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Register a new userCut-Toggling and Cycle-Toggling for Electrical Flow and Other p-Norm Flows
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Added by
Billy Jin
Publication date
2021
fields
Informatics Engineering
and research's language is
English
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We study the problem of finding flows in undirected graphs so as to minimize the weighted $p$-norm of the flow for any $p > 1$. When $p=2$, the problem is that of finding an electrical flow, and its dual is equivalent to solving a Laplacian linear system. The case $p = infty$ corresponds to finding a min-congestion flow, which is equivalent to max-flows. A typical algorithmic construction for such problems considers vertex potentials corresponding to the flow conservation constraints, and has two simple types of update steps: cycle toggling, which modifies the flow along a cycle, and cut toggling, which modifies all potentials on one side of a cut. Both types of steps are typically performed relative to a spanning tree $T$; then the cycle is a fundamental cycle of $T$, and the cut is a fundamental cut of $T$. In this paper, we show that these simple steps can be used to give a novel efficient implementation for the $p = 2$ case and to find near-optimal $p$-norm flows in a low number of iterations for all values of $p > 1$. Compared to known faster algorithms for these problems, our algorithms are simpler, more combinatorial, and also expose several underlying connections between these algorithms and dynamic graph data structures that have not been formalized previously.
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We present faster high-accuracy algorithms for computing $ell_p$-norm minimizing flows. On a graph with $m$ edges, our algorithm can compute a $(1+1/text{poly}(m))$-approximate unweighted $ell_p$-norm minimizing flow with $pm^{1+frac{1}{p-1}+o(1)}$ operations, for any $p ge 2,$ giving the best bound for all $pgtrsim 5.24.$ Combined with the algorithm from the work of Adil et al. (SODA 19), we can now compute such flows for any $2le ple m^{o(1)}$ in time at most $O(m^{1.24}).$ In comparison, the previous best running time was $Omega(m^{1.33})$ for large constant $p.$ For $psimdelta^{-1}log m,$ our algorithm computes a $(1+delta)$-approximate maximum flow on undirected graphs using $m^{1+o(1)}delta^{-1}$ operations, matching the current best bound, albeit only for unit-capacity graphs. We also give an algorithm for solving general $ell_{p}$-norm regression problems for large $p.$ Our algorithm makes $pm^{frac{1}{3}+o(1)}log^2(1/varepsilon)$ calls to a linear solver. This gives the first high-accuracy algorithm for computing weighted $ell_{p}$-norm minimizing flows that runs in time $o(m^{1.5})$ for some $p=m^{Omega(1)}.$ Our key technical contribution is to show that smoothed $ell_p$-norm problems introduced by Adil et al., are interreducible for different values of $p.$ No such reduction is known for standard $ell_p$-norm problems.
The rowmotion action on order ideals or on antichains of a finite partially ordered set has been studied (under a variety of names) by many authors. Depending on the poset, one finds unexpectedly interesting orbit structures, instances of (small order) periodicity, cyclic sieving, and homomesy. Many of these nice features still hold when the action is extended to $[0,1]$-labelings of the poset or (via detropicalization) to labelings by rational functions (the birational setting). In this work, we parallel the birational lifting already done for order-ideal rowmotion to antichain rowmotion. We give explicit equivariant bijections between the birational toggle groups and between their respective liftings. We further extend all of these notions to labellings by noncommutative rational functions, setting an unpublished periodicity conjecture of Grinberg in a broader context.
This paper explores the orbit structure and homomesy (constant averages over orbits) properties of certain actions of toggle groups on the collection of independent sets of a path graph. In particular we prove a generalization of a homomesy conjecture of Propp that for the action of a Coxeter element of vertex toggles, the difference of indicator functions of symmetrically-located vertices is 0-mesic. Then we use our analysis to show facts about orbit sizes that are easy to conjecture but nontrivial to prove. Besides its intrinsic interest, this particular combinatorial dynamical system is valuable in providing an interesting example of (a) homomesy in a context where large orbit sizes make a cyclic sieving phenomenon unlikely to exist, (b) the use of Coxeter theory to greatly generalize the set of actions for which results hold, and (c) the usefulness of Strikers notion of generalized toggle groups.
Directional excitation of guidance modes is central to many applications ranging from light harvesting, optical information processing to quantum optical technology. Of paramount interest, especially, the active control of near-field directionality provides a new paradigm for the real-time on-chip manipulation of light. Here we find that for a given dipolar source, its near-field directionality can be toggled efficiently via tailoring the polarization of surface waves that are excited, for example, via tuning the chemical potential of graphene in a graphene-metasurface waveguide. This finding enables a feasible scheme for the active near-field directionality. Counterintuitively, we reveal that this scheme can transform a circular electric/magnetic dipole into a Huygens dipole in the near-field coupling. Moreover, for Janus dipoles, this scheme enables us to actively flip their near-field coupling and non-coupling faces.
We show that in any graph, the average length of a flow path in an electrical flow between the endpoints of a random edge is $O(log^2 n)$. This is a consequence of a more general result which shows that the spectral norm of the entrywise absolute value of the transfer impedance matrix of a graph is $O(log^2 n)$. This result implies a simple oblivious routing scheme based on electrical flows in the case of transitive graphs.
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