This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A210115 #7 Mar 31 2012 20:17:50
%S A210115 625,50,13,5,3,2,2,2,3,4,5,7,11,17,28,46,78,136,242,441,815,1533,2927,
%T A210115 5669,11123,22090,44363,90027,184482,381499,795686,1672914,3543925,
%U A210115 7561129,16240832,35106812,76346759,166982782,367206632,811693449
%N A210115 Floor of the expected value of number of trials until exactly four cells are empty in a random distribution of n balls in n cells.
%C A210115 Also floor of the expected value of number of trials until we have n-4 distinct symbols in a random sequence on n symbols of length n. A055775 corresponds to zero cells empty.
%D A210115 W. Feller, An Introduction to Probability Theory and its Applications, 2nd ed, Wiley, New York, 1965, (2.4) p. 92. (Occupancy problems)
%H A210115 W. Bomfim, <a href="/A210115/b210115.txt">Table of n, a(n) for n = 5..100</a>
%F A210115 With m = 4, 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 A210115 For n=5, there are 5^5 = 3125 sequences on 5 symbols of length 5. Only 5 sequences has a unique symbol, so a(5) = floor(3125/5) = 625.
%Y A210115 Cf. A055775, A209899, A209900, A210112, A210113, A210114, A210116.
%K A210115 nonn
%O A210115 5,1
%A A210115 _Washington Bomfim_, Mar 18 2012