A085917 Least k such that k*2^n - 1 is a semiprime.

5, 4, 2, 1, 3, 5, 4, 2, 1, 2, 1, 3, 6, 3, 3, 3, 3, 6, 3, 3, 3, 2, 1, 3, 6, 3, 6, 3, 6, 3, 3, 11, 16, 8, 4, 2, 1, 8, 4, 2, 1, 15, 13, 15, 16, 8, 4, 2, 1, 6, 3, 17, 15, 12, 6, 3, 4, 2, 1, 3, 6, 3, 3, 5, 3, 2, 1, 5, 6, 3, 48, 24, 12, 6, 3, 12, 6, 3, 10, 5, 4, 2, 1, 3, 3, 31, 21, 17, 15, 11, 13, 24, 12, 6, 3

Offset: 1

Author

Jason Earls, Aug 16 2003

Keywords

• nonn

Comments

The first few values of n such that 509203*2^n - 1 is a semiprime, where k = 509203 (the conjectured smallest Riesel number), are: 3,4,16,34,61,82,124,142,163,171,... Conjecture: there are infinitely many semiprimes of this form.

Examples

			a(33)=16 because k*2^33 - 1 is not a semiprime for k=1,2,...15, but 16*2^33 - 1 = 223 * 616318177 is.
		

Links

• Sean A. Irvine, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A001358, A085427, A076337, A085245.