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

@constructor
@param {PositiveNumber} [sigma=1.0] - scale parameter
@throws {TypeError} `sigma` must be a positive number
@returns {Rayleigh} distribution instance

@example
var rayleigh = new Rayleigh( 1.0 );

var y = rayleigh.cdf( 0.8 );
// returns ~0.274

var v = rayleigh.mode;
// returns 1.0