A164619 Integers of the form A164577(k)/3.

4, 15, 54, 75, 132, 169, 320, 459, 735, 847, 1104, 1250, 1764, 2175, 2904, 3179, 3780, 4107, 5200, 6027, 7425, 7935, 9024, 9604, 11492, 12879, 15162, 15979, 17700, 18605, 21504, 23595, 26979, 28175, 30672, 31974, 36100, 39039, 43740, 45387, 48804

Offset

1,1

Comments

The sequence members are the third of the average of a set of smallest cubes, if integer.

Links

Table of n, a(n) for n=1..41.

Index entries for linear recurrences with constant coefficients, signature (2,-1,-1,2,-1,2,-4,2,2,-4,2,-1,2,-1,-1,2,-1).

Formula

a(n) = +2*a(n-1) -a(n-2) -a(n-3) +2*a(n-4) -a(n-5) +2*a(n-6) -4*a(n-7) +2*a(n-8) +2*a(n-9) -4*a(n-10) +2*a(n-11) -a(n-12) +2*a(n-13) -a(n-14) -a(n-15) +2*a(n-16) -a(n-17). - R. J. Mathar, Jan 25 2011

G.f.: x*(x^14 +x^13 +16*x^12 +10*x^11 +47*x^10 -22*x^9 +61*x^8 +10*x^7 +88*x^6 +8*x^5 +43*x^4 -14*x^3 +28*x^2 +7*x +4) / ((x -1)^4*(x +1)^3*(x^2 -x +1)^3*(x^2 +x +1)^2). - Colin Barker, Oct 27 2014

Example

A third of the average of the first cube, A164577(1)/3=1/3, is not an integer and does not contribute to the sequence.
A third of the average of the first two cubes, A164577(2)/3=4, is an integer and defines a(1)=4 of the sequence.

Mathematica

s=0; lst={}; Do[a=(s+=(n^3)/3)/n; If[Mod[a, 1]==0, AppendTo[lst, a]], {n, 2*5!}]; lst
LinearRecurrence[{2, -1, -1, 2, -1, 2, -4, 2, 2, -4, 2, -1, 2, -1, -1, 2, -1}, {4, 15, 54, 75, 132, 169, 320, 459, 735, 847, 1104, 1250, 1764, 2175, 2904, 3179, 3780}, 50] (* Harvey P. Dale, Apr 06 2016 *)

Prog

(PARI) Vec(x*(x^14 +x^13 +16*x^12 +10*x^11 +47*x^10 -22*x^9 +61*x^8 +10*x^7 +88*x^6 +8*x^5 +43*x^4 -14*x^3 +28*x^2 +7*x +4) / ((x -1)^4*(x +1)^3*(x^2 -x +1)^3*(x^2 +x +1)^2) + O(x^100)) \\ Colin Barker, Oct 27 2014

Crossrefs

Cf. A050248, A051456, A078617, A078618, A136116, A154293, A164286, A164576, A164577, A164578, A164579.

Sequence in context: A071719 A370034 A289927 * A227382 A090326 A291032

Adjacent sequences: A164616 A164617 A164618 * A164620 A164621 A164622

Keyword

nonn,easy

Author

Vladimir Joseph Stephan Orlovsky, Aug 17 2009

Extensions

Edited by R. J. Mathar, Aug 20 2009

Status

approved