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We consider canonical systems and investigate the Szeg\H{o} class, which isdefined via the finiteness of the associated entropy functional. Noting thatthe canonical system may be studied in a variety of gauges, we choose to workin the Arov gauge, in which we prove that the entropy integral is equal to anintegral involving the coefficients of the canonical system. This sum ruleprovides a spectral theory gem in the sense proposed by Barry Simon.

Szeg\H{o}'s Theorem for Canonical Systems: the Arov Gauge and a Sum Rule