OFFSET
1,2
COMMENTS
a(n) = 2 for prime n. It appears that all terms belong to A100195 (Numbers n such that the denominator of BernoulliB[n] is a record). - Alexander Adamchuk, Sep 09 2006
a(n) = 2 iff n is prime.
a(468) has 1007 decimal digits. - Michael De Vlieger, Sep 12 2018
From Gus Wiseman, Jan 10 2019: (Start)
Number of matrices whose entries are 1,...,n, up to row and column permutations. For example, inequivalent representatives of the a(4) = 8 matrices are:
[1 2 3 4]
.
[1 2] [1 2] [1 3] [1 3] [1 4] [1 4]
[3 4] [4 3] [2 4] [4 2] [2 3] [3 2]
.
[1]
[2]
[3]
[4]
(End)
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..467
Jimmy Devillet, Gergely Kiss, Characterizations of biselective operations, arXiv:1806.02073 [math.RA], 2018.
FORMULA
E.g.f.: Sum_{k>0} (exp(x^k)-1)/k!.
MATHEMATICA
f[n_] := Block[{d = Divisors@n}, Plus @@ (n!/(d! (n/d)!))]; Array[f, 25] (* Robert G. Wilson v, Sep 11 2006 *)
Table[DivisorSum[n, n!/(#!*(n/#)!) &], {n, 25}] (* Michael De Vlieger, Sep 12 2018 *)
PROG
(PARI) a(n) = sumdiv(n, d, n!/(d!*(n/d)!)); \\ Michel Marcus, Sep 13 2018
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Vladeta Jovovic, Sep 09 2006
EXTENSIONS
More terms from Robert G. Wilson v, Sep 11 2006
STATUS
approved