OFFSET
1,2
COMMENTS
a(n) is the number of (nonempty) linear partitions of the linearly ordered set [n] = {1,2,...,n} with blocks of the same size, where each block has exactly one element marked. For instance, for n = 4, we have the following 9 linear partitions (where the marked elements are denoted by *):
. (*)(*)(*)(*), (*2)(*4), (*234),
. (*2)(3*), (1*34),
. (1*)(*4), (12*4),
. (1*)(3*), (123*).
- Emanuele Munarini, Feb 03 2014
LINKS
Nick Hobson, Table of n, a(n) for n = 1..1000
FORMULA
G.f.: Sum_{n>=1} -log(1 - n*x^n)/n = Sum_{n>=0} a(n) x^n/n. - Paul D. Hanna, Aug 04 2002
G.f.: Sum_{n>0} n*x^n/(1-n*x^n). - Vladeta Jovovic, Sep 02 2002
Sum_{k=1..n} a(k) ~ 3^((n + 3 - mod(n,3))/3)/2. - Vaclav Kotesovec, Aug 07 2022
EXAMPLE
a(10) = 10^1 + 5^2 + 2^5 + 1^10 = 68 because positive divisors of 10 are 1, 2, 5, 10.
MATHEMATICA
Table[Total[Quotient[n, x = Divisors[n]]^x], {n, 44}] (* Jayanta Basu, Jul 08 2013 *)
Table[Sum[d^(n/d), {d, Divisors[n]}], {n, 1, 100}] (* Emanuele Munarini, Feb 03 2014 *)
PROG
(PARI) vector(44, n, sumdiv(n, d, (n/d)^d))
(PARI) a(n) = sumdiv(n, d, d^(n/d) ); \\ Joerg Arndt, Apr 14 2013
(Maxima) a(n) := lsum(d^(n/d), d, listify(divisors(n))); makelist(a(n), n, 1, 40); /* Emanuele Munarini, Feb 03 2014 */
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Jun 20 2000
EXTENSIONS
More terms from James A. Sellers, Jul 04 2000
Duplicate g.f. removed by Franklin T. Adams-Watters, Sep 01 2009
STATUS
approved