Here are 50 Functional Equations ( and Inequalities ) from Mathlinks, various contests, own and including basic equations like Cauchy's, Jensen's and DAlembert's.
( There may be some confusion with the definition of range and codomain, Wiki says Range $\subseteq$ Codomain, but somehow not only me, but also a lot of Mathlinker knows the opposite. Anyway, definition is what we define )
Functional Equations PSet : From Very Basic
Functional Equations PSet : From Very Basic
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 Nadim Ul Abrar
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 Phlembac Adib Hasan
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Re: Functional Equations PSet : From Very Basic
Again I am requesting you to use ''image'' instead of ''codomain" and write image$\subseteq $Range.Bacause somewhere codomain refers to range (According to our text book) and somewhere to image (According to mathlinks).That's why Saumitra Vaia advised me not to use "codomain".And a lot of thanks for the note.
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Re: Functional Equations PSet : From Very Basic
Adib is right, your use of codomain is wrong and should be replaced by image/range (as should be done here). It is always best not to use "range" because it can mean either codomain or image.
If $f:A\to B$, then
the codomain of $f$ is $B$
the image of $f$ is $\{f(x):x\in A\}$
So, for example, $f:\mathbb R\to\mathbb R$, $f(x)=x^2$ has codomain $\mathbb R$, and image $\mathbb R_{\ge 0}$. The domain and codomain must be specified when defining a function.
If $f:A\to B$, then
the codomain of $f$ is $B$
the image of $f$ is $\{f(x):x\in A\}$
So, for example, $f:\mathbb R\to\mathbb R$, $f(x)=x^2$ has codomain $\mathbb R$, and image $\mathbb R_{\ge 0}$. The domain and codomain must be specified when defining a function.
"Everything should be made as simple as possible, but not simpler."  Albert Einstein