How to remove (some of) the vertical space between a centered formula and "Theorem"

Question

Some of my theorems contain only a centered formula. In these cases, the vertical space before the entered formula is too large sometimes.

For example:

What would you suggest to do in this case?

(I'm trying to remember a command that "kills" the additional vertical space created after a paragraph)

Here is a MWE.

\documentclass{article}
\usepackage{amsthm}
\newtheorem{theo}{Theorem}
\begin{document}
\begin{theo}
$1+1=2$
\end{theo}
\begin{theo}
[1+1=2]
\end{theo}
\end{document}

Answer 1

I'd not use centering here. Anyway, you can do it.

1. remove the skip above the display
2. move it up one line

Here's with centering and without it.

My preference would be to add explanatory text: what are f and g? Stating the assumptions is very important, in my opinion.

\documentclass{article}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{lipsum}% for mock text
\newtheorem{theo}{Theorem}
\newcommand{\asym}[1][]{\underset{#1}{\sim}}
\newcommand{\smallo}[1][]{\underset{#1}{o}}
\newcommand{\notheoremtext}{%
  \setlength{\abovedisplayskip}{0pt}%
  \mbox{}\vspace{-\baselineskip}%
}
\begin{document}
\lipsum[1][1-4]
\begin{theo}\notheoremtext
\begin{alignat}{3}
f&\asym[\alpha]g
 &\quad\Longleftrightarrow\quad & f&=g+\smallo\alpha \
&&\quad\Longleftrightarrow\quad & f&=g+\smallo\alpha
\end{alignat}
\end{theo}
\lipsum[1][1-4]
\begin{theo}
$\begin{alignedat}[t]{3}
f&\asym[\alpha]g
 &\quad\Longleftrightarrow\quad & f&=g+\smallo\alpha \
&&\quad\Longleftrightarrow\quad & f&=g+\smallo\alpha
\end{alignedat}$
\end{theo}
\lipsum[1][1-4]
\end{document}