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atan(x [, degrees:T or radians:T or cycles:T]), x REAL or a
  structure with REAL components; value in radians (default), cycles, or
  degrees as specified by option 'angles' or the optional keyword
atan(x,y [, degrees:T or radians:T or cycles:T]), y REAL or a structure
  with real components the same size and shape as x

Keywords: transformations
atan(x) transforms the elements of REAL vector, matrix, or array x to
inverse tangents (arctangents).  When x is a structure with components
x1,...,xm, atan(x) is a structure with components atan(x1),...,atan(xm).

When any element of x is MISSING, the corresponding element of atan(x)

atan(x,y) computes theta = arctan(x/y), with the result in the
appropriate quadrant, where x and y must be REAL vectors, matrices, or
arrays with the same dimensions.  Specifically, theta is chosen so that
sin(theta) has the same sign as x, cos(theta) has the same sign as y and
tan(theta) = x/y.

atan(x,y) is also defined when x and y are both structures with the same
number of components, say x is structure(x1,...,xm) and y is
structure(y1,..., ym) .  The result is what would be produced by

The units of the result of atan(x) and atan(x,y) are radians, degrees or
cycles as determined by the value of option 'angles'.  The default is
radians.  See subtopic 'options:"angles"'.

atan(x, radians:T), atan(x, degrees:T), atan(x, cycles:T), atan(x, y,
radians:T), atan(x, y, degrees:T) and atan(x, y, cycles:T) return values
in the specified units, overriding option 'angles'.

See topic 'transformations' for more information about atan(), including
its use with a CHARACTER argument.

See also topics 'structures', 'labels'.

Gary Oehlert 2003-01-15