Equation alignment creating unwanted space

Question

I am defining some text in a align, but no matter how i place the & symbol to create the alignment i get a space between the first chars.

\begin{align*}
H_0:& \text{Something.}\\
H_1:& \text{Something.}\\
\end{align*}

Basically i dont want a space between H_0, but rather after the colon.

Answer 1

The problem you are running in to is that the : is treated as a relational operator and thus with white space around it. The white space after a relational operator disappears if followed by the &-command of the align environment. You will see this problem a lot on this page when seeing equation which are aligned to the right of their =-sign. In order to protect the white space, you would have to type H_0:{}&\text{} or H_0&:\text{} here.

However, it seems as if you do not want to have a relational operator here. There are three possible ways: The relational operator :, the "maps to" symbol \colon or the 'normal' text colon \text{:}. The following shows the differences:

\begin{align*}
&H_0:x\\
&H_0\colon x\\
&H_0\text{: }x
\end{align*} 

You can see the different spacings here. And you may have noticed that the text-colon needs to be followed by a white-space, as there is no space between normal symbols (not operators) in math-mode.

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In the following, I will show an MWE with two possible solutions of your desired alignment. I will use the two cases I believe you would like to have.

% arara: pdflatex

\documentclass{article}
\usepackage{mathtools}

\begin{document}    
\begin{align*}
    H_0\text{: } &\text{Something.}\\
    H_1\text{: } &\text{Something.}
\end{align*}
% or identically
%\begin{align*}
%   H_0&\text{: Something.}\\
%   H_1&\text{: Something.}
%\end{align*}   
\end{document}

\begin{align*}
    H_0\colon &\text{Something.}\\
    H_1\colon &\text{Something.}
\end{align*}
% or identically
%\begin{align*}
%    H_0&\colon \text{Something.}\\
%    H_1&\colon \text{Something.}
%\end{align*}