function ChiSquare() {
	var k;
	if ( !(this instanceof ChiSquare) ) {
		if ( arguments.length === 0 ) {
			return new ChiSquare();
		}
		return new ChiSquare( arguments[ 0 ] );
	}
	if ( arguments.length ) {
		k = arguments[ 0 ];
		if ( !isPositive( k ) ) {
			throw new TypeError( format( 'invalid argument. Rate parameter must be a positive number. Value: `%s`.', k ) );
		}
	} else {
		k = 1.0;
	}
	defineProperty( this, 'k', {
		'configurable': false,
		'enumerable': true,
		'get': function get() {
			return k;
		},
		'set': function set( value ) {
			if ( !isPositive( value ) ) {
				throw new TypeError( format( 'invalid assignment. Must be a positive number. Value: `%s`.', value ) );
			}
			k = value;
		}
	});
	return this;
}
Chi-squared distribution constructor.

@constructor
@param {PositiveNumber} [k=1.0] - degrees of freedom
@throws {TypeError} `k` must be a positive number
@returns {ChiSquare} distribution instance

@example
var chisquare = new ChiSquare( 1.0 );

var y = chisquare.cdf( 0.8 );
// returns ~0.629

var v = chisquare.mode;
// returns 0.0