Guide to LaTeX (4th Edition) (Tools and Techniques

138 Chapter 5. Mathematical Formulas
⎛
⎝
j≥0
k≥0
ajkz k
⎞
⎠ =
n≥0
z n
⎛
⎜
⎝
k0,k1,...≥0
k0+k1+···=0
a0k0
⎞
⎟
a1k1 . . . ⎟
⎠
\[ \prod_{j\ge0}\left( \sum_{k\ge0} a_{jk}zˆk \right) =
\sum_{n\ge0} zˆn \left(\sum_{k_0,k_1\ldots\ge0 \atop
k_0+k_1+\cdots=0} a_{0k_0} a_{1k_1}\ldots \right) \]
Similar structures can be generated with the L AT E X environments
\begin{array}{c} upper line \\ lower line \end{array} (atop)
\left(\begin{array}{c} upper \\ lower \end{array}\right) (choose)
The difference between these array structures and those of the T E X
commands is that the former are always printed in the size and style of
normal text formulas, whereas the latter will have varying sizes depending
on where they appear within the formula.
For comparison
∆p1p2···pn−k
p1p2···pn−k
∆ p1p2 · · · pn−k
p1p2 · · · pn−k
The index array is produced using \atop
The index array is produced using \array
The above TEX commands may also
be
employed
to
produce small
1 0 a b c
matrices within text formulas, such as 0 1 or l m n . Here the first
matrix was typed in with
${1\,0\choose0\,1}$
and the second with
$\left({a\atop l}{b\atop m}{c\atop n}\right)$
The syntax of these Plain T E X commands is radically different from that
! normally used by L AT E X. See page 189 for a way to correct this.
5.4.7 Multiline equations
A multiline equation is one that is developed over several lines in which
the relation symbols (for example, = or ≤) in each line are all vertically
aligned with each other. For this purpose, the environments
\begin{eqnarray} line 1\\ ... \\ line n end{eqnarray}
\begin{eqnarray*} line 1\\ ... \\ line n end{eqnarray*}
are used to set several lines of formulas or equations in displayed math
mode. The individual lines of the equation or formula are separated from
one another by \\. Each entry line has the form