atan(x) or atan(y,x)

Arctangent, result in radians. This function can take one or two arguments. For one argument, the result is between -π/2 and π/2. For two arguments (y, x) the result is arctangent of y/x, ranging between -π and π, with the choice of quadrant taken from the signs of x and y.

For example:

$atan(0,-1) = π, $atan(1,-1) = 3π/4,

$atan(1,0) = π/2, $atan(1,1) = π/4, $atan(0,1) = 0, $atan(-1,1) = -π/4,

$atan(-1,0) = -π/2, $atan(-1,-1) = -3π/4, $atan(-.001,-1) = -3.14