A276641 Number of digits in 2^A000668(n)-1.

1, 3, 10, 39, 2466, 39457, 157827, 646456993, 694127911065419642, 186328542329173367306815834, 48844909400338823199277929902126, 51217599719369681875006054625051616350

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