Embedding properties of network realizations of dissipative reduced order models
Jörn Zimmerling, Mikhail Zaslavsky,Rob Remis, Shasri Moskow, Alexander Mamonov, Murthy Guddati,
Vladimir Druskin, and Liliana Borcea
Mathematical Sciences Department, Worcester Polytechnic Institute
https://www.wpi.edu/people/vdruskin
Abstract
Realizations of reduced order models of passive SISO or MIMO LTI problems can be transformed to tridiagonal and
blocktridiagonal forms, respectively, via dierent modications of the Lanczos algorithm. Generally, such realizations
can be interpreted as ladder resistorcapacitorinductor (RCL) networks. They gave rise to network syntheses in the
rst half of the 20th century that was at the base of modern electronics design and consecutively to MOR that
tremendously impacted many areas of engineering (electrical, mechanical, aerospace, etc.) by enabling ecient
compression of the underlining dynamical systems. In his seminal 1950s works Krein realized that in addition to
their compressing properties, network realizations can be used to embed the data back into the state space of the
underlying continuum problems.
In more recent works of the authors Krein's ideas gave rise to socalled nitedierence Gaussian quadrature rules
(FDGQR), allowing to approximately map the ROM statespace representation to its full order continuum counterpart
on a judicially chosen grid. Thus, the state variables can be accessed directly from the transfer function without
solving the full problem and even explicit knowledge of the PDE coecients in the interior, i.e., the FDGQR directly
learns" the problem from its transfer function. This embedding property found applications in PDE solvers, inverse
problems and unsupervised machine learning.
Here we show a generalization of this approach to dissipative PDE problems, e.g., electromagnetic and acoustic
wave propagation in lossy dispersive media. Potential applications include solution of inverse scattering problems in
dispersive media, such as seismic exploration, radars and sonars.
To x the idea, we consider a passive irreducible SISO ROM
fn(s) = Xn
j=1
yi
s + σj
, (62)
assuming that all complex terms in (62) come in conjugate pairs.
We will seek ladder realization of (62) as
rjuj + vj − vj−1 = −shˆjuj ,
uj+1 − uj + ˆrj vj = −shj vj ,
(63)
for j = 0, . . . , n with boundary conditions
un+1 = 0, v1 = −1,
and 4n real parameters hi, hˆi, ri and rˆi, i = 1, . . . , n, that can be considered, respectively, as the equivalent discrete
inductances, capacitors and also primary and dual conductors. Alternatively, they can be viewed as respectively
masses, spring stiness, primary and dual dampers of a mechanical string. Reordering variables would bring (63)
into tridiagonal form, so from the spectral measure given by (62 ) the coecients of (63) can be obtained via a
nonsymmetric Lanczos algorithm written in Jsymmetric form and fn(s) can be equivalently computed as
fn(s) = u1.
The cases considered in the original FDGQR correspond to either (i) real y, θ or (ii) real y and imaginary θ. Both
cases are covered by the Stieltjes theorem, that yields in case (i) real positive h, hˆ and trivial r, rˆ, and in case (ii) real
positive h,r and trivial hˆ,rˆ. This result allowed us a simple interpretation of (62) as the staggered nitedierence
approximation of the underlying PDE problem [2]. For PDEs in more than one variables (including topologically rich
datamanifolds), a nitedierence interpretation is obtained via a MIMO extensions in block form, e.g., [4, 3].
The main diculty of extending this approach to general passive problems is that the Stieltjes theory is no longer
applicable. Moreover, the tridiagonal realization of a passive ROM transfer function (62) via the ladder network (63)
cannot always be obtained in portHamiltonian form, i.e., the equivalent primary and dual conductors may change
sign [1].
100
Embedding of the Stieltjes problems, e.g., the case (i) was done by mapping h and hˆ into values of acoustic (or
electromagnetic) impedance at grid cells, that required a special coordinate stretching (known as travel time coordinate transform) for continuous problems. Likewise, to circumvent possible nonpositivity of conductors for the
nonStieltjes case, we introduce an additional complex sdependent coordinate stretching, vanishing as s → ∞ [1].
