Table of n, a(n) for n=1..23.
240 doesn't qualify because 2^180-1 has 6291456 divisors but 2^240-1 has only 4718592 divisors.
Sequence in context: A081954 A096903 A222603 * A032458 A079212 A141452
Adjacent sequences: A177707 A177708 A177709 * A177711 A177712 A177713
J. Lowell, May 11 2010
Corrected and extended by Alois P. Heinz, Sep 13 2011