OFFSET
1,2
COMMENTS
a(n) is the number of functional digraphs that are not a solitary rooted tree. - Geoffrey Critzer, Aug 31 2013
For n > 1 a(n) is the number of numbers with n digits in base n. - Gionata Neri, Feb 18 2016
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..300
Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
FORMULA
a(n) = n^n - n^(n-1).
E.g.f.: T^2/(1-T), where T=T(x) is Euler's tree function (see A000169).
For n > 1 a(n)=1/(Integral_{x=n..infinity} 1/x^n dx). - Francesco Daddi, Aug 01 2011
a(n) = sum(i=1..n-1, C(n,i)*(i^i*(n-i)^(n-i-1))). - Vladimir Kruchinin May 15 2013
E.g.f.: x^2*A''(x) where A(x) is the e.g.f. for A000272. - Geoffrey Critzer, Aug 31 2013
EXAMPLE
a(2)=2: [1->2,2->1], [1->2,2->2].
MATHEMATICA
Table[(n-1)*n^(n-1), {n, 1, 20}] (* Vladimir Joseph Stephan Orlovsky, Apr 19 2011 *)
PROG
(Magma) [n^n - n^(n-1): n in [1..20]]; // Vincenzo Librandi, Aug 02 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Len Smiley, Dec 09 2001
STATUS
approved