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

Author

Alois P. Heinz, Jan 09 2017

Keywords

• nonn

Examples

			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.
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 3..600
• Wikipedia, Coprime integers
• Wikipedia, Partition of a set

Crossrefs

Column k=3 of A280880.