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

5.4.2 Ordinary text within a formula
5.4. Additional elements 133
It is often necessary to include some normal text within a formula, for
example single words such as and, or, if, and so on. In this case one
must switch to LR mode (Section 4.7.1) while staying in math mode. This
is carried out with the command \mbox{normal text} given inside the
formula, together with horizontal spacing commands such as \quad or
\hspace. For example:
Xn = Xk if and only if Yn = Yk and Zn = Zk
\[ X_n = X_k \qquad\mbox{if and only if}\qquad
Y_n = Y_k \quad\mbox{and}\quad Z_n = Z_k \]
In order to set a longer piece of text beside a displayed formula, as
in some of the above examples, it is more appropriate to put both the
formula and the text in their own parboxes or minipages, placed side by
side with the proper vertical positioning.
On the other hand, if letters from text fonts are required as mathematical
symbols, they should be entered with the math alphabet commands:
\mathrm \mathtt \mathbf
\mathsf \mathit \mathcal
We have already met \mathcal in Section 5.3.2 on calligraphic letters. All
these commands function the same way: they set their argument in the
corresponding font.
B 0 (x) T i
j
$\mathbf{B}ˆ0(x)$ \quad $\mathsf{T}ˆi_j$
The command \mathnormal in Section 5.3.1 also belongs to this group.
The difference between it and \mathit is that it sets its argument in the
regular math italic font, while the latter uses the normal text italic. The
letters are the same, but the spacing is different.
$\mathnormal{differ} \ne \mathit{differ}$
dif f er = differ
All the math alphabet commands set their text in math mode, which
means that spaces are ignored as usual. This is not the case for text
placed in an \mbox.
5.4.3 Matrices and arrays
a11 a12 · · · a1n
.
.
. ..
.
an1 an2 · · · ann
Structures like the one at the left are the
basis for matrices, determinants, system of
equations, and so on. They will all be referred
to here as arrays.