  It is known that if $T$ is a contraction of class $C_{10}$ and $I-T^\ast T$
is of trace class, then $T$ is a quasiaffine transform of a unilateral shift.
Also it is known that if the multiplicity of a unilateral shift is infinite,
the converse is not true. In this paper the converse for a finite multiplicity
is proved: if $T$ is a contraction and $T$ is a quasiaffine transform of a
unilateral shift of finite multiplicity, then $I-T^\ast T$ is of trace class.
As a consequence we obtain that if a contraction $T$ has finite multiplicity
and its characteristic function has an outer left scalar multiple, then
$I-T^\ast T$ is of trace class.
  Also, it is known that if a contraction $T$ on a Hilbert space $\mathcal H$
is such that $\|b_\lambda(T)x\|\geq\delta\|x\|$ for every $\lambda\in\mathbb
D$, $x\in\mathcal H$, with some $\delta>0$, and $$\sup_{\lambda\in\mathbb
D}\|I-b_\lambda(T)^\ast b_\lambda(T)\|_{\frak S_1}<\infty$$ (here $b_\lambda$
is a Blaschke factor and $\frak S_1$ is the trace class of operators), then $T$
is similar to an isometry. In this paper the converse for a finite multiplicity
is proved: if $T$ is a contraction and $T$ is similar to an isometry of finite
multiplicity, then $T$ satisfies the above conditions.
