# How can I write this formula?

I wrote this it is the first equation it gives me an error I don't understand why

\documentclass[12pt]{report}
\usepackage{enumerate}

\begin{document}
$$maxdd(t) = max\limits_{0\leq\tau\leqt} dd(\tau)$$
\end{document}

• The index of \max should end in \tau\leq t} (with a space). Unrelated: you shouldn't use the plain TeX construct  but the LaTeX one ., to have a better vertical spacing. – Bernard Oct 16 '18 at 9:57
• You should also remove an unnecessary accent ' at the end of your formula. – Sebastiano Oct 16 '18 at 10:43
• As for the "why", \limits are only used with operator names (either those from the package or the ones declared with \DeclareMathOperator*) – Weijun Zhou Apr 28 '19 at 2:17

You have several issues in your code:

1. Use $ and $ for mathematical mode instead .
2. The operater for max in written as \max, for example \max dd(t) or \max\limits.
3. After an operator like \leq you need a blank like \leq t instead your wrong \leqt

So with the complete code

\documentclass[12pt]{report}

\usepackage{enumerate}

\begin{document}
$\max dd(t) = \max\limits_{0\leq\tau\leq t} dd(\tau)$
\end{document}
`

you get the result: