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A276641 Number of digits in 2^A000668(n)-1. 2
1, 3, 10, 39, 2466, 39457, 157827, 646456993, 694127911065419642, 186328542329173367306815834, 48844909400338823199277929902126, 51217599719369681875006054625051616350 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The sequence is a subsequence of A227683.

LINKS

Ivan Panchenko, Table of n, a(n) for n = 1..18

MathWorld, Double Mersenne Number.

Wikipedia, Double Mersenne number.

FORMULA

a(n) = A055642(A000225(A000668(n))).

PROG

(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

terms(n) = my(p=3, i=0); while(i < n, print1(#Str(2^p-1), ", "); i++; p=next_a000668(p+1))

terms(7) \\ print initial seven terms

CROSSREFS

Cf. A227683, A263686.

Sequence in context: A054912 A093463 A302989 * A099763 A318118 A149052

Adjacent sequences:  A276638 A276639 A276640 * A276642 A276643 A276644

KEYWORD

nonn,base,hard

AUTHOR

Felix Fröhlich, Sep 22 2016

EXTENSIONS

a(8)-a(12) from Ivan Panchenko, Apr 11 2018

STATUS

approved

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Last modified April 18 20:41 EDT 2021. Contains 343089 sequences. (Running on oeis4.)