This stretching applied in the discrete setting induces a diagonal factorization, removes oscillating coecients, and
leads to an accurate embedding for moderate variations of the coecients of the continuum problems, i.e., it maps
discrete coecients onto the values of their continuum counterparts.
Not only does this embedding yields an approximate linear algebraic algorithm for the solution of the inverse problems
for dissipative PDEs, it also leads to new insight into the properties of their ROM realizations. We will also discuss
another approach to embedding, based on KreinNudelman theory [5], that results in special datadriven adaptive
grids.
References
[1] Borcea, Liliana and Druskin, Vladimir and Zimmerling, Jörn, A reduced order model approach to
inverse scattering in lossy layered media, Journal of Scientic Computing, V. 89, N1, pp. 136,2021
[2] Druskin, Vladimir and Knizhnerman, Leonid, Gaussian spectral rules for the threepoint second dierences:
I. A twopoint positive denite problem in a semiinnite domain, SIAM Journal on Numerical Analysis, V. 37,
N 2, pp.403422, 1999
[3] Druskin, Vladimir and Mamonov, Alexander V and Zaslavsky, Mikhail, Distance preserving model
order reduction of graphLaplacians and cluster analysis, Druskin, Vladimir and Mamonov, Alexander V
and Zaslavsky, Mikhail, Journal of Scientic Computing, V. 90, N 1, pp 130, 2022
[4] Druskin, Vladimir and Moskow, Shari and Zaslavsky, Mikhail LippmannSchwingerLanczos algorithm
for inverse scattering problems, Inverse Problems, V. 37, N. 7, 2021,
[5] Mark Adolfovich Nudelman The Krein String and Characteristic Functions of Maximal Dissipative Operators, Journal of Mathematical Sciences, 2004, V 124, pp 49184934
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A Combinatorial CutToggling Algorithm for Solving Laplacian Systems
Over the last two decades, a significant line of work in theoretical algorithms has made progress in solving linear systems of the form 𝐋𝐱 = 𝐛, where 𝐋 is the Laplacian matrix of a weighted graph with weights w(i,j) > 0 on the edges. The solution 𝐱 of the linear system can be interpreted as the potentials of an electrical flow in which the resistance on edge (i,j) is 1/w(i,j). Kelner, Orrechia, Sidford, and Zhu [Kelner et al., 2013] give a combinatorial, nearlinear time algorithm that maintains the Kirchoff Current Law, and gradually enforces the Kirchoff Potential Law by updating flows around cycles (cycle toggling).
In this paper, we consider a dual version of the algorithm that maintains the Kirchoff Potential Law, and gradually enforces the Kirchoff Current Law by cut toggling: each iteration updates all potentials on one side of a fundamental cut of a spanning tree by the same amount. We prove that this dual algorithm also runs in a nearlinear number of iterations.
We show, however, that if we abstract cut toggling as a natural data structure problem, this problem can be reduced to the online vectormatrixvector problem (OMv), which has been conjectured to be difficult for dynamic algorithms [Henzinger et al., 2015]. The conjecture implies that the data structure does not have an O(n^{1ε}) time algorithm for any ε > 0, and thus a straightforward implementation of the cuttoggling algorithm requires essentially linear time per iteration.
To circumvent the lower bound, we batch update steps, and perform them simultaneously instead of sequentially. An appropriate choice of batching leads to an Õ(m^{1.5}) time cuttoggling algorithm for solving Laplacian systems. Furthermore, we show that if we sparsify the graph and call our algorithm recursively on the Laplacian system implied by batching and sparsifying, we can reduce the running time to O(m^{1 + ε}) for any ε > 0. Thus, the dual cuttoggling algorithm can achieve (almost) the same running time as its primal cycletoggling counterpart.
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 Award ID(s):
 2007009
 NSFPAR ID:
 10420889
 Editor(s):
 Tauman Kalai, Yael
 Date Published:
 Journal Name:
 Leibniz international proceedings in informatics
 Volume:
 251
 ISSN:
 18688969
 Page Range / eLocation ID:
 69:169:22
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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