A366898 Number of divisors of n*2^n - 1, the Woodall (or Riesel) numbers.

1, 2, 2, 6, 4, 2, 4, 4, 4, 4, 6, 4, 12, 10, 8, 48, 8, 4, 8, 4, 16, 16, 8, 8, 4, 8, 8, 16, 24, 2, 8, 4, 8, 32, 8, 32, 4, 8, 4, 24, 16, 8, 32, 8, 16, 24, 40, 16, 16, 8, 8, 16, 24, 8, 16, 8, 16, 6, 32, 8, 8, 16, 8, 512, 48, 16, 12, 48, 16, 8, 8, 4, 24, 16, 2, 256

Offset: 1

Author

Tyler Busby, Oct 26 2023

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..865

Crossrefs

Cf. A000005, A003261, A002234, A242273, A063515, A056821, A367006, A366899.

Programs

Mathematica

Table[DivisorSigma[0, n*2^n - 1], {n, 1, 100}] (* Amiram Eldar, Dec 11 2023 *)

PARI

a(n) = numdiv(n*2^n - 1); \\ Amiram Eldar, Dec 11 2023

Formula

a(n) = sigma0(n*2^n - 1) = A000005(A003261(n)).