Sstrhsrtj

Alright, here goes, first I know that my title is somewhat vague, but an image is worth a thousand words, here's what I'm trying to achieve :

Now this is the code that I came up with :

 documentclass{article}

 usepackage{amsmath}

 begin{document}

 $P_{CB}$ (ou ${}_{B}stackrel{P}{scriptsize{leftarrow}}_{C}$) telle que

 end{document}

And this is the result of it :

Now as in the picture, I would like to have the capital P inline with the word "ou" and the arrow to be a smaller size and to have the B and C (left and right) of the capital P as subscripts (maybe?)

Why don't you use underset?

documentclass{article}

usepackage{amsmath}



begin{document} 



With verb|underset|:

$P_{CB}$ (ou $underset{Bleftarrow C}{P}$) telle que 



end{document}

Use provided command myfrac mentioned in your preamble part.
documentclass{article}
usepackage{amsmath}

newcommandmyfrac[2]{genfrac{}{}{0pt}{}{#1}{#2}}





defbc{B leftarrow C}



begin{document}



$P_{CB}$ (ou $myfrac{P}{scriptsizebc}$) telle que



end{document}