code: function pdf( x, lambda, k ) {
  	var lnl,
  		lnp;
  	if ( x < 0 ) {
  		return 0;
  	}
  	if ( x === 0 ) {
  		return ( k === 1 ) ? k/lambda : 0;
  	}
  	lnl = ln( lambda );
  	lnp = ln( k ) - lnl;
  	lnp += ( k-1 ) * ( ln(x)-lnl );
  	lnp -= pow( x/lambda, k );
  
  	return exp( lnp );
  }
docs: PDF // 
  FUNCTION: pdf( x, lambda, k )
  Evaluates the probability density function (PDF) for a Weibull distribution with scale parameter `lambda` and shape parameter `k` at a value `x`.
  
  @param {Number} x - input value
  @param {Number} lambda - scale parameter
  @param {Number} k - shape parameter
  @returns {Number} evaluated PDF