> **Possible Duplicate:** 
> [Save interpolated function](http://mathematica.stackexchange.com/questions/1959/save-interpolated-function) 

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In MATLAB, I can save and recall my own function using an M-File(function or script) 
How can I do something similar in *Mathematica*?

I already searched and know how to define functions and modules like the following:

 f[x_]:=Sin[x]+Cos[x] 
 g[x_]:=Module[{a}, a*f[x]]

When I restart *Mathematica*, however, my own function is deleted. 

Of course, I can save my notebook(.nb) file, but it is just saving my script, not saving my definition of the function.

How can I deal with this problem?