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.SC Introduction
.PP
.UC EQN
is a
program for typesetting mathematics
on the Graphics Systems phototypesetters on
.UC UNIX
and
.UC GCOS .
The 
.UC EQN
language was designed to be easy to use
by people who know neither mathematics
nor typesetting.
Thus
.UC EQN
knows relatively little about mathematics.
In particular, mathematical symbols like
+, \(mi, \(mu, parentheses, and so on have no special meanings.
.UC EQN
is quite happy to set garbage (but it will look good).
.PP
.UC EQN
works as a preprocessor for the typesetter formatter,
.UC TROFF [1],
so the normal mode of operation is to prepare
a document with both mathematics and ordinary text
interspersed,
and let
.UC EQN
set the mathematics while
.UC TROFF
does the body of the text.
.PP
On
.UC UNIX ,
.UC EQN
will also produce mathematics on
.UC DASI 
and
.UC GSI
terminals and on
Model 37 teletypes.
The input is identical, but you have to use the programs
.UC NEQN 
and
.UC NROFF
instead of
.UC EQN 
and
.UC TROFF .
Of course, some things won't look as good
because terminals 
don't provide the variety of characters, sizes and fonts
that a typesetter does,
but the output is usually adequate for proofreading.
.PP
To use 
.UC EQN
on
.UC UNIX ,
.P1
eqn files | troff
.P2
.UC GCOS
use is discussed in section 26.
.SC Displayed Equations
.PP
To tell
.UC EQN
where a mathematical expression begins and ends,
we mark it with lines beginning
.UC .EQ
and
.UC .EN .
Thus
if you type the lines
.P1
^EQ
x=y+z
^EN
.P2
your output will look like
.EQ
x=y+z
.EN
The
.UC .EQ
and
.UC .EN
are copied through untouched;
they
are not otherwise processed
by
.UC EQN .
This means that you have to take care
of things like centering, numbering, and so on
yourself.
The most common way is to use the
.UC TROFF
and
.UC NROFF
macro package package `\(mims'
developed by M. E. Lesk[3],
which allows you to center, indent, left-justify and number equations.
.PP
With the `\(mims' package,
equations are centered by default.
To left-justify an equation, use
.UC \&.EQ\ L
instead of
.UC .EQ .
To indent it, use
.UC .EQ\ I .
Any of these can be followed by an arbitrary `equation number'
which will be placed at the right margin.
For example, the input
.P1
^EQ I (3.1a)
x = f(y/2) + y/2
^EN
.P2
produces the output
.EQ I (3.1a)
x = f(y/2) + y/2
.EN
.PP
There is also a shorthand notation so
in-line expressions
like
$pi sub i sup 2$
can be entered without
.UC .EQ
and
.UC .EN .
We will talk about it in section 19.
.SC Input spaces
.PP
Spaces and newlines within an expression are thrown away by
.UC EQN .
(Normal text is left absolutely alone.)
Thus
between
.UC .EQ
and
.UC .EN ,
.P1
x=y+z
.P2
and
.P1
x = y + z
.P2
and
.P1
x   =   y   
   + z
.P2
and so on
all produce the same
output
.EQ
x=y+z
.EN
You should use spaces and newlines freely to make your input equations
readable and easy to edit.
In particular, very long lines are a bad idea,
since they are often hard to fix if you make a mistake.
.SC Output spaces
.PP
To force extra spaces into the 
.ul
output,
use a tilde ``\|~\|''
for each space you want:
.P1
x~=~y~+~z
.P2
gives
.EQ
x~=~y~+~z
.EN
You can also use a circumflex ``^'', 
which gives a space half the width of a tilde.
It is mainly useful for fine-tuning.
Tabs may also be used to position pieces
of an expression,
but the tab stops must be set by 
.UC TROFF
commands.
.SC "Symbols, Special Names, Greek"
.PP
.UC EQN
knows some mathematical symbols,
some mathematical names, and the Greek alphabet.
