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 A210113 Floor of the expected value of number of trials until exactly two cells are empty in a random distribution of n balls in n cells. 5
 9, 3, 2, 1, 2, 3, 4, 7, 12, 21, 40, 75, 147, 292, 594, 1229, 2582, 5499, 11859, 25868, 57008, 126814, 284523, 643401, 1465511, 3360493, 7753730, 17993787, 41982506, 98445184, 231932762, 548839352, 1304155087 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,1 COMMENTS Also floor of the expected value of number of trials until we have n-2 distinct symbols in a random sequence on n symbols of length n. A055775 corresponds to zero cells empty. REFERENCES W. Feller, An Introduction to Probability Theory and its Applications, 2nd ed, Wiley, New York, 1965, (2.4) p. 92. (Occupancy problems) LINKS Washington Bomfim, Table of n, a(n) for n = 3..100 FORMULA With m = 2, a(n) = floor(n^n/(binomial(n,m)*_Sum{v=0..n-m-1}((-1)^v*binomial(n-m,v)*(n-m-v)^n))) EXAMPLE For n=3, there are 3^3 = 27 sequences on 3 symbols of length 3. Only 3 sequences has a unique symbol, so a(3) = floor(27/3) = 9. CROSSREFS Cf. A055775, A209899, A209900, A210112, A210114, A210115, A210116. Sequence in context: A306808 A086232 A133867 * A195704 A176516 A210644 Adjacent sequences:  A210110 A210111 A210112 * A210114 A210115 A210116 KEYWORD nonn AUTHOR Washington Bomfim, Mar 18 2012 STATUS approved

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Last modified September 28 11:24 EDT 2020. Contains 337393 sequences. (Running on oeis4.)