.BG
.FN acos
.FN asin
.FN atan
.TL
Inverse Trigonometric Functions
.CS
acos(x)
asin(x)
atan(x)
atan(x, y)
.AG x
numeric or complex.  Missing values (`NA's) are allowed.
.AG y
numeric, same length as `x'.  Missing values (`NA's) are allowed.
.RT
data transformed by the specified inverse trigonometric function,
with attributes preserved.
.PP
When `atan' is called with two arguments, both arguments should be numeric.
The return value satisfies `cos(atan(x,y))==y'
and `sin(atan(x,y))==x', implying that, for example, `atan(-1,1)' is
`-pi/4', not `3*pi/4'.
This form should be used if `y' is near zero.
Notice that there is a potential confusion in the naming of the
arguments in this form of `atan', as it usual to think of `cos'
being associated with the `x' coordinate and `sin' with the `y'
coordinate.
Thus, for example, `atan(1-1i)' is the same as `atan(-1,1)'
and not `atan(1,-1)'.
.PP
For numeric arguments, the domain of `acos' and `asin' is
the interval [\-1,1], and the range is `0 <= acos(x) <= pi'
and `\-pi/2 <= asin(x) <= pi/2'.
The domain of `atan' is unrestricted and
the range is `\-pi/2 < atan(x) < pi/2' or `\-pi < atan(x,y) <= pi'.
For values of the arguments outside of the appropriate domains,
.Co NA
is returned and a warning is given.
.Tr
For further information on domains and branch cuts in the case
of complex arguments, see section 5.1.5.
.En
.SA
`Arg', `cos', `sin', and `tan'.
.KW math
.WR
