login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A193335 Number of odd divisors of sigma(n). 2
1, 2, 1, 2, 2, 2, 1, 4, 2, 3, 2, 2, 2, 2, 2, 2, 3, 4, 2, 4, 1, 3, 2, 4, 2, 4, 2, 2, 4, 3, 1, 6, 2, 4, 2, 4, 2, 4, 2, 6, 4, 2, 2, 4, 4, 3, 2, 2, 4, 4, 3, 3, 4, 4, 3, 4, 2, 6, 4, 4, 2, 2, 2, 2, 4, 3, 2, 6, 2, 3, 3, 8, 2, 4, 2, 4, 2, 4, 2, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

sigma(n) = sum of divisors of n, A000203.

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A001227(A000203(n)). - Michel Marcus, Jan 14 2014

EXAMPLE

a(8) = 4 because sigma(8) = 15 and the 4 odd divisors are { 1, 3, 5, 15}.

MATHEMATICA

f[n_] := Block[{d = Divisors[DivisorSigma[1, n]]}, Count[OddQ[d], True]]; Table[f[n], {n, 80}]

Table[Count[Divisors[DivisorSigma[1, n]], _?OddQ], {n, 80}] (* Harvey P. Dale, Jul 06 2019 *)

PROG

(PARI) a(n)=sumdiv(sigma(n, 1), d, d%2);

(PARI) a(n)=n=sigma(n); numdiv(n>>valuation(n, 2)) \\ Charles R Greathouse IV, Jul 30 2011

CROSSREFS

Cf. A000203.

Sequence in context: A335283 A126865 A104640 * A016727 A241318 A276064

Adjacent sequences:  A193332 A193333 A193334 * A193336 A193337 A193338

KEYWORD

nonn

AUTHOR

Michel Lagneau, Jul 23 2011

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 4 08:18 EDT 2020. Contains 334825 sequences. (Running on oeis4.)