Markdown inline math rendering not parsing correctly

The following inline latex math markdown renders wrong:

$\,t=0\,$  
$\textbf{r}_i|_{t=0} = \textbf{v}_{i0},\, \textbf{v}_i|_{t=0} = \textbf{v}_{i0}$  
$\textbf{v}_i$  
$ \leqslant i \leqslant N$  
$\textbf{f}_{ij}$

,t=0,
$\textbf{r}i|{t=0} = \textbf{v}{i0},, \textbf{v}i|{t=0} = \textbf{v}{i0}$
$\textbf{v}i$
\leqslant i \leqslant N
$\textbf{f}{ij}$

Also there is no math rendering in the preview mode the edit page.