function factory( k, lambda ) {
	if (
		!isNonNegativeInteger( k ) ||
		isnan( lambda ) ||
		lambda < 0.0
	) {
		return constantFunction( NaN );
	}
	return mgf;

	/**
	* Evaluates the moment-generating function (MGF) for an Erlang distribution.
	*
	* @private
	* @param {number} t - input value
	* @returns {number} evaluated MGF
	*
	* @example
	* var y = mgf( 0.5 );
	* // returns <number>
	*/
	function mgf( t ) {
		if ( isnan( t ) || t >= lambda ) {
			return NaN;
		}
		return pow( 1.0 - (t/lambda), -k );
	}
}
Returns a function for evaluating the moment-generating function (MGF) of an Erlang distribution with shape parameter `k` and rate parameter `lambda`.

@param {NonNegativeInteger} k - shape parameter
@param {PositiveNumber} lambda - rate parameter
@returns {Function} MGF

@example
var mgf = factory( 2, 0.5 );

var y = mgf( 0.2 );
// returns ~2.778

y = mgf( -0.5 );
// returns 0.25