OFFSET
2,1
COMMENTS
a(n) is the denominator of the probability that a sequence of n integers randomly chosen from [n] contains exactly n - 1 different integers (see Brualdi, pp. 57-58).
REFERENCES
Richard A. Brualdi, Introductory Combinatorics, 5th ed. Pearson Education Inc., 2009.
MATHEMATICA
a[n_]:=Denominator[n!^2/(2(n-2)!n^n)]; Array[a, 21, 2]
CROSSREFS
KEYWORD
nonn,frac
AUTHOR
Stefano Spezia, Feb 11 2024
STATUS
approved