code: function gamma( x ) {
  	var w;
  	var y;
  	var v;
  
  	w = 1 / x;
  	w = 1 + w * polyval( w );
  	y = exp( x );
  
  	// Check `x` to avoid `pow()` overflow...
  	if ( x > MAX_STIRLING ) {
  		v = pow( x, 0.5*x - 0.25 );
  		y = v * (v/y);
  	} else {
  		y = pow( x, x-0.5 ) / y;
  	}
  	return SQRT2PI * y * w;
  }
docs: GAMMA // 
  FUNCTION: gamma( x )
  Evaluates the gamma function by using Stirling's formula. The polynomial is valid for 33 <= x <= 172.
  
  @param {Number} x - input value
  @returns {Number} function value