1,1

A candidate for the next term is a(32)=634. 2^634-635 is composite with 191 decimal digits and unknown factorization. - Hugo Pfoertner (hugo(AT)pfoertner.org), Aug 13 2007

Dario Alpern, Factorization using the Elliptic Curve Method.

a(3)=6 because 2^6-6-1=57=3*19 is a semiprime.

Cf. A000295 2^n-n-1 (column 2 of the Eulerian numbers), A099439 2^n-n-1 is prime, A099440 primes in A000295, A099442 semiprimes in A000295.

Sequence in context: A047446 A058065 A164902 this_sequence A129359 A110801 A078339

Adjacent sequences: A099438 A099439 A099440 this_sequence A099442 A099443 A099444

hard,nonn

Hugo Pfoertner (hugo(AT)pfoertner.org), Oct 18 2004

More terms from Hugo Pfoertner (hugo(AT)pfoertner.org), Aug 13 2007