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 A271762 Number of set partitions of [n] with minimal block length multiplicity equal to two. 2
 1, 0, 3, 0, 55, 105, 945, 1218, 15456, 26785, 705573, 2502786, 32988670, 169561483, 1757881723, 10231748010, 84389906941, 540218433147, 6899156019034, 41756989590256, 554960234199955, 4793361957432730, 59690079139252499, 558283841454550850, 7093218105977514525 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,3 LINKS Alois P. Heinz, Table of n, a(n) for n = 2..576 Wikipedia, Partition of a set FORMULA a(n) = A271424(n,2). EXAMPLE a(4) = 3: 12|34, 13|24, 14|23. MAPLE with(combinat): b:= proc(n, i, k) option remember; `if`(n=0, 1,       `if`(i<1, 0, add(multinomial(n, n-i*j, i\$j)         *b(n-i*j, i-1, k)/j!, j={0, \$k..n/i})))     end: a:= n-> b(n\$2, 2)-b(n\$2, 3): seq(a(n), n=2..30); MATHEMATICA multinomial[n_, k_List] := n!/Times @@ (k!); b[n_, i_, k_] := b[n, i, k] = If[n == 0, 1, If[i < 1, 0, Sum[multinomial[n, Join[{n - i*j}, Table[i, j]]]*b[n - i*j, i - 1, k]/j!, {j, Join[{0}, Range[k, n/i]]}]]]; a[n_] := b[n, n, 2] - b[n, n, 3]; Table[a[n], {n, 2, 30}] (* Jean-François Alcover, May 15 2018, after Alois P. Heinz *) CROSSREFS Column k=2 of A271424. Sequence in context: A009786 A012738 A193410 * A264882 A012759 A296621 Adjacent sequences:  A271759 A271760 A271761 * A271763 A271764 A271765 KEYWORD nonn AUTHOR Alois P. Heinz, Apr 13 2016 STATUS approved

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Last modified December 15 01:20 EST 2018. Contains 318141 sequences. (Running on oeis4.)