Computing Effective Sample Size

After a measurement update, a particle filter with 4 particles has the following normalized importance weights: $$w = [\,0.5,\; 0.25,\; 0.125,\; 0.125\,].$$

Using the standard approximation $N_{\text{eff}} = \dfrac{1}{\sum_{i} (w^{(i)})^2}$, compute the effective sample size. Give your answer in number of particles.