Command \pause help

                 \documentclass{beamer}
\usepackage[T1]{fontenc}
%-----

\usepackage{empheq, nccmath} % empheq package loads mathtools
\usepackage{amssymb, amsthm, mathrsfs}
\usepackage{graphicx}
\usepackage{color}

\begin{document}

\begin{frame}{title}
\pause
\begin{block}{}
We have

\begin{equation*}
r(\mathbf{T}):=\inf_{n\in\mathbb{N}^*}\left\|\displaystyle\sum_{\substack{|\alpha|=n,\\\alpha\in \mathbb{N}^d}}\frac{n!}{\alpha!}{\mathbf{T}^*}^{\alpha}\mathbf{T}^{\alpha}\right\|^{\frac{1}{2n}}\pause=\lim_{n\to+\infty}\left\|\displaystyle\sum_{\substack{|\alpha|=n,\\\alpha\in \mathbb{N}^d}}\frac{n!}{\alpha!}{\mathbf{T}^*}^{\alpha}\mathbf{T}^{\alpha}\right\|^{\frac{1}{2n}}.
\end{equation*}
Here for $\alpha = (\alpha_1,\cdots,\alpha_d) \in \mathbb{N}^d$, we used.
\end{block}
\end{frame}
\end{document}


First I want to obtain

Then,

 \begin{frame}{}
\begin{block}{}
We have
\begin{equation*}
r(\mathbf{T}):=\inf_{n\in\mathbb{N}^*}
\left\|
\displaystyle\sum_{\substack{|\alpha|=n,\\\alpha\in
\mathbb{N}^d}}
\frac{n!}{\alpha!}{\mathbf{T}^*}^{\alpha}\mathbf{T}^{\alpha}
\right\|^{\frac{1}{2n}}
\onslide<2>{
=\lim_{n\to+\infty}
\left\|
\displaystyle\sum_{\substack{|\alpha|=n,\\\alpha\in
\mathbb{N}^d}}
\frac{n!}{\alpha!}{\mathbf{T}^*}^{\alpha}\mathbf{T}^{\alpha}
\right\|^{\frac{1}{2n}}
}.
\end{equation*}
Here for $\alpha = (\alpha_1,\cdots,\alpha_d) \in \mathbb{N}^d$,
we used.
\end{block}
\end{frame}


PS While I was writing it, @leandrils proposed another solution, with \only command. If you are interested, there is a subtle difference between these solutions: \only does not typeset a hidden text while \onslide typesets and hides it, leaving the space on the slides where the text is not present.

In your example \only typesets the first equation centered, and on the second slide it moves it to the left. On the other hand, \onslide typesets the first equation with the space for the second one. Depending on your taste you might want either solution. Some people like each slide to have correctly centered equations, while other people dislike the surrounding text on slides "jumping" giving the way to newly inserted fragments.

• You might want to point out that this approach will not horizontally center the expression on the first slide (as shown in the sketch in the question). Also, the code is not compilable as some }s seem to be missing. – leandriis Jun 16 '19 at 17:13
• Done - thanks for noticing – Boris Jun 16 '19 at 17:24
• Thanks however juste if I add  \begin{frame}{title} \pause I have the same problem. Please see my edit. – Student Jun 16 '19 at 17:31
• \pause is a very simple command intended for simple sequential uncovering of text. If you need anything more complex, \only and \onslide must be used instead. – Boris Jun 16 '19 at 17:33
• If you want silde 1 with title only, slide 2 with the first equality, slide 3 with all text, use \onslide<2-3> and \onslide<3> correspondingly – Boris Jun 16 '19 at 17:37

Use \only<2> instead of \pause:

\documentclass{beamer}
\usepackage[T1]{fontenc}

\usepackage{empheq, nccmath} % empheq package loads mathtools
\usepackage{amssymb, amsthm, mathrsfs}
Here for $\alpha = (\alpha_1,\cdots,\alpha_d) \in \mathbb{N}^d$, we used.