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

%I A280881 #6 Jan 12 2017 14:44:11
%S A280881 1,6,10,75,21,476,540,4185,1375,47850,10374,249431,395955,2572680,
%T A280881 278392,30877389,5000211,214159070,291693150,1465649955,1224138223,
%U A280881 23284864476,13175039700,157019230225,198060711381,1510657970346,1118209769530,18176060999625
%N A280881 Number of set partitions of [n] into exactly three blocks where sizes of distinct blocks are coprime.
%H A280881 Alois P. Heinz, <a href="/A280881/b280881.txt">Table of n, a(n) for n = 3..600</a>
%H A280881 Wikipedia, <a href="https://en.wikipedia.org/wiki/Coprime_integers">Coprime integers</a>
%H A280881 Wikipedia, <a href="https://en.wikipedia.org/wiki/Partition_of_a_set">Partition of a set</a>
%e A280881 a(3) = 1: 1|2|3.
%e A280881 a(4) = 6: 12|3|4, 13|2|4, 1|23|4, 14|2|3, 1|24|3, 1|2|34.
%e A280881 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.
%e A280881 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.
%Y A280881 Column k=3 of A280880.
%K A280881 nonn
%O A280881 3,2
%A A280881 _Alois P. Heinz_, Jan 09 2017