OFFSET
0,3
REFERENCES
K. H. Rosen, Discrete Mathematics and Its Application, 6th Edition, McGraw-Hill, 2007, Ex. 26 of section 2.4.
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
FORMULA
a(0) = 0, a(1) = 1, a(n) = 2*a(n-1) - a(n-2) if n not a perfect cube, else a(n) = 2*a(n-1) - a(n-2) + 1 if n is a perfect cube.
a(n) = -1/4*floor(n^(1/3))*(floor(n^(1/3))^3+2*floor(n^(1/3))^2+floor(n^(1/3))-4*(n+1)). - John M. Campbell, Mar 22 2016
G.f.: Sum_{k>=1} x^(k^3)/(1 - x)^2. - Ilya Gutkovskiy, Dec 22 2016
a(n) = (3/4)*n^(4/3) + O(n). - Charles R Greathouse IV, Aug 23 2017
MATHEMATICA
Accumulate[Floor[Surd[Range[0, 70], 3]]] (* Harvey P. Dale, Nov 03 2013 *)
Table[Sum[Floor[i^(1/3)], {i, 0, n}], {n, 0, 50}] (* G. C. Greubel, Dec 22 2016 *)
PROG
(PARI) a(n) = sum(k=1, n, sqrtnint(k, 3)); \\ Michel Marcus, Mar 12 2016
(PARI) a(n)=my(t=sqrtnint(n, 3)); t*(4*n-t^3-2*t^2-t+4)/4 \\ Charles R Greathouse IV, Aug 23 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Michel Tixier (tixier(AT)dyadel.net)
STATUS
approved