OFFSET
0,2
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..140
FORMULA
For n > 1, a(n) = 8*(9297776*n^10 + 17051200*n^9 - 11545329*n^8 - 20688255*n^7 + 7760028*n^6 + 7548270*n^5 - 2879537*n^4 - 619195*n^3 + 326046*n^2 - 30420*n + 216) * (4*n-5)! / (3 * (2*n-1) * (2*n+1) * (2*n+3) * (9*n^2-9*n+2) * (9*n^2+9*n+2) * (n-2)! * (n+1)! * (n+2)! * (n+3)!). - Vaclav Kotesovec, Sep 02 2014
EXAMPLE
a(0) = 1: the empty word.
a(1) = 24: abcd, abdc, ..., dcab, dcba, (all permutations of 4 letters).
a(2) = 105: aabbccdd, aabbcdcd, aabbdccd, ..., dcaabbcd, dcababcd, dcbaabcd.
MATHEMATICA
Flatten[{1, 24, Table[8*(9297776*n^10 + 17051200*n^9 - 11545329*n^8 - 20688255*n^7 + 7760028*n^6 + 7548270*n^5 - 2879537*n^4 - 619195*n^3 + 326046*n^2 - 30420*n + 216) * (4*n-5)! / (3 * (2*n-1) * (2*n+1) * (2*n+3) * (9*n^2-9*n+2) * (9*n^2+9*n+2) * (n-2)! * (n+1)! * (n+2)! * (n+3)!), {n, 2, 20}]}] (* Vaclav Kotesovec, Sep 02 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Jun 23 2012
STATUS
approved