login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A121860 a(n) = Sum_{d|n} n!/(d!*(n/d)!). 6
1, 2, 2, 8, 2, 122, 2, 1682, 10082, 30242, 2, 7318082, 2, 17297282, 3632428802, 36843206402, 2, 2981705126402, 2, 1690185726028802, 3379030566912002, 28158588057602, 2, 76941821303636889602, 1077167364120207360002 (list; graph; refs; listen; history; text; internal format)
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

Cf. A038041, A057625, A061095, A100195, A236696, A320444.

Sequence in context: A274139 A283995 A056189 * A283990 A021442 A296851

Adjacent sequences:  A121857 A121858 A121859 * A121861 A121862 A121863

KEYWORD

easy,nonn

AUTHOR

Vladeta Jovovic, Sep 09 2006

EXTENSIONS

More terms from Robert G. Wilson v, Sep 11 2006

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 19 19:09 EST 2019. Contains 329323 sequences. (Running on oeis4.)