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Sep 4, 2012 at 6:56 comment added Rurik Thank you very much. As a matter of fact I think I managed: infact my 27 polynomials, even if scary-looking in the whole, turned out to be quite simple. For example, for many of them both $F$ and $G$ are of prime degree, and for the others, they have a degree written as the product of just to prime... this allowed me to esclude at once many decompositions...
Aug 29, 2012 at 12:37 comment added Michael Zieve @Rurik: in practice, even if one only has partial or approximation information about two polynomials $F(x)$ and $G(x)$, one can usually prove that $F(x)-G(y)$ is irreducible. The reason is that one can usually show that both $F$ and $G$ are indecomposable, for instance just by writing $F = A \circ B$ where $A,B$ have undetermined coefficients, and then solving for the coefficients of $A$ and $B$. Once this has been done, then Fried's "same splitting field" result will imply irreducibility. Please feel free to email me about how to do this for your specific polynomials.
Aug 29, 2012 at 12:23 history answered Michael Zieve CC BY-SA 3.0