math equation does not show properly when using curly brackets as protected subindex

I am experiencing some problems using editorial markdown when formatting math equations. For instance, if I type
$ N_1 = \sigma_1 dA $The equation is properly rendered using the “Preview with math” workflow. However, if I type
$ N_{1} = \sigma_{1} dA $
The equation is not rendered at lo, and it appears as italic text. The problem becomes worst if
I try to use the curly brackets to put more complex subíndices, for instance$ N_{12} = \sigma_{12} dA $
shows the same problem, it does not show properly.
Any help is sincerely appreciated.
I will be also grateful if you could provide any pointers to latex math syntax documentation which is currently supported by editorial. My interest in editorial is related to the possibility of using markdown math typing capabilities.
Oscar