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# rational()

Usage:
 ```rational(x, a, b) or rational(x, a) or rational(x,,b), x REAL or a structure with REAL components, a and b REAL vectors ```

Keywords: transformations
```rational(x,a,b) computes a rational function of the REAL vector, matrix,
or array x, with coefficients for the numerator and denominator
polynomials in REAL vectors a and b.  The result is REAL with the same
size and shape as x.

The rational function computed is
(a[1] + a[2]*x + a[3]*x^2+ ... )/(b[1] + b[2]*x + b[3]*x^2+ ... ) .

If a or b is omitted, it is construed to represent the constant 1.  For
example, rational(x,a), rational(x,a,), and rational(x,a,1) are
equivalent and compute a polynomial in x, and rational(x,,b), and
rational(x,1,b) are equivalent and compute the reciprocal of a
polynomial in x.

x can also be a structure, all of whose non-structure components are
REAL, in which case the result is a similar structure.

An important use of rational() is in writing macros to compute
mathematical functions that are not directly available in MacAnova but
which can be approximated by rational functions.  For example

Cmd> cumnor1 <- macro("@x <- \$1; @posx <- @x > 0
@posx - (@posx - .5)*rational(abs(@x),1,\\
vector(1,.196854,.115194,.000355,.019527))^4", dollar:T)

creates macro cumnor1() which uses an approximation due to Hastings to
cumpute cumulative normal probabilities.  For more elaborate use, see
macros i0() and i1() in file Math.mac distributed with MacAnova.

See topics macro() and 'macros'.
```

Gary Oehlert 2003-01-15