# Fast computation of optimal damping parameters for linear vibrational systems

We formulate the quadratic eigenvalue problem underlying the mathematical model of a linear vibrational system as an eigenvalue problem of a diagonal-plus-low-rank matrix A. The eigenvector matrix of A has a Cauchy-like structure. Optimal viscosities are those for which trace(X) is minimal, where X is the solution of the Lyapunov equation AX+XA^*=GG^*. Here G is a low-rank matrix which depends on the eigenfrequencies that need to be damped. After initial eigenvalue decomposition of linearized problem which requires O(n^3) operations, our algorithm computes optimal viscosities for each choice of external dampers in O(n^2) operations, provided that the number of dampers is small. Hence, the subsequent optimization is order of magnitude faster than in the standard approach which solves Lyapunov equation in each step, thus requiring O(n^3) operations. Our algorithm is based on O(n^2) eigensolver for complex symmetric diagonal-plus-rank-one matrices and fast O(n^2) multiplication of linked Cauchy-like matrices.

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