Singular Bifurcations : a Regularization Theory

Several nonlinear and nonequilibrium driven as well as active systems (e.g. microswimmers) show bifurcations from one state to another (for example a transition from a non motile to motile state for microswimmers) when some control parameter reaches a critical value. Bifurcation analysis relies either on a regular perturbative expansion close to the critical point, or on a direct numerical simulation. While many systems exhibit a regular bifurcation such as a pitchfork one, other systems undergo a singular bifurcation not falling in the classical nomenclature, in that the bifurcation normal form is not analytic. We present a swimmer model which offers an exact solution showing a singular normal form, and serves as a guide for the general theory. We provide an adequate general regularization theory that allows us to handle properly the limit of singular bifurcations, and provide several explicit examples of normal forms of singular bifurcations. This study fills a longstanding gap in bifurcations theory.