Solution
= Solution
Since $T_i$ has <cumulative hazard function> $\phi_iH_0(t)$,
$$
\mathbb P\{H_0(T_i)>u\}
=\mathbb P\{T_i>H_0^{-1}(u)\}
=e^{-\phi_i u}.
$$
Thus the cumulative-hazard time change $U_i=H_0(T_i)$ has an <exponential distribution> of rate $\phi_i$.