Telescopic Relative Entropy--II Triangle inequalities

In previous work (see arxiv:1102.3040), we have defined the telescopic relative entropy (TRE), which is a regularisation of the quantum relative entropy $S(ρ||σ)=\traceρ(\logρ-\logσ)$, by replacing the second argument $σ$ by a convex combination of the first and the second argument, $τ=aρ+(1-a)σ$ and dividing the result by $-\log a$. We also explored some basic properties of the TRE. In this follow-up paper we state and prove two upper bounds on the variation of the TRE when either the first or the second argument changes. These bounds are close in spirit to a triangle inequality. For the ordinary relative entropy no such bounds are possible due to the fact that the variation could be infinite.

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