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A027777 a(n) = 2*(n+1)*C(n+2,4). 3
6, 40, 150, 420, 980, 2016, 3780, 6600, 10890, 17160, 26026, 38220, 54600, 76160, 104040, 139536, 184110, 239400, 307230, 389620, 488796, 607200, 747500, 912600, 1105650, 1330056, 1589490, 1887900, 2229520, 2618880 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

Number of 7-subsequences of [ 1, n ] with just 2 contiguous pairs.

LINKS

Table of n, a(n) for n=2..31.

Index entries for linear recurrences with constant coefficients, signature (6,-15,20,-15,6,-1).

FORMULA

G.f.: 2*(3+2x)*x^2/(1-x)^6.

a(n) = 2*A006411(n+1).

a(n) = C(n+1, 3)*C(n+2, 2) - Zerinvary Lajos, May 13 2005, corrected by R. J. Mathar, Feb 13 2016

MATHEMATICA

Table[2(n+1)Binomial[n+2, 4], {n, 2, 35}]  (* Harvey P. Dale, Feb 03 2011 *)

CROSSREFS

Equals second right hand column of A163934. - Johannes W. Meijer, Oct 16 2009

Sequence in context: A210291 A089207 A318169 * A227013 A073773 A001919

Adjacent sequences:  A027774 A027775 A027776 * A027778 A027779 A027780

KEYWORD

nonn,easy

AUTHOR

thi ngoc dinh (via R. K. Guy)

STATUS

approved

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Last modified April 8 08:04 EDT 2020. Contains 333313 sequences. (Running on oeis4.)