Someone wrote in ![[info]](http://l-stat.livejournal.com/img/userinfo.gif){kind=small} alsuren, |
You're trying to say what your set is first and then check that it is closed under f. However this probably won't work as you have found. I think you can just declare your set to be the set a) containing the rationals, e, pi and anything else you like b) f(x,y) for any x, y in the set. I.e. you just declare that your set is closed under f (except: you can't divide by zero) and as long as you can prove the set is still countable then you're fine.
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