Theorem. Consider a continuous map and suppose that the autonomous dynamical system has a semiflow . Let . If is nonempty, compact, convex and forward-invariant, then contains an equilibrium of the dynamical system, i.e. a zero of the map .
According to a reliable source, the above theorem is a standard result everyone uses in dynamical systems without proof. I propose a proof in “Equilibria Exist in Compact Convex Forward-Invariant Sets“. I am interested in comments on this proof, in references to this or other proofs in the literature, and in new/better proofs. Please contribute here or on MathOverflow at “Equilibria Exist in Compact Convex Forward-Invariant Sets“.
Hi Gilles, I also struggled to find references for this when writing up my thesis a few years ago. Strange when it’s such a fundamental result!
You might like to take a look at “A fixed point theorem for bounded dynamical systems” by Richeson and Wiseman in the Illinois Journal of Mathematics (issue 46(2) pp. 491-495), which I think contains a proof. The authors mention other proofs in an addendum to their paper, in issue 48(3) pp. 1079-1080 of the same journal.
Thank you, Pete. I will look for these two papers. A colleague recently told me about another paper that also has the result: “The Brouwer Fixed Point Theorem Applied to Rumour Transmission” by William Basener, Bernard P. Brooks, and David Ross; http://dx.doi.org/10.1016/j.aml.2006.02.007.