The number of limit cycles that can be born from periodic solutions of a polynomial Hamiltonian planar system by a small polynomial perturbation

not increasing the degree , is explicitly bounded by a double exponent , where is an explicit polynomial in of degree not exceeding 60 (fine tuning of the proof gives a better value around 5 or so, which hypothetically could be reduced to just 2). For hyperelliptic Hamiltonians of the form the bound can be improved to with an explicit constant in the term . This assertion constitutes an **explicit constructive solution of the so called “Infinitesimal” Hilbert 16th Problem** which first implicitly appeared in the works of Petrovskii and Landis in the 1950-s. Since mid-1960-s the problem was repeatedly formulated in many sources (starting with Arnold’s problems and as recently as in Ilyashenko’s 2008 list) as the natural step towards a still evasive solution of the complete Hilbert 16th Problem.

*“J’ai** Nous (i.e., ***Gal Binyamini, Dmitry Novikov et moi-même**) avons trouvé une merveilleuse démonstration de cette proposition, mais je ne peux l’écrire dans cette marge car elle est trop longue.”

*La démonstration* is indeed a bit too long to be reproduced here: the complete exposition is available on **arXiv** (50+ pages) and strongly uses **another paper** of 30+ pages which establishes *non-uniform* explicit double exponential upper bound on the number of isolated complex zeros of functions satisfying linear systems of Fuchsian differential equations, provided that all residue matrices have only real eigenvalues.

Our proof is based solely on the fact that Abelian integrals of polynomial 1-forms along cycles on complexified level curves of the Hamiltonian, satisfy an integrable system of regular Pfaffian differential equations defined over with quasiunipotent monodromy along all small loops.

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