Existence of Invariant Measures for Delay Equations with Stochastic Negative Feedback

Abstract: We provide sufficient conditions for the existence of invariant probability measures for generic stochastic differential equations with finite time delay. Applications include the Mackey--Glass equations and Nicholson's blowflies equation, each perturbed by a (small) multiplicative noise term. Solutions to these stochastic negative feedback systems persist globally and all solutions are bounded above in probability. It turns out that the occurrence of finite time blowups and boundedness in probability of solutions and solution segments are closely related. A non-trivial invariant measure is shown to exist if and only if there is at least one initial condition for which the solution remains bounded away from zero in probability. The noise driving the dynamical system is allowed to be an integrable L\'evy process.