Sry this the second question from the following article, I am asking in this week.

At page 6 (126), 3th line, of the following article.

[THE HEAT EQUATION WITH A SINGULAR POTENTIAL][1]

the authors say by positivity preserving property of the semigroup $\{e^{t \Delta}\}$, we have:

$$ e^{\delta (\Delta+ V_n)}v_0 = \lim_{m \to \infty} (e^{\delta \Delta/m } e^{(\delta/m) V_n})^m v_0 \leq e^{\delta \lambda} e^{\delta \Delta} v_0$$

Here $\delta >0$, $V_n$ is a non-negative bounded function, $\| V_n\|_{\infty}\leq \lambda$ and $v_0$ is an initial value.

I can't understand this inequality. Why this limiting procedure is necessary and where the positivity preserving property is used?

  [1]: https://www.ams.org/journals/tran/1984-284-01/S0002-9947-1984-0742415-3/S0002-9947-1984-0742415-3.pdf