A new infinite game in Banach spaces with applications

TL;DR

This paper introduces a novel infinite two-player game in Banach spaces, analyzing strategies and embeddings related to sequence domination, with implications for universal space constructions.

Contribution

It develops a new game-theoretic framework in Banach space theory to study embeddings and sequence domination, extending understanding of reflexive spaces and FDDs.

Findings

Player S can force P's sequences to be dominated by fixed sequences.

Reflexive spaces where S wins both games embed into spaces with FDDs satisfying specific estimates.

Universal space consequences are derived from the game strategies.

Abstract

We consider the following two-player game played on a separable, infinite-dimensional Banach space X. Player S chooses a positive integer k_1 and a finite-codimensional subspace X_1 of X. Then player P chooses x_1 in the unit sphere of X_1. Moves alternate thusly, forever. We study this game in the following setting. Certain normalized, 1-unconditional sequences (u_i) and (v_i) are fixed so that S has a winning strategy to force P to select x_i's so that if the moves are (k_1,X_1,x_1,k_2,X_2,x_2,...), then (x_i) is dominated by (u_{k_i}) and/or (x_i) dominates (v_{k_i}). In particular, we show that for suitable (u_i) and (v_i) if X is reflexive and S can win both of the games above, then X embeds into a reflexive space Z with an FDD which also satisfies analogous block upper (u_i) and lower (v_i) estimates. Certain universal space consequences ensue.