%I #15 Feb 23 2017 23:02:43
%S 0,0,1,3,11,26,58,110,199,331,532,811,1206,1729,2432,3331,4494,5942,
%T 7760,9975,12692,15944,19858,24473,29941,36308,43752,52327,62239,
%U 73551,86499,101155,117788,136479,157532,181038,207338,236534,269005,304865,344534
%N Number of partitions of 3n into exactly 6 parts.
%C Also the number of partitions of 3*(n-2) into at most 6 parts. - _Colin Barker_, Apr 01 2015
%H Colin Barker, <a href="/A256315/b256315.txt">Table of n, a(n) for n = 0..1000</a>
%H <a href="/index/Rec#order_15">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-4,2,1,-4,3,3,-4,1,2,-4,1,2,-1).
%F G.f.: x^2*(x^8+x^7+4*x^6+5*x^5+5*x^4+5*x^3+4*x^2+x+1) / ((x-1)^6*(x+1)^3*(x^2+1)*(x^4+x^3+x^2+x+1)).
%e For n=3 the 3 partitions of 3*3 = 9 are [1,1,1,1,1,4], [1,1,1,1,2,3] and [1,1,1,2,2,2].
%t CoefficientList[Series[x^2*(x^8 + x^7 + 4*x^6 + 5*x^5 + 5*x^4 + 5*x^3 + 4*x^2 + x + 1)/((x - 1)^6*(x + 1)^3*(x^2 + 1)*(x^4 + x^3 + x^2 + x + 1)), {x, 0, 50}], x] (* _Wesley Ivan Hurt_, Feb 22 2017 *)
%t Table[Length@ IntegerPartitions[3 n, {6}], {n, 0, 40}] (* _Michael De Vlieger_, Feb 22 2017 *)
%o (PARI) concat(0, vector(40, n, k=0; forpart(p=3*n, k++, , [6,6]); k))
%o (PARI) concat([0,0], Vec(x^2*(x^8+x^7+4*x^6+5*x^5+5*x^4+5*x^3+4*x^2+x+1) / ((x-1)^6*(x+1)^3*(x^2+1)*(x^4+x^3+x^2+x+1)) + O(x^100)))
%Y Cf. A077043, A256313, A256314.
%K nonn,easy
%O 0,4
%A _Colin Barker_, Mar 23 2015