Prove for each function $f:\mathbb Q\times\mathbb Q\mapsto\mathbb R$ there exists a function $g:\mathbb Q\mapsto\mathbb R$ such that $f(x,y)\le g(x)+g(y)\,\forall x,y\in\mathbb Q$ Find a function $f:\mathbb R\times\mathbb R\mapsto\mathbb R$ for which there is no function $g:\mathbb R\mapsto\mathbb R$ such that $f (x,y)\le g (x)+g (y)\,\forall x,y\in\mathbb R $