Fra\"iss\'e and Ramsey properties of Fr\'echet spaces

Abstract

We develop the theory of Fra\"iss\'e limits for classes of finite-dimensional multi-seminormed spaces, which are defined to be vector spaces equipped with a finite sequence of seminorms. We define a notion of a Fra\"iss\'e Fr\'echet space and we use the Fra\"iss\'e correspondence in this setting to obtain many examples of such spaces. This allows us to give a Fra\"iss\'e-theoretic construction of $(\mathbb{G}^\omega, (\|\cdot\|_n)_{n<\omega})$, the separable Fr\'echet space of almost universal disposition for the class of all finite-dimensional Fr\'echet spaces with an infinite sequence of seminorms. We then identify and prove an approximate Ramsey property for various classes of finite-dimensional multi-seminormed spaces using known approximate Ramsey properties of normed spaces. A version of the Kechris-Pestov-Todor\v{c}evi\'c correspondence for approximately ultrahomogeneous Fr\'echet spaces is also established and is used to obtain new examples of extremely amenable groups. In particular, we show that the group of surjective linear seminorm-preserving isometries of $(\mathbb{G}^\omega, (\|\cdot\|_n)_{n<\omega})$ is extremely amenable.