OFFSET
0,3
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..10000
FORMULA
Expansion of eta(q^6)^2 * eta(q^9) / (eta(q) * eta(q^2) * eta(q^18)) in powers of q.
a(n) ~ 5^(1/4) * exp(2*Pi*sqrt(5*n/3)/3) / (2 * 3^(7/4)* n^(3/4)). - Vaclav Kotesovec, Oct 13 2017
EXAMPLE
n = 3 n = 4 n = 5
---------- -------------- ------------------
2'+ 1' 4' 5'
1'+ 1 + 1 2'+ 2 4'+ 1'
1 + 1'+ 1 2 + 2' 2'+ 2 + 1'
1 + 1 + 1' 2'+ 1'+ 1 2 + 2'+ 1'
2'+ 1 + 1' 2'+ 1'+ 1 + 1
1'+ 1 + 1 + 1 2'+ 1 + 1'+ 1
1 + 1'+ 1 + 1 2'+ 1 + 1 + 1'
1 + 1 + 1'+ 1 1'+ 1 + 1 + 1 + 1
1 + 1 + 1 + 1' 1 + 1'+ 1 + 1 + 1
1 + 1 + 1'+ 1 + 1
1 + 1 + 1 + 1'+ 1
1 + 1 + 1 + 1 + 1'
---------- -------------- ------------------
a(3) = 4. a(4) = 9. a(5) = 12.
MATHEMATICA
nmax = 50; CoefficientList[Series[Product[(1-x^(6*k))^2 / ( (1-x^k)^2 * (1+x^k) * (1+x^(9*k)) ), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Oct 13 2017 *)
PROG
(Ruby)
def partition(n, min, max)
return [[]] if n == 0
[max, n].min.downto(min).flat_map{|i| partition(n - i, min, i).map{|rest| [i, *rest]}}
end
def A(k, n)
partition(n, 1, n).select{|i| i.all?{|j| j % k > 0}}.map{|a| a.each_with_object(Hash.new(0)){|v, o| o[v] += 1}.values.inject(:*)}.inject(:+)
end
def A293569(n)
[1] + (1..n).map{|i| A(3, i)}
end
p A293569(40)
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Oct 12 2017
STATUS
approved