%I #7 Mar 31 2012 20:17:50
%S 9,3,2,1,2,3,4,7,12,21,40,75,147,292,594,1229,2582,5499,11859,25868,
%T 57008,126814,284523,643401,1465511,3360493,7753730,17993787,41982506,
%U 98445184,231932762,548839352,1304155087
%N 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.
%C 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.
%D W. Feller, An Introduction to Probability Theory and its Applications, 2nd ed, Wiley, New York, 1965, (2.4) p. 92. (Occupancy problems)
%H Washington Bomfim, <a href="/A210113/b210113.txt">Table of n, a(n) for n = 3..100</a>
%F 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)))
%e 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.
%Y Cf. A055775, A209899, A209900, A210112, A210114, A210115, A210116.
%K nonn
%O 3,1
%A _Washington Bomfim_, Mar 18 2012