prove $\dim(\operatorname{range}(T)) = \dim(\operatorname{range}(\sqrt{T^*T}))$ I'm a student and I'm studying linear algebra. in Polar Decomposition we have:

for a linear operator $T$, there exist a linear isometry $S$ that:
  $$ T =S\sqrt{T^*T}$$

so if $S$ is a linear transformation then it must be $\dim(\operatorname{range}(T)) \leq  \dim(\operatorname{range}(\sqrt{T^*T}))$.
But why? 
edit: I think it must be equal, I mean: 
$$\dim(\operatorname{range}(T)) = \dim(\operatorname{range}(\sqrt{T^*T}))$$
I know that $\dim(\operatorname{range}(T)) = \dim(\operatorname{range}(T^*))$ but because of the square root I cannot prove that $\dim(\operatorname{range}(T)) = \dim(\operatorname{range}(\sqrt{T^*T}))$.

As @egreg pointed out, for any linear maps $S,T$, the rank of $ST$ is always less than or equal to that of $T$. To see this, note that by dimension theorem $$\dim \text{ran} L =\dim \operatorname{dom} L-\dim\ker L\le\dim \operatorname{dom} L$$ for any linear map $L$. Now, the image of $ST$ can be seen as the image of
$$
S\big|_{\text{ran} T} :\text{ran} T\to V,
$$ we can see that $\dim \text{ran} (ST)\le \dim \text{ran} T$ as wanted. Moreover, equality holds if $\dim \ker S\big|_{\text{ran}T}=\dim [\ker S\cap\text{ran}T]=0$. (Here, $V$ denotes the vector space where $S,T$ are defined.)
To see that $\dim \text{ran}T =\dim \text{ran}(T^*T)$, note that $Tx = 0$ if and only if $T^*Tx =0$, which is saying that $\ker T = \ker (T^*T)$. Now, by dimension theorem, we have
$$
\dim \text{ran} T = \dim V - \dim \ker T = \dim V - \dim \ker (T^*T) = \dim\text{ran}(T^*T).
$$ (Or we can use the fact that $S$ is an isometry.)
