How can I write something above and below the signs?

Question

I have a relation called Likelihood Ratio Test which has a special format. As you see in the picture, there are two non-equality signs and also somethings above and below them. How can I write these symbols?

Answer 1

You can use a \Vectorstack, where the baselineskip of the stack may be set to the desired value. As Bernard points out, a \Vectorstack is actually a \Centerstack that is automatically processed in math mode.

\documentclass{article}
\usepackage{stackengine}
\begin{document}
\[
  \setstackgap{L}{.7\baselineskip}
  \Lambda(y) = \frac{p(y{\mid}H_1)}{p(y{\mid}H_0)}
  \Vectorstack{H_1 > < H_0}
  \lambda.
\]
\end{document}

Answer 2

I propose using \gtrless of amssymb, to begin with.

The “official” way would be

\underset{H_0}{\overset{H_1}{\gtrless}}

but the effect is the same with more efficient code:

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}

\newcommand{\LRT}[2]{%
  \mathrel{\mathop\gtrless\limits^{#1}_{#2}}%
}

\begin{document}

\begin{equation*}
\Lambda(y)=\frac{p(y\mid H_1)}{p(y\mid H_0)} \LRT{H_1}{H_0} \lambda
\end{equation*}

\end{document}

Answer 3

You can stack these using a regular array:

\documentclass{article}

\begin{document}

\[
  \Lambda(y) = \frac{p(y \mid H_1)}{p(y \mid H_0)}
  \begin{array}{c}
    H_1 \\ > \\ < \\ H_0
  \end{array}
  \lambda.
\]

\end{document}