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A027780 a(n) = 7*(n+1)*C(n+2,7)/2. 0
21, 196, 1008, 3780, 11550, 30492, 72072, 156156, 315315, 600600, 1089088, 1893528, 3174444, 5155080, 8139600, 12534984, 18877089, 27861372, 40378800, 57557500, 80810730, 111891780, 152956440, 206633700 (list; graph; refs; listen; history; text; internal format)
OFFSET

5,1

LINKS

Table of n, a(n) for n=5..28.

Index entries for linear recurrences with constant coefficients, signature (9,-36,84,-126,126,-84,36,-9,1).

FORMULA

Number of 10-subsequences of [ 1, n ] with just 2 contiguous pairs; g.f. 7*(3+x)*x^5/(1-x)^9.

a(n) = C(n+1, 6)*C(n+2, 2). - Zerinvary Lajos, Apr 28 2005, corrected by R. J. Mathar, Feb 10 2016

MATHEMATICA

Table[7(n+1) Binomial[n+2, 7]/2, {n, 30}] (* Harvey P. Dale, Feb 25 2013 *)

PROG

(PARI) a(n)=7*(n+1)*binomial(n+2, 7)/2 \\ Charles R Greathouse IV, Feb 26 2013

CROSSREFS

Sequence in context: A022713 A163718 A164606 * A108679 A200825 A058086

Adjacent sequences:  A027777 A027778 A027779 * A027781 A027782 A027783

KEYWORD

nonn,easy

AUTHOR

thi ngoc dinh (via R. K. Guy)

EXTENSIONS

Offset corrected by Harvey P. Dale, Feb 26 2013

STATUS

approved

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Last modified June 19 05:18 EDT 2021. Contains 345125 sequences. (Running on oeis4.)