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 A019472 Weak preference orderings of n alternatives, i.e., orderings that have indifference between at least two alternatives. 13
 0, 0, 1, 7, 51, 421, 3963, 42253, 505515, 6724381, 98618763, 1582715773, 27612565995, 520631327581, 10554164679243, 228975516609853, 5294731892093355, 130015079601039901, 3379132289551117323, 92679942218919579133, 2675254894236207563115, 81073734056332364441821 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS From Gus Wiseman, Jun 24 2020: (Start) Equivalently, a(n) is number of (1,1)-matching sequences of length n that cover an initial interval of positive integers. For example, the a(2) = 1 and a(3) = 7 sequences are:   (1,1)  (1,1,1)          (1,1,2)          (1,2,1)          (1,2,2)          (2,1,1)          (2,1,2)          (2,2,1) Missing from this list are:   (1,2)  (1,2,3)   (2,1)  (1,3,2)          (2,1,3)          (2,3,1)          (3,1,2)          (3,2,1) (End) LINKS Wikipedia, Weak ordering Wikipedia, Permutation pattern FORMULA a(n) = A000670(n) - n!. - corrected by Eugene McDonnell, May 12 2000 a(n) = Sum_{j=0..n-1} Sum_{i=0..n-1} (-1)^(j-i)*C(j, i)*i^n. - Peter Luschny, Jul 22 2014 MATHEMATICA a[n_] := Sum[(-1)^(j-i)*Binomial[j, i]*i^n, {i, 0, n-1}, {j, 0, n-1}]; Table[a[n], {n, 0, 21}] (* Jean-François Alcover, Feb 26 2016, after Peter Luschny *) PROG (Sage) def A019472(n):     return add(add((-1)^(j-i)*binomial(j, i)*i^n for i in range(n)) for j in range(n)) [A019472(n) for n in range(21)] # Peter Luschny, Jul 22 2014 CROSSREFS Cf. A000670, A052875. (1,1)-avoiding patterns are counted by A000142. (1,2)-matching patterns are counted by A056823. (1,1)-matching compositions are counted by A261982. (1,1)-matching compositions are ranked by A335488. Patterns matched by patterns are counted by A335517. Cf. A056986, A333217, A335454, A335456, A335515. Sequence in context: A332936 A222849 A273055 * A219306 A246572 A230883 Adjacent sequences:  A019469 A019470 A019471 * A019473 A019474 A019475 KEYWORD nonn,easy,nice AUTHOR Robert Ware (bware(AT)wam.umd.edu) STATUS approved

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Last modified June 14 12:04 EDT 2021. Contains 345025 sequences. (Running on oeis4.)