The OpenD Programming Language

atan

Calculates the arc tangent of x, returning a value ranging from -π/2 to π/2.

Special Values
x	atan(x)	invalid?
±0.0	±0.0	no
±∞	NaN	yes

real atan(real x)
double atan(double x)
float atan(float x)
pragma(inline, true) @safe pure nothrow @nogc
float
atan
(
float x
)

Examples

import std.math.operations : isClose;
import std.math.traits : isIdentical;
import std.math.constants : PI;
import std.math.algebraic : sqrt;

assert(isIdentical(atan(0.0), 0.0));
assert(atan(sqrt(3.0)).isClose(PI / 3));

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Source

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std math trigonometry functions