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

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A121860 a(n) = Sum_{d|n} n!/(d!*(n/d)!). 6

%I

%S 1,2,2,8,2,122,2,1682,10082,30242,2,7318082,2,17297282,3632428802,

%T 36843206402,2,2981705126402,2,1690185726028802,3379030566912002,

%U 28158588057602,2,76941821303636889602,1077167364120207360002

%N a(n) = Sum_{d|n} n!/(d!*(n/d)!).

%C 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

%C a(n) = 2 iff n is prime.

%C a(468) has 1007 decimal digits. - _Michael De Vlieger_, Sep 12 2018

%C From _Gus Wiseman_, Jan 10 2019: (Start)

%C 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:

%C [1 2 3 4]

%C .

%C [1 2] [1 2] [1 3] [1 3] [1 4] [1 4]

%C [3 4] [4 3] [2 4] [4 2] [2 3] [3 2]

%C .

%C [1]

%C [2]

%C [3]

%C [4]

%C (End)

%H Michael De Vlieger, <a href="/A121860/b121860.txt">Table of n, a(n) for n = 1..467</a>

%H Jimmy Devillet, Gergely Kiss, <a href="https://arxiv.org/abs/1806.02073">Characterizations of biselective operations</a>, arXiv:1806.02073 [math.RA], 2018.

%F E.g.f.: Sum_{k>0} (exp(x^k)-1)/k!.

%t f[n_] := Block[{d = Divisors@n}, Plus @@ (n!/(d! (n/d)!))]; Array[f, 25] (* _Robert G. Wilson v_, Sep 11 2006 *)

%t Table[DivisorSum[n, n!/(#!*(n/#)!) &], {n, 25}] (* _Michael De Vlieger_, Sep 12 2018 *)

%o (PARI) a(n) = sumdiv(n, d, n!/(d!*(n/d)!)); \\ _Michel Marcus_, Sep 13 2018

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

%K easy,nonn

%O 1,2

%A _Vladeta Jovovic_, Sep 09 2006

%E More terms from _Robert G. Wilson v_, Sep 11 2006

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 December 13 09:54 EST 2019. Contains 329968 sequences. (Running on oeis4.)