A280881 Number of set partitions of [n] into exactly three blocks where sizes of distinct blocks are coprime.

1, 6, 10, 75, 21, 476, 540, 4185, 1375, 47850, 10374, 249431, 395955, 2572680, 278392, 30877389, 5000211, 214159070, 291693150, 1465649955, 1224138223, 23284864476, 13175039700, 157019230225, 198060711381, 1510657970346, 1118209769530, 18176060999625

Offset

3,2

Links

Alois P. Heinz, Table of n, a(n) for n = 3..600

Wikipedia, Coprime integers

Wikipedia, Partition of a set

Example

a(3) = 1: 1|2|3.
a(4) = 6: 12|3|4, 13|2|4, 1|23|4, 14|2|3, 1|24|3, 1|2|34.
a(5) = 10: 123|4|5, 124|3|5, 125|3|4, 134|2|5, 135|2|4, 1|234|5, 1|235|4, 145|2|3, 1|245|3, 1|2|345.
a(7) = 21: 12345|6|7, 12346|5|7, 12347|5|6, 12356|4|7, 12357|4|6, 12367|4|5, 12456|3|7, 12457|3|6, 12467|3|5, 12567|3|4, 13456|2|7, 13457|2|6, 13467|2|5, 13567|2|4, 1|23456|7, 1|23457|6, 1|23467|5, 1|23567|4, 14567|2|3, 1|24567|3, 1|2|34567.

Crossrefs

Column k=3 of A280880.

Sequence in context: A119041 A105157 A166386 * A256246 A246800 A269341

Adjacent sequences: A280878 A280879 A280880 * A280882 A280883 A280884

Keyword

nonn

Author

Alois P. Heinz, Jan 09 2017

Status

approved