6.1 Reduced Order Models Based on the Open-loop Balanced Realization6.1.1 Direct Truncation Method6.1.2 Singular Perturbation Method6.2 Reduced Order Model Exercises6.3 Symmetric Systems6.3.1 Reduced Order Models for SISO Systems6.3.2 Properties of Symmetric Systems6.3.3 The Cross-gramian Matrix6.3.4 Relations Between Wc2, Wo2 and Wco6.3.5 Open-loop Parameterization6.3.6 Relation Between the Cauchy Index and the Hankel Matrix6.3.7 Singular Values for a FIR Filter6.3.8 Singular Values of All-pass Systems6.4 Exercises