Commutant of positive invertible operator

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‎Let‎ ‎$ H‎^{N} $ be a ‎$ N $-dimensional Hilbert space, ‎$ S, T‎\in ‎L(H‎^{N})‎. $ Suppose that ‎‎$ T $ ‎is invertible and ‎positive ‎operator ‎$‎‎(<Tx,x>‎\geq0 ) ‎$‎ such that ‎$ ST=TS. $ I‎ ‎can't ‎prove ‎that‎ ‎$ ST‎^{r}‎=T‎^{r}S$‎, for ay real number ‎$ r‎\in‎ \mathbb {R}$‎. We know since ‎$ T $ ‎is ‎positive and invertible‎, then ‎$ T‎^{r}‎ $ ‎is ‎well-defined ‎for ‎any ‎real ‎number‎ ‎$ r‎\in‎ \mathbb {R}$‎.