A070529 Number of divisors of repunit 111...111 (with n digits).

1, 2, 4, 4, 4, 32, 4, 16, 12, 16, 4, 128, 8, 16, 64, 64, 4, 384, 2, 128, 128, 96, 2, 1024, 32, 64, 64, 256, 32, 8192, 8, 2048, 64, 64, 128, 3072, 8, 8, 64, 2048, 16, 24576, 16, 1536, 768, 64, 4, 8192, 16, 1024, 256, 512, 16, 8192, 256, 4096

Offset: 1

Author

Henry Bottomley, May 02 2002

Keywords

• base
• nonn

Examples

			a(9) = 12 since the divisors of 111111111 are 1, 3, 9, 37, 111, 333, 333667, 1001001, 3003003, 12345679, 37037037, 111111111.
		

Links

• Table of n, a(n) for n = 1..352

Crossrefs

Cf. A000005, A002275, A070528, A051064, A004023, A046413, A003020, A095370, A176973, A046053, A046412, A102380.

Programs

PARI

a(n) = numdiv(10^n\9); \\ Michel Marcus, Sep 09 2015

Formula

a(n) = A000005(A002275(n)).

a(n) = A070528(n)*A051064(n)/(A051064(n)+2).

a(A004023(n)) = 2. - Michel Marcus, Sep 09 2015

a(A046413(n)) = 4. - Bruno Berselli, Sep 09 2015

Extensions

Terms to a(280) in b-file from Hans Havermann, Aug 20 2011

a(281)-a(322) in b-file from Ray Chandler, Apr 22 2017

a(323)-a(352) ib b-file from Max Alekseyev, May 04 2022