“…We do so by means of a multiscale analysis of the balanced equations of motion which are shown to collapse to a dimension reduced system when some of the Hankel singular values go to zero; see, e.g., [35,36] for a related approach or [22,47,59,29] in which low-rank perturbative approximations of transfer functions of linear systems are sought. To the best of our knowledge our approach is new, and, although it is based on computing an appropriate balanced form of the system equations which certainly becomes infeasible if the system is extremely high-dimensional (n ∼ 10 6 or larger), we see it not merely as an alternative, but rather as an extension to existing projectionbased methods such as Krylov subspace [54,4,15,45,46,12,10] or interpolation (moment-matching) methods [9,23,67], and empirical POD [37,44,13,58]; see also [7,60]. A common feature of these methods is that they identify a subspace which contains the "essential" part of the dynamics, and we suggest to combine the numerically cheap identification of Krylov or POD subspaces with singular perturbation methods that have certain advantages in terms of structure preservation.…”