An inverse approach to the center problem

  • Jaume LlibreEmail author
  • Rafael Ramírez
  • Valentín Ramírez


We consider analytic or polynomial vector fields of the form \({\mathcal {X}}=\left( -y+X\right) \dfrac{\partial }{\partial x}+\left( x+Y\right) \dfrac{\partial }{\partial y},\) where \(X=X(x,y))\) and \(Y=Y(x,y))\) start at least with terms of second order. It is well-known that \({\mathcal {X}}\) has a center at the origin if and only if \({\mathcal {X}}\) has a Liapunov–Poincaré local analytic first integral of the form \(H=\dfrac{1}{2}(x^2+y^2)+\sum _{j=3}^ {\infty } H_j\), where \(H_j=H_j(x,y)\) is a homogenous polynomial of degree j. The classical center-focus problem already studied by Poincaré consists in distinguishing when the origin of \({\mathcal {X}}\) is either a center or a focus. In this paper we study the inverse center problem, i.e. for a given analytic function H of the previous form defined in a neighborhood of the origin, we determine the analytic or polynomial vector field \({\mathcal {X}}\) for which H is a first integral. Moreover, given an analytic function \(V=1+\sum _{j=1}^ {\infty } V_j\) in a neighborhood of the origin, where \(V_j\) is a homogenous polynomial of degree j, we determine the analytic or polynomial vector field \({\mathcal {X}}\) for which V is a Reeb inverse integrating factor. We study the particular case of centers which have a local analytic first integral of the form \( H=\dfrac{1}{2}(x^2+y^2)\,\left( 1+ \sum _{j=1}^{\infty } \Upsilon _j\right) , \) in a neighborhood of the origin, where \(\Upsilon _j\) is a homogenous polynomial of degree j for \(j\ge 1.\) These centers are called weak centers, they contain the uniform isochronous centers and the isochronous holomorphic centers, but they do not coincide with the class of isochronous centers. We have characterized the expression of an analytic or polynomial differential system having a weak center at the origin We extended to analytic or polynomial differential systems the weak conditions of a center given by Alwash and Lloyd for linear centers with homogeneous polynomial nonlinearities. Furthermore the centers satisfying these weak conditions are weak centers.


Center-focus problem Analytic planar differential system Liapunov’s constants Isochronous center Darboux’s first integral Weak condition for a center Weak center 

Mathematics Subject Classification

34C05 34C07 



The first author is partially supported by MINECO Grants MTM2016-77278-P and MTM2013-40998-P, and an AGAUR Grant Number 2017SGR-1617. The second author was partly supported by the Spanish Ministry of Education through Projects TIN2014-57364-C2-1-R, TSI2007-65406-C03-01 “AEGIS”.


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Copyright information

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Authors and Affiliations

  1. 1.Departament de MatemàtiquesUniversitat Autònoma de BarcelonaBellaterra, BarcelonaSpain
  2. 2.Departament d’Enginyeria Informàtica i MatemàtiquesUniversitat Rovira i VirgiliTarragonaSpain
  3. 3.Universitat Central de BarcelonaBarcelonaSpain

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