login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A247109 Number of permutations of length n with two 4-sequences. 1
0, 0, 0, 0, 1, 2, 11, 65, 433, 3271, 27741, 261231, 2708064, 30671367, 377034018, 5001404982, 71229862678, 1084282429946, 17571257417630, 302064161086250, 5490937395703435, 105243824522368960, 2121386876912041845, 44863116021267642255, 993272322666679219071, 22977273619066571708457 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,6
LINKS
FORMULA
a(n) ~ n! / n^3. - Vaclav Kotesovec, Nov 23 2014
MATHEMATICA
Table[Sum[(-1)^k * k*(k-1)/2 * Sum[Sum[Binomial[k-1, p] * Binomial[k-p-1, i-2*p] * Binomial[n-2*k+p-1, n-3*k+i-1] * (n-3*k+i)!, {p, 0, k-1}], {i, Max[0, 3*k-n], 2*(k-1)}], {k, 2, n-3}], {n, 1, 20}] (* Vaclav Kotesovec, Nov 23 2014 after Tani Akinari *)
PROG
(PARI) a(n)=sum(k=2, n-3, (-1)^k*k*(k-1)/2*sum(i=max(0, 3*k-n), 2*(k-1), sum(p=0, k-1, binomial(k-1, p)*binomial(k-p-1, i-2*p)*binomial(n-2*k+p-1, n-3*k+i-1)*(n-3*k+i)!)))
CROSSREFS
Cf. A002630 (permutations with two 3-sequences).
Sequence in context: A357845 A199412 A074613 * A039632 A143816 A220783
KEYWORD
nonn
AUTHOR
Tani Akinari, Nov 21 2014
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 20:05 EDT 2024. Contains 371254 sequences. (Running on oeis4.)