R help - May 2009 - image, domain, codomain

Hi all!
i'm trying to write a particular function:
it is as follows:

Pi greek (x):
- has values from R(n) to the [0, 1] interval
- the function Pi Greek(-x)=1-Pi Greek(x)

now the question:
how i could set the codomain to that interval??

thanks very much!

..fede!

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