code: static void computePPpX( DMatrixRMaj P , DMatrixRMaj P_plus, DMatrixRMaj X , int offsetX,  DMatrixRMaj output ) {
  		final int N = P.numRows;
  		output.reshape(N,1);
  
  		// A=P_plus * X <-- 2x1
  		double a00=0,a10=0;
  		for (int i = 0,indexX=offsetX; i < N; i++, indexX += 2) {
  			double x = -X.data[indexX];
  			a00 += x*P_plus.data[i];
  			a10 += x*P_plus.data[i+N];
  		}
  
  		// P*A
  		for (int i = 0,indexP=0; i < N; i++) {
  			output.data[i] = a00*P.data[indexP++] + a10*P.data[indexP++];
  		}
  
  	}
docs: P*P_plus*X*1