code: public double getRMSRigidity(){
  
  		rigidity = new double[this.getVariables().length];
  		for (int i = 0; i < this.getVariables().length; i++) {
  			if ( ((TimePoint)this.getVariables()[i]).isUsed() ) {
  			rigidity[i] = (
  					((double)1 / 
  							((double)(1 + ((TimePoint)this.getVariables()[i]).getUpperBound() - ((TimePoint)this.getVariables()[i]).getLowerBound()))
  							));
  			//System.out.println(i + " " + j + " -> " + distance[this.getVariables()[i].getID()][this.getVariables()[j].getID()]);
  			//System.out.println(i + " " + j + " -> " + rigidity[i][j]);
  			}
  		}
  		double sigma = 0;
  		for (int i = 0; i < this.getVariables().length; i++) {
  			if ( ((TimePoint)this.getVariables()[i]).isUsed() ) {
  				sigma += Math.pow(rigidity[i], 2.0);
  			}
  		}			
  
  		return Math.sqrt(((double)sigma) * ((double)2/(this.getVariables().length * (this.getVariables().length + 1))));
  	}
docs: This method computes the root mean square rigidity of a consistent STN (the inverse concept of flexibility of a STN).
  If the STN is completely rigid, then its rigidity is 1. If the STN has no constraints, its rigidity is 0.
  This measure in proposed in [Luke Hunsberger, 2002].
  @return Root mean square rigidity of a consistent STN.