### Superscripts and Subscripts

Subscripts and superscripts are obtained using the special
characters `_`

and `^`

respectively. Thus the
identity

is obtained by typing
\[ ds^2 = dx_1^2 + dx_2^2 + dx_3^2 - c^2 dt^2 \]

It can also be obtained by typing
\[ ds^2 = dx^2_1 + dx^2_2 + dx^2_3 - c^2 dt^2 \]

since, when a superscript is to appear above a subscript, it is
immaterial whether the superscript or subscript is the first to be
specified.
Where more than one character occurs in a superscript or subscript,
the characters involved should be enclosed in braces.
For example, the polynomial
is obtained by typing `$x^{17} - 1$`

.

One may not type expressions such as `$s^n^j$`

since this is
ambiguous and could be interpreted either as
or as
.
The first of these alternatives is
obtained by typing `$s^{n j}$`

, the second by typing
`$s^{n^j}$`

. A similar remark applies to subscripts.
Note that one can obtain in this way double superscripts (where a
superscript is placed on a superscript) and double subscripts.

It is sometimes necessary to obtain expressions in which the horizontal
ordering of the subscripts is significant. One can use an `empty group'
`{}`

to separate superscripts and subscripts that must follow
one another. For example, the identity

can be obtained by typing
\[ R_i{}^j{}_{kl} = g^{jm} R_{imkl}
= - g^{jm} R_{mikl} = - R^j{}_{ikl} \]