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A258457 Number of partitions of n into parts of exactly 2 sorts which are introduced in ascending order. 2
1, 4, 12, 30, 72, 160, 351, 743, 1561, 3219, 6616, 13456, 27312, 55139, 111166, 223472, 448902, 900305, 1804838, 3615137, 7239325, 14490368, 29000050, 58025059, 116090823, 232234573, 464554483, 929220024, 1858618215, 3717468189, 7435305664, 14871092926 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

LINKS

Alois P. Heinz, Table of n, a(n) for n = 2..1000

FORMULA

a(n) ~ c * 2^n, where c = 1/Product_{n>=2} (1-1/2^n) = 1/(2*A048651) = 1.7313733097275318... . - Vaclav Kotesovec, Jun 01 2015

MAPLE

b:= proc(n, i, k) option remember; `if`(n=0, 1, `if`(i<1, 0,

      b(n, i-1, k) +`if`(i>n, 0, k*b(n-i, i, k))))

    end:

T:= (n, k)-> add(b(n$2, k-i)*(-1)^i/(i!*(k-i)!), i=0..k):

a:= n-> T(n, 2):

seq(a(n), n=2..35);

CROSSREFS

Column k=2 of A256130.

Sequence in context: A006802 A068055 A221855 * A279152 A317780 A274217

Adjacent sequences:  A258454 A258455 A258456 * A258458 A258459 A258460

KEYWORD

nonn

AUTHOR

Alois P. Heinz, May 30 2015

STATUS

approved

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Last modified November 17 03:06 EST 2019. Contains 329216 sequences. (Running on oeis4.)