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 A011970 Apply (1+Shift)^3 to Bell numbers. 4
 1, 4, 8, 15, 37, 114, 409, 1657, 7432, 36401, 192713, 1094076, 6618379, 42436913, 287151994, 2042803419, 15229360185, 118645071202, 963494800557, 8138047375093, 71351480138824, 648222594284197, 6092330403828749 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Starting with n=3 (a(3)=15), number of set partitions of n+2 with at least one singleton and the smallest element in any singleton is exactly n-2. The maximum number of singletons is therefore 5. Alternatively, starting with n=3, number of set partitions of n+2 with at least one singleton and the largest element in any singleton is exactly 5. - Olivier Gérard, Oct 29 2007 Let V(N)={v(1),v(2),...,v(N)} denote an ordered set of increasing positive integers containing 3 pairs of adjacent elements that differ by at least 2, that is, v(i),v(i+1) with v(i+1)-v(i)>1. Then for n>2, a(n) is the number of partitions of V(n+1) into blocks of nonconsecutive integers. - Augustine O. Munagi, Jul 17 2008 REFERENCES Olivier Gérard and Karol Penson, A budget of set partitions statistics, in preparation, unpublished as of Sep 22 2011 LINKS Chai Wah Wu, Table of n, a(n) for n = 0..500 Cohn, Martin; Even, Shimon; Menger, Karl, Jr.; Hooper, Philip K.; On the Number of Partitionings of a Set of n Distinct Objects, Amer. Math. Monthly 69 (1962), no. 8, 782--785. MR1531841. Cohn, Martin; Even, Shimon; Menger, Karl, Jr.; Hooper, Philip K.; On the Number of Partitionings of a Set of n Distinct Objects, Amer. Math. Monthly 69 (1962), no. 8, 782--785. MR1531841. [Annotated scanned copy] Augustine O. Munagi, Extended set partitions with successions, European J. Combin. 29(5) (2008), 1298--1308. Jocelyn Quaintance and Harris Kwong, A combinatorial interpretation of the Catalan and Bell number difference tables, Integers, 13 (2013), #A29. FORMULA If n>2, then bell(n)+3*bell(n-1)+3*bell(n-2)+bell(n-3). - Augustine O. Munagi, Jul 17 2008 EXAMPLE a(3)=15 because the set {1,3,5,7} has 15 different partitions which are necessarily into blocks of nonconsecutive integers. MAPLE with(combinat): 1, 4, 8, seq(`if`(n>2, bell(n)+3*bell(n-1)+3*bell(n-2)+bell(n-3), NULL), n=3..22); # Augustine O. Munagi, Jul 17 2008 PROG (Python) # requires python 3.2 or higher. Otherwise use def'n of accumulate in python docs. from itertools import accumulate A011970_list, blist, b, b2, b3 = [1, 4, 8], [1, 2], 2, 1, 1 for _ in range(498): ....blist = list(accumulate([b]+blist)) ....A011970_list.append(3*(b+b2)+b3+blist[-1]) ....b3, b2, b = b2, b, blist[-1] # Chai Wah Wu, Sep 02 2014, updated Chai Wah Wu, Sep 20 2014 CROSSREFS Cf. A000110. Cf. A011968, A011969. A diagonal of A011971 and A106436. - N. J. A. Sloane, Jul 31 2012 Sequence in context: A305218 A321500 A103536 * A111988 A110652 A059373 Adjacent sequences:  A011967 A011968 A011969 * A011971 A011972 A011973 KEYWORD nonn AUTHOR STATUS approved

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Last modified September 17 14:07 EDT 2021. Contains 347478 sequences. (Running on oeis4.)