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A293204
G.f.: Product_{m>0} (1+x^m+2!*x^(2*m)).
6
1, 1, 3, 2, 6, 7, 12, 13, 22, 26, 42, 46, 73, 80, 116, 139, 194, 226, 306, 358, 482, 558, 735, 856, 1108, 1300, 1657, 1926, 2426, 2834, 3530, 4110, 5082, 5898, 7234, 8409, 10216, 11860, 14304, 16568, 19891, 22990, 27470, 31670, 37630, 43382, 51274, 58982, 69450
OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..5000 (first 501 terms from Seiichi Manyama)
FORMULA
a(n) ~ c^(1/4) * exp(2*sqrt(c*n)) / (4 * sqrt(Pi) * n^(3/4)), where c = Pi^2/3 - arctan(sqrt(7))^2 + log(2)^2/4 + polylog(2, -1/4 - I*sqrt(7)/4) + polylog(2, -1/4 + I*sqrt(7)/4) = 1.323865936864425754643630663383779192757247984691212163137... - Vaclav Kotesovec, Oct 02 2017
Equivalently, c = -polylog(2, -1/2 + I*sqrt(7)/2) - polylog(2, -1/2 - I*sqrt(7)/2). - Vaclav Kotesovec, Oct 05 2017
EXAMPLE
Let's consider the partitions of n where no positive integer appears more than twice. (See A000726)
For n = 5,
partition | |
--------------------------------------------------------------
5 -> one 5 -> 1! (= 1)
= 4 + 1 -> one 4 and one 1 -> 1!*1! (= 1)
= 3 + 2 -> one 3 and one 2 -> 1!*1! (= 1)
= 3 + 1 + 1 -> one 3 and two 1 -> 1!*2! (= 2)
= 2 + 2 + 1 -> two 2 and one 1 -> 2!*1! (= 2)
--------------------------------------------------------------
a(5) = 7.
For n = 6,
partition | |
--------------------------------------------------------------
6 -> one 6 -> 1! (= 1)
= 5 + 1 -> one 5 and one 1 -> 1!*1! (= 1)
= 4 + 2 -> one 4 and one 2 -> 1!*1! (= 1)
= 4 + 1 + 1 -> one 4 and two 1 -> 1!*2! (= 2)
= 3 + 3 -> two 3 -> 2! (= 2)
= 3 + 2 + 1 -> one 3, one 2 and one 1 -> 1!*1!*1! (= 1)
= 2 + 2 + 1 + 1 -> two 2 and two 1 -> 2!*2! (= 4)
--------------------------------------------------------------
a(6) = 12.
MAPLE
b:= proc(n, i) option remember; `if`(n=0, 1, `if`(i<1, 0,
add(b(n-i*j, i-1)*j!, j=0..min(2, n/i))))
end:
a:= n-> b(n$2):
seq(a(n), n=0..50); # Alois P. Heinz, Oct 02 2017
MATHEMATICA
nmax = 100; CoefficientList[Series[Product[1 + x^k + 2*x^(2*k), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Oct 02 2017 *)
CROSSREFS
Column k=2 of A293202.
Cf. A293072.
Sequence in context: A286226 A269852 A329691 * A273344 A370665 A127717
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Oct 02 2017
STATUS
approved