function acosh( x ) {
	if ( x < 1.0 ) {
		return NaN;
	}
	if ( isnan( x ) || isinfinite( x ) ) {
		return x;
	}
	return ln( x + (sqrt( x+1.0 )*sqrt( x-1.0 )) );
}
Computes the hyperbolic arccosine of a number.

@param {number} x - input value
@returns {number} hyperbolic arccosine (in radians)

@example
var v = acosh( 1.0 );
// returns 0.0

@example
var v = acosh( 2.0 );
// returns ~1.317

@example
var v = acosh( NaN );
// returns NaN