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Confusing results with N[expr]?


I have the impression that since some time (since Vers. 5.0?) N[expr] does 
not give anymore the results I expect.

The following code should illustrate this:

N[a] = 2.;

k = z*(a*Pi);
N[k]
> 6.28319 z       (* "a" and "Pi" are replaced by their numerical value as 
> expected *)

k = z^(a*Pi)
N[k]
> z^(3.14159 a)    (* "Pi" is replaced by its numerical value but not "a", 
> why not? *)


There are more examples of this behaviour: If the expression is Log[z*a*Pi], 
the numerical values of "a" and "Pi" are evaluated, if the expression is 
Exp[z*a*Pi] the numerical value of "a" is not evaluated. Why not?

The motivation behind my question is that I was used to write expressions 
symbolically and assign numerical values with N[expr] = num. value. This way 
you could keep those expressions symbolically and just apply //N when you 
needed numerical values. Now it seems that I have lost this possibility.

Could you please comment my observation and maybe suggest a way out?

Thank you very much for any idea.

Kind regards,
Thomas.



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