# Eqnarray nested matrices produces alignment error

I wrote the following equation array.

\begin{eqnarray}
w_{0}     & < & 1 \\

w^{m}_{i}     & = & \left \{
\begin{matrix}
w_{0}           & i = 0 \\
\frac{1-w_{0}}{2L}  & i > 0
\end{matrix} \right. \\

w^{c}_{ij}    & = & \left \{
\begin{matrix}
0           & i \neq 0 \\
\frac{1-w_{0}}{2L}  & i = 0
\end{matrix} \right. \\

\zeta     & = & \sqrt{\frac{L}{1-w_{0}}}
\end{eqnarray}


This produces the following Error messages when compiled with pdflatex. I triple checked the alignment characters. Please Help me!

! Missing $inserted. <inserted text>$
l.286

?
! Missing } inserted.
<inserted text>
}
l.286

?
! Missing } inserted.
<inserted text>
}
l.286

?
! Missing \cr inserted.
<inserted text>
\cr
l.286

?
! Missing { inserted.
<inserted text>
{
l.286

?
! Missing $inserted. <inserted text>$
l.286

?
! Missing inserted. <to be read again> \par l.286 ? ! Missing  inserted. <inserted text>  l.287 w^ {m}_{i} & = & \left \{ ? ! Misplaced alignment tab character &. l.287 w^{m}_{i} & = & \left \{ ? ! Misplaced alignment tab character &. l.287 w^{m}_{i} & = & \left \{ ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.292 ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.292 ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.292 ? ! Misplaced \cr. \@@eqncr ...l \@eqnswtrue \global \@eqcnt \z@ \cr l.292 ? ! Misplaced \noalign. \@xeqncr [#1]->\ifnum 0={\fi }\@@eqncr \noalign {\penalty \@eqpen \vskip \j... l.292 ? ! Missing  inserted. <inserted text>  l.292 ? ! Missing } inserted. <inserted text> } l.292 ? ! Extra }, or forgotten \endgroup. \@xeqncr ... \@eqpen \vskip \jot \vskip #1\relax } l.292 ? ! Missing  inserted. <inserted text>  l.293 w^ {c}_{ij} & = & \left \{ ? ! Misplaced alignment tab character &. l.293 w^{c}_{ij} & = & \left \{ ? ! Misplaced alignment tab character &. l.293 w^{c}_{ij} & = & \left \{ ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.298 ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.298 ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.298 ? ! Misplaced \cr. \@@eqncr ...l \@eqnswtrue \global \@eqcnt \z@ \cr l.298 ? ! Misplaced \noalign. \@xeqncr [#1]->\ifnum 0={\fi }\@@eqncr \noalign {\penalty \@eqpen \vskip \j... l.298 ? ! Missing  inserted. <inserted text>  l.298 ? ! Missing } inserted. <inserted text> } l.298 ? ! Extra }, or forgotten \endgroup. \@xeqncr ... \@eqpen \vskip \jot \vskip #1\relax } l.298 ? ! Missing  inserted. <inserted text>  l.299 \zeta & = & \sqrt{\frac{L}{1-w_{0}}} ? ! Misplaced alignment tab character &. l.299 \zeta & = & \sqrt{\frac{L}{1-w_{0}}} ? ! Misplaced alignment tab character &. l.299 \zeta & = & \sqrt{\frac{L}{1-w_{0}}} ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.300 \end{eqnarray} ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.300 \end{eqnarray} ? ! Misplaced alignment tab character &. \reserved@a ->& & & l.300 \end{eqnarray} ? ! Misplaced \cr. \@@eqncr ...l \@eqnswtrue \global \@eqcnt \z@ \cr l.300 \end{eqnarray} ? ! Extra }, or forgotten . \H@endeqnarray ->\@@eqncr \egroup \global \advance \c@equation \m@ne\@ign...
l.300 \end{eqnarray}

?
! Missing $inserted. <inserted text>$
l.300 \end{eqnarray}

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Welcome to TeX.SX! I think you should have a look to eqnarray vs align. – Claudio Fiandrino May 9 '14 at 12:46

1. Never ever use blank lines in a math display environment
2. Never ever use eqnarray: eqnarray vs align

Example

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
w_{0}      & <  1 \\
w^{m}_{i}  & =
\begin{cases}
w_{0}              & i = 0 \\
\frac{1-w_{0}}{2L} & i > 0
\end{cases} \\
w^{c}_{ij} & =
\begin{cases}
0                  & i \neq 0 \\
\frac{1-w_{0}}{2L} & i = 0
\end{cases} \\
\zeta   & = \sqrt{\frac{L}{1-w_{0}}}
\end{align}

\end{document}


A possible refinement with mathtools (that automatically loads amsmath)

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{align}
w_{0}      & <  1 \\
w^{m}_{i}  & =
\begin{dcases}
w_{0}              & i = 0 \\
\frac{1-w_{0}}{2L} & i > 0
\end{dcases} \\
w^{c}_{ij} & =
\begin{dcases}
0                  & i \neq 0 \\
\frac{1-w_{0}}{2L} & i = 0
\end{dcases} \\
\zeta   & = \sqrt{\frac{L}{1-w_{0}}}
\end{align}
\end{document}


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