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I want to write the shown mathematical equation in LaTeX but I am getting an error. Here this is not eta but it is n. The code which I am writing is

\begin{equation}
\Delta {E} +  \frac{\omega^2 n$_{i}$^2}{c^2} E = 0
\end{equation}

enter image description here

3
  • 4
    What are the dollar signs for? You cannot use them within the equation environment. Without them, it works.
    – Chris
    Feb 4, 2014 at 12:08
  • \omega^{2} n_{i}^{2} will do.
    – egreg
    Feb 4, 2014 at 12:17
  • 1
    Moreover, it's not n but \eta.
    – Manuel
    Feb 4, 2014 at 12:25

1 Answer 1

9

Your code incorrectly contains dollar signs, which are not allowed directly in a math display:

Sample output

\documentclass{article}

\begin{document}

Suppose \( E \) satisfies the following differential equation
\begin{equation}
  \Delta E + \frac{\omega^2 \eta_i^2}{c^2} E = 0.
\end{equation}

\end{document}

The curly brackets {...} around the first E are unnecessary. On the other hand you might consider writing brackets around super- and subscripts:

\omega^{2} \eta_{i}^{2}

so you don't get surprised when you have more complicated material in these positions.

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