OFFSET
0,3
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..1000
A. Ahmad, A. Magidin and R. F. Morse, Two generator p-groups of nilpotency class 2 and their conjugacy classes, Publ. Math. Debrecen 81 (2012), no. 1-2, 145-166.
C. Voll, Enumerating finite class-2-nilpotent groups on 2 generators, C. R. Math. Acad. Sci. Paris 347 (2009), no. 23-24, 1347-1350.
FORMULA
a(n) = sum_{r+s+t=n, r >= s >= t >= 0}( (t+1)+(1/2)*min{t,r-s}*(2t+1-min{t,r-s} ).
G.f.: 1/((1-x)*(1-x^2)*(1-x^3)^2*(1-x^4)). - Tani Akinari, Jun 28 2014
a(n) = floor( (n^4+26*n^3+234*n^2+(909-64*(n mod 3))*n+1701)/1728+(n+1)*(-1)^n/64 ). [Tani Akinari, Jun 28 2014 - see PARI code]
MAPLE
A213506 := proc(n)
a := 0 ;
for t from 0 to n do
for s from t to n-t do
r := n-s-t ;
if r >= s then
m := min(t, r-s) ;
a := a+t+1+m*(2*t+1-m)/2 ;
end if;
end do:
end do:
return a;
end proc:
seq(A213506(n), n=0..70) ; # R. J. Mathar, Jun 26 2012
MATHEMATICA
CoefficientList[Series[1/((1 - x)*(1 - x^2)*(1 - x^3)^2*(1 - x^4)), {x, 0, 50}], x] (* Wesley Ivan Hurt, Jun 28 2014 *)
PROG
(PARI) a(n)=floor((n^4+26*n^3+234*n^2+(909-64*(n%3))*n+1701)/1728+(n+1)*(-1)^n/64) \\ Tani Akinari, Jun 28 2014
(PARI) Vec( 1/((1-x)*(1-x^2)*(1-x^3)^2*(1-x^4)) + O(x^100)) \\ Michel Marcus, Jun 28 2014
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Arturo Magidin, Jun 12 2012
STATUS
approved