function factory( lambda ) {
	if ( isnan( lambda ) || lambda < 0.0 ) {
		return constantFunction( NaN );
	}
	if ( lambda === 0.0 ) {
		return degenerate( 0.0 );
	}
	return logpmf;

	/**
	* Evaluates the natural logarithm of the probability mass function (PMF) for a Poisson distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logPMF
	*
	* @example
	* var y = logpmf( 2.0 );
	* // returns <number>
	*/
	function logpmf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( isNonNegativeInteger( x ) && x !== PINF ) {
			return ( x * ln( lambda ) ) - lambda - factorialln( x );
		}
		return NINF;
	}
}
Returns a function for evaluating the natural logarithm of the probability mass function (PMF) for a Poisson distribution with mean parameter `lambda`.

@param {NonNegativeNumber} lambda - mean parameter
@returns {Function} logPMF

@example
var logpmf = factory( 1.0 );
var y = logpmf( 3.0 );
// returns ~-2.792

y = logpmf( 1.0 );
// returns ~-1.0