Constant functions

Forum for the GRE subject test in mathematics.
Hom
Posts: 39
Joined: Sat Oct 01, 2011 3:22 am

Constant functions

Postby Hom » Wed Nov 09, 2011 10:11 am

If f(g(x)) is a constant, is it necessary that either f or g is a constant function?

yoyobarn
Posts: 80
Joined: Sun Dec 19, 2010 7:01 am

Re: Constant functions

Postby yoyobarn » Wed Nov 09, 2011 10:39 am

I think it is not necessarily so.

Example: f(x)=e^{ix}
g(x)=2x\pi, if x is an integer, and 0 otherwise.

Then, f(g(x))=1 but neither f nor g are constant functions.

Hom
Posts: 39
Joined: Sat Oct 01, 2011 3:22 am

Re: Constant functions

Postby Hom » Wed Nov 09, 2011 10:53 am

yoyobarn wrote:I think it is not necessarily so.

Example: f(x)=e^{ix}
g(x)=2x\pi, if x is an integer, and 0 otherwise.

Then, f(g(x))=1 but neither f nor g are constant functions.


Thank you yoyobarn. You're right. You inspired me to think of another case in R.
f(x) = sinx
g(x)= [x]*pi
f(g(x))=0

owlpride
Posts: 204
Joined: Fri Jan 29, 2010 2:01 am

Re: Constant functions

Postby owlpride » Thu Nov 10, 2011 2:51 am

Or g = f^{-1}.

blitzer6266
Posts: 61
Joined: Sun Apr 04, 2010 1:08 pm

Re: Constant functions

Postby blitzer6266 » Thu Nov 10, 2011 3:57 am

The identity? (gotta be honest, that was the first thing that came to my mind too.. not right though)

owlpride
Posts: 204
Joined: Fri Jan 29, 2010 2:01 am

Re: Constant functions

Postby owlpride » Thu Nov 10, 2011 4:26 am

Ooops. Lol!

That's what happens when you use 1_X to denote the identity function on X...




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