OFFSET
1,1
COMMENTS
Number of n permutations (n>=4) of 5 objects u, v, z, x, y with repetition allowed, containing n-4 u's. Example: if n=4 then n-4 = zero (0) u, a(1)=256, if n=5 then n-4 = one (1) u, a(2)=1280, if n=6 then n-4 = two (2) u, a(3)=3840, etc.
LINKS
Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).
FORMULA
G.f.: 256/(1-x)^5.
a(n) = C(n+3,4)*4^4, n>=1.
From Amiram Eldar, Sep 01 2022: (Start)
Sum_{n>=1} 1/a(n) = 1/192.
Sum_{n>=1} (-1)^(n+1)/a(n) = log(2)/8 - 1/12. (End)
MAPLE
seq(binomial(n+3, 4)*4^4, n=1..36);
MATHEMATICA
256*Binomial[Range[30]+3, 4] (* or *) LinearRecurrence[{5, -10, 10, -5, 1}, {256, 1280, 3840, 8960, 17920}, 30] (* Harvey P. Dale, Jul 19 2018 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Zerinvary Lajos, Aug 07 2008
STATUS
approved