How we can proof that the complexity:
  $\sum _{k=1}^{clogn-1}\:k\cdot \left(\frac{1}{2}\right)^{\frac{k}{3}}\in O\left(1\right)$

How we can prove that: $$ \sum_{k=1}^{c \log n-1}\:k\cdot \left(\frac{1}{2}\right)^{\frac{k}{3}}\in O\left(1\right) \quad \mbox{?} $$