Piecewise Interpretable Hilbert Spaces

We study Hilbert spaces $H$ interpreted, in an appropriate sense, in a first-order theory. Under a new finiteness hypothesis that we call {\em scatteredness} we prove that $H$ is a direct sum of {\em asymptotically free} components, where short-range interactions are controlled by algebraic closure and long-range interactions vanish. Examples include $L^2$-spaces relative to Macpherson-Steinhorn definable measures; $L^2$ spaces relative to the Haar measure of the absolute Galois groups; irreducible unitary representations of $p$-adic Lie groups; and unitary representations of the automorphism group of an $ω$-categorical theory. In the last case, our main result specialises to a theorem of Tsankov. New methods are required, making essential use of local stability theory in continuous logic.

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