
In this interesting paper, the authors study the phenomenon of stability breakdown for nonautonomous ordinary differential equations whose time dependence is determined by a strictly ergodic flow. More precisely, an approach for a nonautonomous version of the classical transcritical bifurcation is provided. The authors work in a skew-product setting and, therefore, to consider systems of the form \[ y'=\omega(y),\quad x'=| \varepsilon| f(y,x,\varepsilon),\tag{*} \] on \(Y\times{\mathbb R}\). Here, \(Y\) is a compact subset of \({\mathbb R}^m\) invariant under the flow \(\{\tau_t\}\) induced by \(y'=\omega(y)\), \(f:{\mathbb R}^m\times{\mathbb R}\times I\to{\mathbb R}\), \(I\) denotes an open interval containing \(0\), is assumed to possess continuous partial derivatives w.r.t.~\(x\) up to order \(3\), and one has \(f(y,0,\varepsilon)\equiv 0\). Moreover, the hypotheses that the flow \((Y,{\mathbb R})\) is strictly ergodic holds throughout the paper. In order to investigate a transcritical bifurcation for \((\ast)\), i.e., its nonautonomous generalization, the authors write \[ f(y,x,\varepsilon)=a(y,\varepsilon)x+b(y,\varepsilon)x^2+p(y,\varepsilon,x) \] and assume \(\bar{a}(0)=0\), \(\bar{b}(0)0\), \(| x_0| \) and arbitrary initial values \(y_0\), the \(\omega\)-limit set of \((y_0,x_0)\) in \(Y\times{\mathbb R}\) contains at least one and at most two minimal subsets, and there are almost automorphic extensions of \((Y,\{\tau_t\})\) [cf. \textit{W.~Shen} and \textit{Y.~Yi}, Mem. Am. Math. Soc. 647 (1998; Zbl 0913.58051)]. Under further assumptions on \(a\), there exists a local attractor \(A_{\varepsilon}\subset Y\times{\mathbb R}\) such that \(A_{\varepsilon}\cap(Y\times\{0\})=\emptyset\), and also a statement on the Hausdorff distance between \(A_{\varepsilon}\) and \(Y\times\{0\}\) holds. These results are sharpened in the quasiperiodic case of a \(k\)-dimensional torus \(Y\) with angular components \((y_1,\ldots,y_k)\), \(\tau_t(y_1,\ldots,y_k)=(y_1+\omega_1t,\ldots,y_k+\omega_kt)\,mod\,1\) and rationally independent \(\omega=(\omega_1,\ldots, \omega_k)\). Then, writing \[ a(y,\varepsilon)=a_0(y)+ \varepsilon a_1(y)+\varepsilon^2 a_2(y,\varepsilon) \] and under the conditions \(a_0(0)\equiv 0\), \(\bar{a}_1<0\) for the mean value of \(a_1\), the local attractor \(A_{\varepsilon}\) tends to \(Y\times\{0\}\) in the Hausdorff distance as \(\varepsilon\searrow 0\). In effect, one can also apply these results inside the parameter space ``bubbles'', occurring in a bifurcation theory for ODEs with quasiperiodic coefficients [cf. \textit{H. W. Broer}, \textit{G. B. Huitema}, \textit{F.~Takens} and \textit{B. L. J. Braaksma}, Mem. Am. Math. Soc. 421 (1990; Zbl 0717.58043)]. These ``bubbles'' lead to a very limited information on the bifurcating objects and are caused by Diophantine conditions for \(\omega\), whereas the present paper assumes only that \(\omega\) is rationally independent or satisfies a Liouville-type condition. Using an integral manifold theorem, these results are applied to a nonlinearly forced ``inverted pendulum'' given by a second-order nonautonomous ODE.
Averaging method for ordinary differential equations, local attractor, Dynamical aspects of attractors and their bifurcations, Hausdorff distance, averaging method, Topological dynamics of nonautonomous systems, Periodic and quasi-periodic flows and diffeomorphisms, nonautonomous bifurcation, ergodic flow, ODEs with quasiperiodic, parameter intermittence, almost automorphic minimal set
Averaging method for ordinary differential equations, local attractor, Dynamical aspects of attractors and their bifurcations, Hausdorff distance, averaging method, Topological dynamics of nonautonomous systems, Periodic and quasi-periodic flows and diffeomorphisms, nonautonomous bifurcation, ergodic flow, ODEs with quasiperiodic, parameter intermittence, almost automorphic minimal set
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