Abstract
Let g(x) be a fixed non-constant complex polynomial. It was conjectured by Schinzel that if g(h(x)) has boundedly many terms, then h(x)∈ℂ[x] must also have boundedly many terms. Solving an older conjecture raised by Rényi and by Erdös, Schinzel had proved this in the special cases g(x)=x d; however that method does not extend to the general case. Here we prove the full Schinzel’s conjecture (actually in sharper form) by a completely different method. Simultaneously we establish an “algorithmic” parametric description of the general decomposition f(x)=g(h(x)), where f is a polynomial with a given number of terms and g,h are arbitrary polynomials. As a corollary, this implies for instance that a polynomial with l terms and given coefficients is non-trivially decomposable if and only if the degree-vector lies in a suitable element of the Boolean algebra generated by the subgroups of ℤl.
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Zannier, U. On composite lacunary polynomials and the proof of a conjecture of Schinzel. Invent. math. 174, 127–138 (2008). https://doi.org/10.1007/s00222-008-0136-8
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DOI: https://doi.org/10.1007/s00222-008-0136-8