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A156870 Number of nondecreasing sequences of 2n positive integers with reciprocals adding up to n (1 <= n). 1
1, 4, 18, 168, 3648, 298165, 159628076 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Conjecture: A156869(2n + m, n + m) = a(n) if and only if m >= 0.

LINKS

Table of n, a(n) for n=1..7.

EXAMPLE

For n = 2, the a(2) = 4 sequences are (1, 2, 3, 6), (1, 2, 4, 4), (1, 3, 3, 3) and (2, 2, 2, 2) because 1/1 + 1/2 + 1/3 + 1/6 = 1/1 + 1/2 + 1/4 + 1/4 = 1/1 + 1/3 + 1/3 + 1/3 = 1/2 + 1/2 + 1/2 + 1/2 = 2.

CROSSREFS

a(n) = A156869(2n, n).

Sequence in context: A222766 A007153 A239839 * A240317 A145075 A058924

Adjacent sequences:  A156867 A156868 A156869 * A156871 A156872 A156873

KEYWORD

more,nonn

AUTHOR

Jens Voß, Feb 17 2009

EXTENSIONS

a(6) from M. F. Hasler, Feb 21 2009

a(7) from Max Alekseyev, Jan 04 2017

STATUS

approved

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Last modified September 26 10:38 EDT 2017. Contains 292518 sequences.