delim $$

eqn – typeset mathematics

eqn [ option ... ] [ file ... ]

Eqn is a troff(1) preprocessor for typesetting mathematics on a typesetter. Usage is almost always
eqn file ... | troff
If no files are specified, eqn reads from the standard input. Eqn prepares output for the typesetter named in the −Tdest option (default −Tutf; see troff(1)). When run with other preprocessor filters, eqn usually comes last.
A line beginning with .EQ marks the start of an equation; the end of an equation is marked by a line beginning with .EN. Neither of these lines is altered, so they may be defined in macro packages to get centering, numbering, etc. It is also possible to set two characters as ‘delimiters’; text between delimiters is also eqn input. Delimiters may be set to characters x and y with the option −dxy or (more commonly) with delim xy between .EQ and .EN. Left and right delimiters may be identical. (They are customarily taken to be $font L "$$" )$. Delimiters are turned off by delim off. All text that is neither between delimiters nor between .EQ and .EN is passed through untouched.
Tokens within eqn are separated by spaces, tabs, newlines, braces, double quotes, tildes or circumflexes. Braces {} are used for grouping; generally speaking, anywhere a single character like x could appear, a complicated construction enclosed in braces may be used instead. Tilde ~ represents a full space in the output, circumflex ^ half as much.
Subscripts and superscripts are produced with the keywords sub and sup. Thus x sub i makes $x sub i$, a sub i sup 2 produces $a sub i sup 2$, and e sup {x sup 2 + y sup 2} gives $e sup {x sup 2 + y sup 2}$.
Over makes fractions: a over b yields $a over b$.
Sqrt produces square roots: 1 over sqrt {ax sup 2 +bx+c} results in $1 over sqrt {ax sup 2 +bx+c}$ .
The keywords from and to introduce lower and upper limits on arbitrary things: $lim from {n -> inf} sum from 0 to n x sub i$ is made with lim from {n −> inf} sum from 0 to n x sub i.
Left and right brackets, braces, etc., of the right height are made with left and right: left [ x sup 2 + y sup 2 over alpha right ] ~=~1 produces $left [ x sup 2 + y sup 2 over alpha right ] ~=~1$. The right clause is optional. Legal characters after left and right are braces, brackets, bars, c and f for ceiling and floor, and "" for nothing at all (useful for a right-side-only bracket).
Vertical piles of things are made with pile, lpile, cpile, and rpile: pile {a above b above c} produces $pile {a above b above c}$. There can be an arbitrary number of elements in a pile. lpile left-justifies, pile and cpile center, with different vertical spacing, and rpile right justifies.
Matrices are made with matrix: matrix { lcol { x sub i above y sub 2 } ccol { 1 above 2 } } produces $matrix { lcol { x sub i above y sub 2 } ccol { 1 above 2 } }$. In addition, there is rcol for a right-justified column.
Diacritical marks are made with prime, dot, dotdot, hat, tilde, bar, under, vec, dyad, and under: x sub 0 sup prime = f(t) bar + g(t) under is $x sub 0 sup prime = f(t) bar + g(t) under$, and x vec = y dyad is $x vec = y dyad$.
Sizes and fonts can be changed with prefix operators size n, size ±n, fat, roman, italic, bold, or font n. Size and fonts can be changed globally in a document by gsize n and gfont n, or by the command-line arguments −sn and −fn.
Normally subscripts and superscripts are reduced by 3 point sizes from the previous size; this may be changed by the command-line argument −pn.
Successive display arguments can be lined up. Place mark before the desired lineup point in the first equation; place lineup at the place that is to line up vertically in subsequent equations.
Shorthands may be defined or existing keywords redefined with define: define thing % replacement % defines a new token called thing which will be replaced by replacement whenever it appears thereafter. The % may be any character that does not occur in replacement.
Keywords like sum ( sum ), int ( int ), inf ( inf ), and shorthands like >= (>=), −> (->), and != ( != ) are recognized. Greek letters are spelled out in the desired case, as in alpha or GAMMA. Mathematical words like sin, cos, log are made Roman automatically. Troff(1) four-character escapes like \(lh (<=) can be used anywhere. Strings enclosed in double quotes " " are passed through untouched; this permits keywords to be entered as text, and can be used to communicate with troff when all else fails.

/sys/lib/troff/font/devutf   font descriptions for PostScript


troff(1), tbl(1)
J. F. Ossanna and B. W. Kernighan, “Troff User’s Manual”.
B. W. Kernighan and L. L. Cherry, “Typesetting Mathematics--User’s Guide”, Unix Research System Programmer’s Manual, Tenth Edition, Volume 2.

To embolden digits, parens, etc., it is necessary to quote them, as in bold "12.3". delim off

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