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Let be given a map $F:(x,y)\in\mathbb{R}\times\mathbb{R}^n\to F(x,y)\in\mathbb{R}^n$.
Let us denote by $\mathcal{P}$ the set whose elements are the solutions of the ode $y'=F(x,y)$, i.e. the differentiable maps $u:J\to\mathbb{R}^n$, where $J\ $ is some open interval in $\mathbb{R}\ $, s.t. $u'(t)=F(t,u(t))$ for all $t\in J$.
Let $\mathcal{P}$ be endowed with the ordering by extension.

In order to prove that any element of $\mathcal{P}$ is extendable to a (not unique) maximal element, without particular hypothesis on $F$, I was wondering if the Zorn lemma can be used.

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    @Giuseppe, okay, since you keep insisting. But it's not really much of an answer -- all I did was deny that the proof you had already sketched in the question has any problems with it.2011-10-22

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Yes, Zorn's Lemma should be all you need. Take the set of partial solutions that extend your initial solution, and order them by the subset relation under the common definition of a function as the set of pairs $\langle x, f(x)\rangle$. Then the union of all functions in a chain will be another partial solution, so Zorn's Lemma applies.