OFFSET
4,1
COMMENTS
Number of 15-subsequences of [ 1, n ] with just 5 contiguous pairs.
LINKS
T. D. Noe, Table of n, a(n) for n = 4..1000
Index entries for linear recurrences with constant coefficients, signature (11,-55,165,-330,462,-462,330,-165,55,-11,1).
FORMULA
G.f.: 126*(1+x)*x^4/(1-x)^11.
a(n) = C(n+1, 5)*C(n+5, 5). - Zerinvary Lajos, Apr 18 2005; corrected by R. J. Mathar, Feb 10 2016
From Amiram Eldar, Feb 03 2022: (Start)
Sum_{n>=4} 1/a(n) = 25*Pi^2/6 - 1160419/28224.
Sum_{n>=4} (-1)^n/a(n) = 25*Pi^2/12 - 27625/1344. (End)
MATHEMATICA
Table[126(n+1)Binomial[n+5, 9]/5, {n, 4, 40}] (* Harvey P. Dale, Mar 13 2011 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Thi Ngoc Dinh (via R. K. Guy)
STATUS
approved