# How to write this equation in publication?

I have the equation in the twocolumn view

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\begin{document}

\begin{aligned} X(m) &= \frac{b-a}{N} \sum_{k=1}^{N} e^{-i2\pi (k-1)(m-1)/N} x(a + (b-a)\frac{k-1}{N}) \\ &= \frac{b-a}{N} \sum_{k=1}^{N} e^{-i2\pi (a + (b-a)\frac{k-1}{N})*\frac{m-1}{b-a}} \\ & x(a + (b-a)\frac{k-1}{N})e^{i2\pi a \frac{m-1}{b-a}} \\ & \xrightarrow{ N\to\infty } \int_{a}^{b} e^{-2\pi t \frac{m-1}{b-a}} x(t) dt\\ & e^{i2\pi a \frac{m-1}{b-a}} Qx(\frac{m-1}{b-a}). \end{aligned}
\end{document}


Output

How would you present this equation in a twocolumn view?

• Please don't just include snippets but a minimal code that can be typeset, from \documentclass{...} to \end{document}, removing the unnecessary code. – Alenanno Oct 18 '13 at 14:55
• You mean aligned and not align, don't you? – karlkoeller Oct 18 '13 at 14:59
• Are you saying that you don't like how it looks and you would want suggestions on how to have it look better? – karlkoeller Oct 18 '13 at 15:03
• Don't nest align and equation environments. Both enter math-mode on their own, see this related question. – Qrrbrbirlbel Oct 18 '13 at 15:03
• @Masi Use a_{k} instead of a+(b-a)\frac{k-1}{N}, to begin with. – egreg Oct 18 '13 at 15:36

You should be using split not align inside equation. Introducing new variables for large repeating expressions and shifting your indexing by one will reduce the size of the equation. Finally I introduce an eqbreak command to shift a split expression:

\documentclass[twocolumn]{article}
\usepackage{amsmath, amsfonts, amssymb, textcomp}

\newcommand{\eqbreak}[1][2]{\\&\hskip#1em}

\begin{document}

$$\begin{split} X(m+1) &= \frac{b-a}N \sum_{k=0}^{N-1} e^{-i2\pi km/N}\, x(a_k) \\ &= \frac{b-a}N \sum_{k=0}^{N-1} e^{-i2\pi a_km/(b-a)} \eqbreak[6] \times x(a_k)\,e^{i2\pi a m/(b-a)} \\ &\xrightarrow{N\to\infty} e^{i2\pi a m/(b-a)} \,Qx\Bigl(\frac m{b-a}\Bigr), \end{split}$$
where $$a_k = a + (b-a)k/N$$.
\end{document}

• As I just told to Marienplatz, I'm not sure that \times is the right symbol to use... – karlkoeller Oct 18 '13 at 15:47
• @karlkoeller An alternative would be \cdot, but it depends on the wider context. – Andrew Swann Oct 18 '13 at 15:49
• @Marienplatz There is no integral in the poster's example, nor is Qx explained. – Andrew Swann Oct 18 '13 at 15:52
• I would go with \cdot... Anyway, as a mathematician, I would say that this is the only answer that looks good, IMHO. +1. – karlkoeller Oct 18 '13 at 15:59
• @Masi I will comment on that question - you are missing alignment points. – Andrew Swann Aug 10 '16 at 12:47

I would propose you split the first two lines at a reasonably natural point. I would use explicit sizing instructions for the parentheses, and use \exp(...) expressions.

\documentclass[twocolumn]{article}
\usepackage{amsmath, amsfonts, amssymb, textcomp}
\begin{document}
\begin{align}
&X(m) \notag\\
&= \frac{b-a}{N} \sum_{k=1}^{N} \exp\bigl(-i2\pi (k-1)(m-1)/N\bigr) \notag\\
&\qquad  \times x\Bigl[a + (b-a)\frac{k-1}{N}\Bigr] \\
&= \frac{b-a}{N} \sum_{k=1}^{N} \exp\Bigl[-i2\pi \Bigl(a + (b-a)\frac{k-1}{N}\Bigr)
\frac{m-1}{b-a}\Bigr] \notag \\
\exp\Bigl(i2\pi a \frac{m-1}{b-a}\Bigr) \\
& \xrightarrow{ N\to\infty } \exp\Bigl(i2\pi a \frac{m-1}{b-a}\Bigr)
Qx\Bigl(\frac{m-1}{b-a}\Bigr)\,.
\end{align}
\end{document}


Just for fun!

\documentclass{article}
\usepackage[a4paper,margin=2cm,twocolumn]{geometry}
\usepackage{mathtools}
\begin{document}
$$\begin{split} X(m) &= \! \begin{multlined}[t] \frac{b-a}{N} \sum_{k=1}^{N} \Bigg[ x\left(a + \frac{k-1}{N}(b-a)\right)\\ \times e^{-\frac{2\pi i (k-1)(m-1)}{N}} \Bigg] \end{multlined}\\ &= \! \begin{multlined}[t] \frac{b-a}{N} \sum_{k=1}^{N}\Bigg[ x\left(a + \frac{k-1}{N}(b-a)\right) \\ \times e^{2\pi i a \frac{m-1}{b-a}}\\ \times e^{-2\pi i \left(a + \frac{k-1}{N}(b-a)\right)\times \frac{m-1}{b-a}}\Bigg] \end{multlined}\\ & \xrightarrow{ N\to\infty } \! \begin{multlined}[t] e^{2\pi i a \frac{m-1}{b-a}} \\ \times \int_{a}^{b} e^{2\pi i a \frac{m-1}{b-a}} Qx\left(\frac{m-1}{b-a}\right). \end{multlined} \end{split}$$
\end{document}


• I'm not sure that \times is the right symbol to use... – karlkoeller Oct 18 '13 at 15:45