code: private double densityThresholdFunction(int tg, double cl, double decayFactor, int N)
  	{
  		return (cl * (1.0 - Math.pow(decayFactor, (this.getCurrTime()-tg+1.0))))/(N * (1.0 - decayFactor));
  	}
docs: Implements the function pi given in Definition 4.1 of Chen and Tu 2007
  
  @param tg - the update time in the density grid's characteristic vector
  @param cl - user defined parameter which controls the threshold for sparse grids
  @param decayFactor - user defined parameter which is represented as lambda in Chen and Tu 2007
  @param N - the number of density grids, defined after eq 2 in Chen and Tu 2007