text: /**
  * @license Apache-2.0
  *
  * Copyright (c) 2018 The Stdlib Authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *    http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
  
  'use strict';
  
  // MODULES //
  
  var isnan = require( '@stdlib/math-base-assert-is-nan' );
  var pow = require( '@stdlib/math-base-special-pow' );
  var SQRT_PI = require( '@stdlib/constants-float64-sqrt-pi' );
  var PI = require( '@stdlib/constants-float64-pi' );
  
  
  // VARIABLES //
  
  var SKEWNESS = 2.0 * SQRT_PI * ( PI-3.0 ) / pow( 4.0-PI, 1.5 );
  
  
  // MAIN //
  
  /**
  * Returns the skewness of a Rayleigh distribution.
  *
  * @param {NonNegativeNumber} sigma - scale parameter
  * @returns {NonNegativeNumber} skewness
  *
  * @example
  * var v = skewness( 9.0 );
  * // returns ~0.631
  *
  * @example
  * var v = skewness( 2.0 );
  * // returns ~0.631
  *
  * @example
  * var v = skewness( -0.2 );
  * // returns NaN
  *
  * @example
  * var v = skewness( NaN );
  * // returns NaN
  */
  function skewness( sigma ) {
  	if ( isnan( sigma ) || sigma < 0.0 ) {
  		return NaN;
  	}
  	return SKEWNESS;
  }
  
  
  // EXPORTS //
  
  module.exports = skewness;
  