OFFSET
1,2
COMMENTS
Coupon collector's problem for two persons.
FORMULA
a(n) = round(1 - Sum_{j=0..n} Sum_{k=0..n} ( (-1)^(2*n-j-k) * binomial(n,j) * binomial(n,k) * j * k / (n^2-j*k) )) excluding term with j=k=n in summation.
EXAMPLE
a(1)=1, a(2)=round(11/3)=3, a(3)=round(1909/280)=7, a(4)=round(4687/455)=10, a(5)=round(7517050763/535422888)=14.
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo Pfoertner, Mar 07 2018
STATUS
approved