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A049452 Pentagonal numbers with even index. 25
0, 5, 22, 51, 92, 145, 210, 287, 376, 477, 590, 715, 852, 1001, 1162, 1335, 1520, 1717, 1926, 2147, 2380, 2625, 2882, 3151, 3432, 3725, 4030, 4347, 4676, 5017, 5370, 5735, 6112, 6501, 6902, 7315, 7740, 8177, 8626, 9087, 9560, 10045, 10542 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

If Y is a 3-subset of an (2n+1)-set X then, for n>=4, a(n-1) is the number of 4-subsets of X having at least two elements in common with Y. - Milan Janjic, Dec 16 2007

Sequence found by reading the line (one of the diagonal axes) from 0, in the direction 0, 5,..., in the square spiral whose vertices are the generalized pentagonal numbers A001318. - Omar E. Pol, Sep 08 2011

LINKS

Harvey P. Dale, Table of n, a(n) for n = 0..1000

Index entries for linear recurrences with constant coefficients, signature (3,-3,1).

FORMULA

a(n) = n*(6*n-1).

G.f.: x*(5+7*x)/(1-x)^3.

a(n) = C(6*n,2)/3, n>=0. - Zerinvary Lajos, Jan 02 2007

a(n) = A001105(n) + A033991(n) = A033428(n) + A049450(n) = A022266(n) + A000326(n). - Zerinvary Lajos, Jun 12 2007

a(n) = 12*n + a(n-1) - 7. - Vincenzo Librandi, Aug 06 2010

a(n) = 4*A000217(n) + A001107(n). - Bruno Berselli, Feb 11 2011

a(0)=0, a(1)=5, a(2)=22, a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3). - Harvey P. Dale, Mar 07 2012

MAPLE

[seq(binomial(6*n, 2)/3, n=0..42)]; # Zerinvary Lajos, Jan 02 2007

seq(n*(6*n-1), n=0..42); # Zerinvary Lajos, Jun 12 2007

MATHEMATICA

s=0; lst={s}; Do[s+=n++ +5; AppendTo[lst, s], {n, 0, 7!, 12}]; lst (* Vladimir Joseph Stephan Orlovsky, Nov 16 2008 *)

Table[n(6n-1), {n, 0, 50}] (* or *) LinearRecurrence[{3, -3, 1}, {0, 5, 22}, 50] (* Harvey P. Dale, Mar 07 2012 *)

PROG

(PARI) a(n)=n*(6*n-1) \\ Charles R Greathouse IV, Sep 24 2015

CROSSREFS

Cf. A000326, A033570, A049453, A001318, A033568, A185019.

See index to sequences with numbers of the form n*(d*n+10-d)/2 in A140090.

Sequence in context: A273648 A272993 A273581 * A273075 A272824 A273677

Adjacent sequences:  A049449 A049450 A049451 * A049453 A049454 A049455

KEYWORD

nonn,easy

AUTHOR

Joe Keane (jgk(AT)jgk.org)

STATUS

approved

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Last modified June 25 02:50 EDT 2017. Contains 288708 sequences.