OFFSET
0,5
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..10000
FORMULA
G.f.: Product_{k>=1} 1/((1 - x^prime(k))*(1 - x^(prime(k)^2))). - Ilya Gutkovskiy, Dec 26 2016
EXAMPLE
G.f. = 1 + x^2 + x^3 + 2*x^4 + 2*x^5 + 3*x^6 + 4*x^7 + 5*x^8 + 7*x^9 + 8*x^10 + ...
a(10) = #{ 7+3, 5+5, 5+3+2, 2^2+2^2+2, 2^2+3+3, 2^2+2+2+2, 3+3+2+2, 2+2+2+2+2 } = 8.
MAPLE
with(numtheory):
a:= proc(n) option remember; `if`(n=0, 1, add(a(n-j)*add(
`if`(tau(d) in [2, 3], d, 0), d=divisors(j)), j=1..n)/n)
end:
seq(a(n), n=0..60); # Alois P. Heinz, Mar 30 2017
MATHEMATICA
a[n_] := a[n] = If[n == 0, 1, Sum[a[n - j]*DivisorSum[j, If[2 <= DivisorSigma[0, #] <= 3, #, 0]&], {j, 1, n}]/n];
Table[a[n], {n, 0, 60}] (* Jean-François Alcover, Apr 06 2017, after Alois P. Heinz *)
PROG
(PARI) {a(n) = if(n < 0, 0, polcoeff( 1 / prod(k=1, primepi(n), (1 - x^prime(k)^2 + x*O(x^n)) * (1 - x^prime(k))), n))}; /* Michael Somos, Dec 26 2016 */
CROSSREFS
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, Aug 20 2005
STATUS
approved