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Here is a good reference for this

Michael Taylor: Pseudodifferential Operators, Princeton University PRess, 1981

In Chapter 12 it explains how to construct $f(A)$ when $A$ is elliptic selfadjoint of order $1$, $A\geq 0$, and $f$ is a smooth symbol of order $m$, i.e., $f$ is smooth and

$$ f^{(k)}(\lambda)= O(|\lambda|^{m-k}),\;\;\lambda\to\infty $$,

for any nonnegative integer $k$

show/hide this revision's text 2 clarified exposition

Here is a good reference for this

Michael Taylor: Pseudodifferential Operators, Princeton University PRess, 1981

In Chapter 12 it explains how to construct $f(A)$ when $A$ is selfadjoint of order $1$, $A\geq 0$, and $f$ is a smooth symbol of order $m$.m$, i.e., $f$ is smooth and

$$ f^{(k)}(\lambda)= O(|\lambda|^{m-k}),\;\;\lambda\to\infty $$,

for any nonnegative integer $k$

show/hide this revision's text 1

Here is a good reference for this

Michael Taylor: Pseudodifferential Operators, Princeton University PRess, 1981

In Chapter 12 it explains how to construct $f(A)$ when $A$ is selfadjoint of order $1$, $A\geq 0$, and $f$ is a symbol of order $m$.