#once #create message
.nf
.EQ
delim $$
.EN
Several of the examples we did earlier had Greek letters
and other mathematical symbols in them.
The way to get things like $pi$ and $sum$ and $int$ is
rather easy - you just spell out their names, like this:

$pi$ is pi

$sum$ is sum

$int$ is int

and so on.
The main thing you must always remember about these names
is that when they appear in an equation, they must be
separated from surrounding symbols by blanks or tildes,
OR THEY WILL NOT BE RECOGNIZED.

For practice, modify "Example" so the symbol $partial$
is replaced by $sum$ everywhere it appears.
Type "ready" when you're done.
.pl 1
#once #create Ref
.PP
The symbol
.EQ
sum
.EN
often appears in lines like
.EQ
{ sum x } over { sum y } = y over x
.EN
.pl 1
#
#once neqn Ref | nroff >X1  &
#once neqn message | nroff -T$term
#once #create Example
.PP
The symbol
.EQ
partial
.EN
often appears in lines like
.EQ
{ partial x } over { partial y } = y over x
.EN
.pl 1
#user
neqn Example | nroff >X2
#cmp X1 X2
#log
#next
2.1d 10