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.
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
KEYWORD
nonn,base,hard
AUTHOR
Felix Fröhlich, Sep 22 2016
EXTENSIONS
a(8)-a(12) from Ivan Panchenko, Apr 11 2018
STATUS
approved