Non-commutative Lebesgue decomposition of non-commutative measures

Abstract: A positive non-commutative (NC) measure is a positive linear functional on the free disk operator system which is generated by a $d$-tuple of non-commuting isometries. By introducing the hybrid forms, their Cauchy transforms, and techniques from NC reproducing kernel Hilbert spaces (RKHS), we construct a natural Lebesgue decomposition for any positive NC measure against any other such measure. Our work extends the Jury-Martin decomposition, which originally decomposes positive NC measures against the standard NC Lebesgue measure. In fact, we give a more generalized definition of absolute continuity and singularity, which reduces to their definition when the splitting measure is the standard NC Lebesgue measure. This generalized definition makes it possible to extend Jury-Martin theory for any splitting NC measure, and it recovers their decomposition when the splitting NC measure is the Lebesgue one. Our work implies a Lebesgue decomposition for representations of the Cuntz-Toeplitz C*-algebra. Furthermore, our RKHS method gives a new proof of the classical Lebesgue decomposition when applied to the classical one dimensional setting, i.e., $d=1$.