underfull \hbox (badness 1342) in paragraph

This message of warning appears at the line 967, which says:

Las ecuaciones en \eqref{eq1.32} sugieren un m\'etodo para calcular las factorizaciones \QR reducidas. Dados $a^1,a^2,\dotsc$, podemos construir los vectores $\seq[q]{n}$ y los escalares $r_{ij}$ por un proceso de ortogonalizaci\'on sucesiva. Esta es una idea antigua, conocida como la \textit{ortogonalizaci\'on de Gram-Schimd}.

In the log file:

Underfull \hbox (badness 1342) in paragraph at lines 967--968
[]\OT1/cmr/m/n/12 Las ecua-ciones en [] su-gie-ren un m^^Setodo para cal-cu-lar las fac-to-ri-za-cio-nes
[]
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that just means that TeX couldn't find a very good place to break the line and so that line is stretched more than specified. 1342 isn't that bad though. Hard to say more without seeing the whole paragraph and language setuop. – David Carlisle Dec 10 '12 at 0:29
You should say in your preamble \usepackage[T1]{fontenc} or words with accented characters will not be hyphenated. Probably "ortogonalización" is the culprit. – egreg Dec 10 '12 at 0:30

Since you're writing in Spanish, which has many accented words, it's necessary to employ fonts in T1 encoding, that have glyphs for the accented characters. Otherwise TeX won't hyphenate words with accented letters past them.

\documentclass[a4paper,twocolumn]{article}
\usepackage[T1]{fontenc}
\usepackage[spanish]{babel}
\usepackage{amsmath,xspace}
\newcommand{\QR}{\textbf{QR}\xspace}
\newcommand{\seq}[2][x]{#1_1,#1_2,\dots,#1_{#2}}

\textwidth=.8\textwidth

\begin{document}

$$\label{eq1.32} a=b$$

Las ecuaciones en \eqref{eq1.32} sugieren un m\'etodo para calcular las
factorizaciones \QR reducidas. Dados $a^1,a^2,\dotsc$, podemos construir
los vectores $\seq[q]{n}$ y los escalares $r_{ij}$ por un proceso de
ortogonalizaci\'on sucesiva. Esta es una idea antigua, conocida como
la \textit{ortogonalizaci\'on de Gram-Schimd}.

\end{document}

If I remove the line \usepackage[T1]{fontenc}, then I get the warning

Underfull \hbox (badness 1147) in paragraph at lines 16--21
[]\OT1/cmr/m/n/10 Las ecua-cio-nes en [] su-gie-ren un

and the result is as follows (notice the bad spacing):

PS: It's "Gram-Schmidt"! I find it written in many different ways in my students' papers. :)

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Thanks, good response, but I dont want use fontenc package, I solved my problem by changing the wording ... regards – juanuni May 3 '15 at 18:10
@juanquispe You'll have several problems with hyphenation, if you don't use T1. – egreg May 3 '15 at 18:13
I'm carefull with wording :) ... is no major problem ... regards – juanuni May 3 '15 at 18:15