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A130841 Number of ways to write n as a sum of oterms, where an oterm is an ordered product of (1+oterm), sorted by size and an empty product has value 1. 0
1, 1, 1, 2, 2, 3, 3, 5, 6, 8, 8, 12, 12, 15, 17, 23, 23, 31, 31, 41, 44, 52, 52, 69, 73, 85, 91, 109, 109, 136, 136, 162, 170, 193, 199, 248, 248, 279, 291, 344, 344, 406, 406, 466, 493, 545, 545, 646, 655, 740, 763, 860, 860, 986, 1002, 1132, 1163, 1272, 1272, 1484 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

Every oterm is at least 1 (implicit) and every 1+oterm is at least 2. Therefore to write 1 as a product of (1+oterms) can only be done as an empty product, which has value 1. Therefore a(1) = 1.

LINKS

Table of n, a(n) for n=1..60.

FORMULA

a(n) = sum over sequences (n_1,n_2,...,n_k) such that 2 <= n_1 <= n_2 <= ... <= n_k and n1*n2*...*nk=n of the product of j from 1 to k of a(n_j-1). The program, in J, implements this formula. (It works by factorizing n and then grouping the factors in all distinct ways. This J code handles the a(1) case without requiring any exception case.)

EXAMPLE

a(8)=5 because we can write 8 as one of (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)

PROG

(J) belly =: ~. @ (i."1~) @ (#~ #: (i.@ ^~))

bell =: (<"1@belly@#) </.&.> <

bells =: [: ~. [: /:~&.> [: /:~&.>&.> bell

fax =: [: >&.> [: */&.>&.> [: bells q:

weird =: [: +/ [: > [: */&.> [: $:"0&.> [: <:&.> fax

w =: weird"0

CROSSREFS

Sequence in context: A227426 A229950 A050318 * A002095 A029017 A035371

Adjacent sequences:  A130838 A130839 A130840 * A130842 A130843 A130844

KEYWORD

nonn

AUTHOR

Daniel R. L. Brown, Jul 19 2007, revised Nov 23 2007

STATUS

approved

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Last modified November 16 12:40 EST 2018. Contains 317272 sequences. (Running on oeis4.)