,base,hard
%O 1,2
%A _Felix Fröhlich_, Sep 22 2016
%E a(8)-a(12) from _Ivan Panchenko_, Apr 11 2018
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