OFFSET
1,1
FORMULA
EXAMPLE
a(7)=12 since the divisors of 9999999 are 1, 3, 9, 239, 717, 2151, 4649, 13947, 41841, 1111111, 3333333, 9999999.
MATHEMATICA
DivisorSigma[0, #]&/@(10^Range[60]-1) (* Harvey P. Dale, Jan 14 2011 *)
Table[DivisorSigma[0, 10^n - 1], {n, 60}] (* T. D. Noe, Aug 18 2011 *)
PROG
(PARI) a(n) = numdiv(10^n - 1); \\ Michel Marcus, Sep 08 2015
CROSSREFS
Cf. A004023 (indices of 6's).
KEYWORD
base,nonn
AUTHOR
Henry Bottomley, May 02 2002
EXTENSIONS
Terms to a(280) in b-file from Hans Havermann, Aug 19 2011
a(281)-a(322) in b-file from Ray Chandler, Apr 22 2017
a(323)-a(352) in b-file from Max Alekseyev, May 04 2022
STATUS
approved