function pdf( x, a, b ) {
	if (
		isnan( x ) ||
		isnan( a ) ||
		isnan( b ) ||
		a >= b
	) {
		return NaN;
	}
	if ( x < a || x > b ) {
		return 0.0;
	}
	return 1.0 / ( PI * sqrt( ( x-a ) * ( b-x ) ) );
}
Evaluates the probability density function (PDF) for an arcsine distribution with minimum support `a` and maximum support `b` at a value `x`.

@param {number} x - input value
@param {number} a - minimum support
@param {number} b - maximum support
@returns {number} evaluated PDF

@example
var y = pdf( 2.0, 0.0, 4.0 );
// returns ~0.159

@example
var y = pdf( 5.0, 0.0, 4.0 );
// returns 0.0

@example
var y = pdf( 0.25, 0.0, 1.0 );
// returns ~0.735

@example
var y = pdf( NaN, 0.0, 1.0 );
// returns NaN

@example
var y = pdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = pdf( 0.0, 0.0, NaN );
// returns NaN

@example
var y = pdf( 2.0, 3.0, 1.0 );
// returns NaN