A076506 Expansion of x*(1+3*x+12*x^2)/(1-24*x^3).

1, 3, 12, 24, 72, 288, 576, 1728, 6912, 13824, 41472, 165888, 331776, 995328, 3981312, 7962624, 23887872, 95551488, 191102976, 573308928, 2293235712, 4586471424, 13759414272, 55037657088, 110075314176, 330225942528

Offset

1,2

Comments

Previous name was: 3 people at a party are saying Hello to each other. Person 1 says Hello. Person 2 counts the times Hello has been said and says Hello twice that number of times. Person 3 says Hello 3 times the sum of Hello's and then it is Person 1's turn again. This is how the sum of Hello's.

Links

Table of n, a(n) for n=1..26.

Index entries for linear recurrences with constant coefficients, signature (0,0,24).

Formula

G.f.: x*(1+3*x+12*x^2)/(1-24*x^3). [Colin Barker, Jun 07 2012]

Mathematica

a[n_] := Product[Mod[k, 3]+2, {k, 1, n-1}]; Array[a, 26] (* Jean-François Alcover, Oct 05 2016, adapted from PARI *)

Prog

(PARI) a(n)=prod(k=1, n-1, k%3+2)

Crossrefs

Cf. A076505.

Sequence in context: A272173 A028725 A047166 * A206917 A320334 A009776

Adjacent sequences: A076503 A076504 A076505 * A076507 A076508 A076509

Keyword

nonn,easy

Author

Jon Perry, Oct 15 2002

Status

approved