How I can mathematically convert from CIELAB (D65,10) to XYZ (D65,2) or RGB?
You can’t. CIELAB can be converted back to XYZ using the simple inversion of the CIELAB equations, but there is no way to change from the 10 observer to the 2 observer in the process. As far as conversion to RGB, that requires a characterization for the monitor you are using. A common choice is to convert to standard RGB (sRGB) values and there is a set of equations defined for that conversion.
