Thursday, 17 December 2015

logic - Is the (first order theory) of Hilbert spaces categorical?

Suppose the axioms of a Hilbert space (i.e. vector space, inner product, completeness and separability) are formulated as a first order theory.




It can be shown that any infinite dimensional Hilbert space is isomorphic to l2 (the space of infinite sequences of complex numbers the sum of whose absolute squares converges). In other words, any model satisfying the (infinite dimensional) Hilbert space axioms is isomorphic to l2.




  1. Since all the models are isomorphic to l2 (and hence to each other), the Hilbert space theory is categorical.


  2. But by Lowenheim-Skolem theorem, any first order theory that has an infinite model of cardinality c, also has a model of any cardinality larger c.




So 2 seems to contradict 1, since all models of a categorical theory have to have the same cardinality (in order to be isomorphic).




How is this resolved?

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