# Identically distributed and same characteristic function

If $X,Y$ are identically distributed random variables, then I know that their characteristic functions $\phi_X$ and $\phi_Y$ are the same. Does the converse also hold?

• Yes it does, the characteristic function completely defines the probability distribution. – Ian May 15 '14 at 23:31

Yes, it is true as a consequence of the inversion formula $$\mu(a,b) +\frac 12\mu(\{a,b\}) = \frac 1{2\pi}\lim_{ t\to +\infty}\int_{-T}^T\frac{e^{ita} -e^{itb} }{it}\varphi_\mu (t)\mathrm dt,$$ valid for $a\lt b$.
If $\mu$ and $\nu$ have the same characteristic function, then $\mu([a,b])=\nu([a,b])$ for each $a\lt b$. This is true for finite disjoint unions of half-open intervals, and these sets characterize Borel probability measures on the real line.