OFFSET
0,3
COMMENTS
If the additional constraint was added that b_j does not equal to b_{j+1}, the sequence generated would be the compositions (ordered partitions) of integers.
This is a variant of compositions of compositions: for each composition of n, write it in value^repetition form, and then choose a composition for each repetition factor. - Franklin T. Adams-Watters, May 27 2010
INVERT transform of tau (A000005). - Alois P. Heinz, Feb 11 2021
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = 0..1000
Sylvie Corteel and Paweł Hitczenko, Generalizations of Carlitz Compositions, Journal of Integer Sequences, Vol. 10 (2007), Article 07.8.8.
FORMULA
G.f.: 1/(1 - Sum_{k>0} z^k/(1-z^k)).
G.f.: 1/(1 - Sum_{k>0} tau(k) x^k), where tau(k) is the number of divisors of k. - Franklin T. Adams-Watters, May 27 2010
G.f.: 1/(1 + x * (d/dx) log(Product_{k>=1} (1 - x^k)^(1/k))). - Ilya Gutkovskiy, Oct 18 2018
a(n) = Sum_{k=0..n-1} tau(n-k)*a(k) for n>0 with a(0) = 1. - Ridouane Oudra, Mar 13 2020
EXAMPLE
a(3)=7 because we can write
3^{1},
1^{2} 2^{1},
2^{1} 1^{1},
1^{3},
1^{2} 1^{1},
1^{1} 1^{2},
1^{1} 1^{1} 1^{1}.
MAPLE
a:= proc(n) option remember; `if`(n=0, 1,
add(add(a(n-i*j), j=1..n/i), i=1..n))
end:
seq(a(n), n=0..40); # Alois P. Heinz, Jul 22 2017
# second Maple program:
a:= proc(n) option remember; `if`(n=0, 1,
add(a(n-i)*numtheory[tau](i), i=1..n))
end:
seq(a(n), n=0..40); # Alois P. Heinz, Feb 11 2021
MATHEMATICA
nmax = 50; CoefficientList[Series[1/(1 - Sum[DivisorSigma[0, k]*x^k, {k, 1, nmax}]), {x, 0, nmax}], x] (* Vaclav Kotesovec, Jan 06 2017 *)
PROG
(PARI) N=66; A=vector(66); A[0+1]=1;
for (n=1, N-1, A[n+1] = sum(k=0, n-1, A[k+1]*sigma(n-k, 0)) );
A /* Joerg Arndt, Apr 28 2013 */
(PARI) N = 66; x = 'x + O('x^N);
gf = 1/(1-sum(k=1, N, x^k/(1-x^k)) );
Vec(gf) /* Joerg Arndt, Apr 28 2013 */
CROSSREFS
KEYWORD
nonn
AUTHOR
Pawel Hitczenko (phitczenko(AT)math.drexel.edu), Jun 05 2007
EXTENSIONS
Edited by Franklin T. Adams-Watters, May 27 2010
More terms from Joerg Arndt, Apr 28 2013
STATUS
approved