For example,
.P1
x=2 pi int sin ( omega t)dt
.P2
produces
.EQ
x = 2 pi int sin ( omega t)dt
.EN
Here the spaces in the input are
.B
necessary
.R
to tell
.UC EQN
that
.ul
int,
.ul
pi,
.ul
sin
and
.ul
omega
are separate entities that should get special treatment.
The
.ul
sin,
digit 2, and parentheses are set in roman type instead of italic;
.ul
pi
and
.ul
omega
are made Greek;
and
.ul
int
becomes the integral sign.
.PP
When in doubt, leave spaces around separate parts of the input.
A
.ul
very
common error is to type
.ul
f(pi)
without leaving spaces on both sides of the
.ul
pi.
As a result,
.UC EQN
does not recognize
.ul
pi
as a special word, and it appears as
$f(pi)$
instead of
$f( pi )$.
.PP
A complete list of
.UC EQN
names appears in section 23.
Knowledgeable users can also use
.UC TROFF
four-character names
for anything 
.UC EQN
doesn't know about,
like
.ul
\\(bs
for the Bell System sign \(bs.
.SC "Spaces, Again"
.PP
The only way
.UC EQN
can deduce that some sequence
of letters might be special
is if that sequence is separated from the letters
on either side of it.
This can be done by surrounding a special word by ordinary spaces
(or tabs or newlines),
as we did in the previous section.
.PP
.tr ~~
You can also make special words stand out by surrounding them
with tildes or circumflexes:
.P1
x~=~2~pi~int~sin~(~omega~t~)~dt
.P2
is much the same as the last example,
except that the tildes
not only
separate the magic words
like
.ul
sin,
.ul
omega,
and so on,
but also add extra spaces,
one space per tilde:
.EQ
x~=~2~pi~int~sin~(~omega~t~)~dt
.EN
.PP
Special words can also be separated by braces { }
and double quotes "...",
which have special meanings that we will
see soon.
.tr ~
.SC "Subscripts and Superscripts"
.PP
Subscripts and superscripts are
obtained with the words
.ul
sub
and
.ul
sup.
.P1
x sup 2 + y sub k
.P2
gives
.EQ
x sup 2 + y sub k
.EN
.UC EQN
takes care of all the size changes and vertical motions
needed to make the output look right.
The words
.ul
sub
and
.ul
sup
must be surrounded by spaces;
.ul
x sub2
will give you
$x sub2$ instead of $x sub 2$.
Furthermore, don't forget to leave a space
(or a tilde, etc.)
to mark the end of a subscript or superscript.
A common error is to say
something like
.P1
y = (x sup 2)+1
.P2
which causes
.EQ
y = (x sup 2)+1
.EN
instead of
the intended
.EQ
y = (x sup 2 )+1
.EN
.PP
Subscripted subscripts and superscripted superscripts
also work:
.P1
x sub i sub 1
.P2
is
.EQ
x sub i sub 1
.EN
A subscript and superscript on the same thing
are printed one above the other
if the subscript comes
.ul
first:
.P1
x sub i sup 2
.P2
is
.EQ
x sub i sup 2
.EN
.PP
Other than this special case,
.ul
sub
and
.ul
sup
group to the right, so
.ul
x\ sup\ y\ sub\ z
means
$x sup {y sub z}$, not ${x sup y} sub z$.
.SC "Braces for Grouping"
.PP
Normally, the end of a subscript or superscript is marked
simply by a blank (or tab or tilde, etc.)
What if the subscript or superscript is something that has to be typed
with blanks in it?
In that case, you can use the braces
{ and } to mark the
beginning and end of the subscript or superscript:
.P1
e sup {i omega t}
.P2
is
.\" This equation got lost in the V7 Volume 2 manual!
.EQ
e sup {i omega t}
.EN
.sp
Rule:  Braces can
.ul
always
be used to force 
.UC EQN
to treat something as a unit,
or just to make your intent perfectly clear.
