OFFSET
1,3
FORMULA
It appears that a(n) = n^3/8 if n is even, a(n) = (n-1)*(n+1)^2/8 if n is odd.
Empirical g.f.: x^2*(x^3+x^2+3*x+1) / ((x-1)^4*(x+1)^3). - Colin Barker, Jul 08 2014
PROG
(PARI) isok(k) = {cbr = sqrtnint(k, 3); if (cbr^3 == k, 1, cbr*(cbr+1)^2 == k); } \\ Michel Marcus, Jul 08 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
John W. Layman, Mar 21 2012
STATUS
approved