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 A005425 a(n) = 2*a(n-1) + (n-1)*a(n-2). (Formerly M1461) 32
 1, 2, 5, 14, 43, 142, 499, 1850, 7193, 29186, 123109, 538078, 2430355, 11317646, 54229907, 266906858, 1347262321, 6965034370, 36833528197, 199037675054, 1097912385851, 6176578272782, 35409316648435, 206703355298074, 1227820993510153, 7416522514174082 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Switchboard problem with n subscribers, where a subscriber who is not talking can be of either of two sexes. Subscribers who are talking cannot be distinguished by sex. See also A000085. - Karol A. Penson, Apr 15 2004 John W. Layman observes that computationally this agrees with the binomial transform of A000085. Number of self-inverse partial permutations. Number of '12-3 and 214-3'-avoiding permutations. Number of matchings of the corona K'(n) of the complete graph K(n) and the complete graph K(1); in other words, K'(n) is the graph constructed from K(n) by adding for each vertex v a new vertex v' and the edge vv'. Example: a(3)=14 because in the graph with vertex set {A,B,C,a,b,c} and edge set {AB,AC,BC,Aa,Bb,Cc} we have the following matchings: (i) the empty set (1); (ii) the edges as singletons (6); (iii) {Aa,BC},{Bb,AC},{Cc,AB},{Aa,Bb},{Aa,Cc}, {Bb,Cc} (6); (iv) {Aa,Bb,Cc} (1). Row sums of A100862. - Emeric Deutsch, Jan 10 2005 Consider finite sequences of positive integers of length n with b(1)=1 and with the constraint that b(m) <= max_{0

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Last modified June 30 14:51 EDT 2022. Contains 354943 sequences. (Running on oeis4.)