Thus:
.P1
x sub {i sub 1} sup 2
.P2
is
.EQ
x sub {i sub 1} sup 2
.EN
with braces, but
.P1
x sub i sub 1 sup 2
.P2
is
.EQ
x sub i sub 1 sup 2
.EN
which is rather different.
.PP
Braces can occur within braces if necessary:
.P1
e sup {i pi sup {rho +1}}
.P2
is
.EQ
e sup {i pi sup {rho +1}}
.EN
The general rule is that anywhere you could use some single
thing like
.ul
x,
you can use an arbitrarily complicated thing if you enclose
it in braces.
.UC EQN
will look after all the details of positioning it and making
it the right size.
.PP
In all cases, make sure you have the
right number of braces.
Leaving one out or adding an extra will cause 
.UC EQN
to complain bitterly.
.PP
Occasionally you will have to
print braces.
To do this,
enclose them in double quotes,
like "{".
Quoting is discussed in more detail in section 14.
.SC Fractions
.PP
To make a fraction,
use the word
.ul
over:
.P1
a+b over 2c =1
.P2
gives
.EQ
a+b over 2c =1
.EN
The line is made the right length and positioned automatically.
Braces can be used to make clear what goes over what:
.P1
{alpha + beta} over {sin (x)}
.P2
is
.EQ
{alpha + beta} over {sin (x)}
.EN
What happens when there is both an
.ul
over
and a
.ul
sup
in the same expression?
In such an apparently ambiguous case,
.UC EQN
does the
.ul
sup
before the
.ul
over,
so
.P1
\(mib sup 2 over pi
.P2
is
$-b sup 2 over pi$
instead of
$-b sup {2 over pi}$
The rules
which decide which operation is done first in cases like this
are summarized in section 23.
When in doubt, however,
.ul
use braces
to make clear what goes with what.
.SC "Square Roots"
.PP
To draw a square root, use
.ul
sqrt:
.P1 2
sqrt a+b + 1 over sqrt {ax sup 2 +bx+c}
.P2
is
.EQ
sqrt a+b + 1 over sqrt {ax sup 2 +bx+c}
.EN
.KS
Warning _ square roots of tall quantities look lousy,
because a root-sign 
big enough to cover the quantity is
too dark and heavy:
.P1
sqrt {a sup 2 over b sub 2}
.P2
is
.EQ
sqrt{a sup 2 over b sub 2}
.EN
.KE
Big square roots are generally better written as something
to the power \(12:
.EQ
(a sup 2 /b sub 2 ) sup half
.EN
which is
.P1
(a sup 2 /b sub 2 ) sup half
.P2
.SC "Summation, Integral, Etc."
.PP
Summations, integrals, and similar constructions
are easy:
.P1
sum from i=0 to {i= inf} x sup i
.P2
produces
.EQ
sum from i=0 to {i= inf} x sup i
.EN
Notice that we used
braces to indicate where the upper
part
$i= inf$
begins and ends.
No braces were necessary for the lower part $i=0$,
because it contained no blanks.
The braces will never hurt,
and if the 
.ul
from
and
.ul
to
parts contain any blanks, you must use braces around them.
.PP
The
.ul
from
and
.ul
to
parts are both optional,
but if both are used,
they have to occur in that order.
.PP
Other useful characters can replace the
.ul
sum
in our example:
.P1
int   prod   union   inter
.P2
become, respectively,
.EQ
int ~~~~~~ prod ~~~~~~ union ~~~~~~ inter
.EN
Since the thing before the 
.ul
from
can be anything,
even something in braces,
.ul
from-to
can often be used in unexpected ways:
.P1
lim from {n \(mi> inf} x sub n =0
.P2
is
.\" Correct missing space before "->" for GNU eqn.  GBR, 2022.
.EQ
lim from {n -> inf} x sub n =0
.EN