The arc-tangent of two arguments `atan2(y, x)`

returns the angle
between the x-axis and the vector from the origin to \((x, y)\),
i.e., for positive arguments `atan2(y, x) == atan(y/x)`

.

Angles are in radians, not degrees, for the standard versions (i.e., a
right angle is \(\pi/2\)), and in ‘half-rotations’ for
`cospi`

etc.

`cospi(x)`

, `sinpi(x)`

, and `tanpi(x)`

are accurate
for `x`

values which are multiples of a half.

All except `atan2`

are internal generic primitive
functions: methods can be defined for them individually or via the
`Math`

group generic.

These are all wrappers to system calls of the same name (with prefix
`c`

for complex arguments) where available. (`cospi`

,
`sinpi`

, and `tanpi`

are part of a C11 extension
and provided by e.g.macOS and Solaris: where not yet
available call to `cos`

*etc* are used, with special cases
for multiples of a half.)