 Solution

Assume that $f(x)=0$ wherever $h(x)=0$ on the part of the space relevant to $g$. The importance sampling algorithm draws independent and identically distributed random variables $Y_1,\dots ,Y_m$ from $h$, forms the weightsand returnsA change of measure givesHence the estimator is unbiased whenever the expectation exists, and, if ${\mathbb{E}}_h|g(Y)w(Y)|<\infty$, the strong law of large numbers gives ${\hat{\mu }}_m\to {\mathbb{E}}_f[g(X)]$ almost surely.