OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (11,-55,165,-330,462,-462,330,-165,55,-11,1).
FORMULA
a(n) = n*(-6044030 +17209877*n -19851310*n^2 +12422410*n^3 -4715472*n^4 +1139127*n^5 -176832*n^6 +17190*n^7 -960*n^8 +24*n^9)/24.
G.f.: x*(1+312*x +1670*x^2 -1255*x^3 +15327*x^4 +38264*x^5 +81248*x^6 +406785*x^7 +520730*x^8 +2565718*x^9)/(1-x)^11.
EXAMPLE
a(0) = 0: no word of length 10 is possible for an empty alphabet.
a(1) = 1: aaaaaaaaaa for alphabet {a}.
a(2) = 323: aaaaaaaaaa, aaaaaaaaab, ..., baabababab, bbbbbbbbbb for alphabet {a,b}.
MAPLE
a:= n-> n*(-6044030+ (17209877+ (-19851310+ (12422410+ (-4715472+ (1139127+ (-176832+ (17190+(-960+24*n) *n)*n)*n)*n)*n)*n)*n)*n)/24:
seq(a(n), n=0..40);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Alois P. Heinz, Jun 08 2012
STATUS
approved