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!)
A138011 a(n) = number of positive divisors, k, of n where d(k) divides d(n). (d(m) = number of positive divisors of m, A000005) 4
1, 2, 2, 2, 2, 4, 2, 3, 2, 4, 2, 5, 2, 4, 4, 2, 2, 5, 2, 5, 4, 4, 2, 6, 2, 4, 3, 5, 2, 8, 2, 4, 4, 4, 4, 4, 2, 4, 4, 6, 2, 8, 2, 5, 5, 4, 2, 5, 2, 5, 4, 5, 2, 6, 4, 6, 4, 4, 2, 11, 2, 4, 5, 2, 4, 8, 2, 5, 4, 8, 2, 10, 2, 4, 5, 5, 4, 8, 2, 5, 2, 4, 2, 11, 4, 4, 4, 6, 2, 11, 4, 5, 4, 4, 4, 9, 2, 5, 5, 4 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..10000

EXAMPLE

12 has 6 divisors (1,2,3,4,6,12). The number of divisors of each of these divisors of 12 form the sequence (1,2,2,3,4,6). Of these, five divide d(12)=6: 1,2,2,3,6. So a(12) = 5.

MATHEMATICA

Table[Length[Select[Divisors[n], Mod[Length[Divisors[n]], Length[Divisors[ # ]]] == 0 &]], {n, 1, 100}] (* Stefan Steinerberger, Feb 29 2008 *)

PROG

(PARI) A138011(n) = sumdiv(n, d, if(!(numdiv(n)%numdiv(d)), 1, 0)); \\ Antti Karttunen, May 25 2017

(Python)

from sympy import divisors, divisor_count

def a(n): return sum([1*(divisor_count(n)%divisor_count(d)==0) for d in divisors(n)]) # Indranil Ghosh, May 25 2017

CROSSREFS

Cf. A000005, A138010, A138012.

Sequence in context: A173439 A322483 A061389 * A036555 A046927 A084718

Adjacent sequences:  A138008 A138009 A138010 * A138012 A138013 A138014

KEYWORD

nonn

AUTHOR

Leroy Quet, Feb 27 2008

EXTENSIONS

More terms from Stefan Steinerberger, Feb 29 2008

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 September 16 07:19 EDT 2021. Contains 347469 sequences. (Running on oeis4.)