A099441 Numbers n such that A000295(n) = 2^n-n-1 is a semiprime.

3, 5, 6, 8, 9, 12, 30, 32, 33, 46, 52, 62, 74, 76, 86, 98, 130, 134, 154, 228, 230, 242, 256, 266, 346, 352, 382, 412, 428, 474, 488, 634, 650, 662, 688, 704, 722, 772, 896, 986, 1108, 1222, 1246, 1326

Offset: 1

Author

Hugo Pfoertner, Oct 18 2004

Keywords

• hard
• more
• nonn

Comments

A candidate for the next term is a(45) = 1736. 2^1736-1737 is composite with 523 decimal digits and unknown factorization. - Tyler Busby, Jan 05 2025

Examples

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

Links

• Dario Alpern, Factorization using the Elliptic Curve Method.
• Status of 2^1736-1737 in factordb.com.

Crossrefs

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).

Extensions

More terms from Hugo Pfoertner, Aug 13 2007

a(24) corrected by Hugo Pfoertner, Sep 07 2017

a(32)-a(44) from Tyler Busby, Jan 05 2025