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 Feb 2 revised Find all the functions which satisfy a given functional equation edited body Feb 2 comment Find all the functions which satisfy a given functional equation @DejanGovc I was editing while I received your post. Please check it again Feb 2 revised Find all the functions which satisfy a given functional equation added 123 characters in body Feb 2 comment Find all the functions which satisfy a given functional equation @Alex did you assume $f(0)=0$ in your solution? Because then I'm pretty sure that $f(0)=-1$ is also possible. My apologies since I inverted $x,y$ in the first term of the equation. Please check the edited version. Still my apologies. BTW in this case setting $x=y=0$ gives $f(f(0))=f(0)^2+f(f(0))+f(0)$ from which $f(0)\in \{-1,0\}$. Feb 2 asked Find all the functions which satisfy a given functional equation Dec 5 asked Unusual Compact Embeddings Nov 13 asked solution of Lagrange differential equation are square integrable Nov 5 revised Ordering of two weak star limits added 492 characters in body Nov 5 comment Ordering of two weak star limits lol you are right... Nov 5 asked Ordering of two weak star limits Oct 29 accepted At most three different eigenvalues Oct 29 comment At most three different eigenvalues @GerryMyerson I'm following the language introduced in the Birkhoff Rota Ordinary differential equation book and you can define the operator $L[u]=u'+q(x)u$ so for $\lambda$ to be an eigenvalue it means that $L[u]=\lambda u$ has a non trivial solution $u$. Oct 29 comment At most three different eigenvalues @JuliánAguirre it is $u'$, it is a first order DE Oct 29 asked At most three different eigenvalues Oct 29 comment airy equation vanish infinitely many times nice. Didn't know about this one. Can we solve the exercise without using that theorem? Oct 29 accepted airy equation vanish infinitely many times Oct 28 asked airy equation vanish infinitely many times Oct 17 accepted Quick criterion to decide whether a limit of functions in $W^{1,p}(\Omega)$ is in that space Oct 17 asked Quick criterion to decide whether a limit of functions in $W^{1,p}(\Omega)$ is in that space Oct 4 accepted How to evaluate this limit?