arXiv subjects

Sara Ahmadian

Publications and source records attributed to Sara Ahmadian.

23 records · Page 2Linked to original sources

arXiv2017

Further Approximations for Demand Matching: Matroid Constraints and Minor-Closed Graphs

We pursue a study of the Generalized Demand Matching problem, a common generalization of the $b$-Matching and Knapsack problems. Here, we are given a graph with vertex capacities, edge profits, and asymmetric demands on the edges. The goal is to find a maximum-profit subset of edges so the demands of chosen edges do not violate vertex capacities. This problem is APX-hard and constant-factor approximations are known. Our results fall into two categories. First, using iterated relaxation and various filtering strategies, we show with an efficient rounding algorithm if an additional matroid structure $\mathcal M$ is given and we further only allow sets $F \subseteq E$ that are independent in $\mathcal M$, the natural LP relaxation has an integrality gap of at most $\frac{25}{3} \approx 8.333$. This can be improved in various special cases, for example we improve over the 15-approximation for the previously-studied Coupled Placement problem [Korupolu et al. 2014] by giving a $7$-approximation. Using similar techniques, we show the problem of computing a minimum-cost base in $\mathcal M$ satisfying vertex capacities admits a $(1,3)$-bicriteria approximation. This improves over the previous $(1,4)$-approximation in the special case that $\mathcal M$ is the graphic matroid over the given graph [Fukanaga and Nagamochi, 2009]. Second, we show Demand Matching admits a polynomial-time approximation scheme in graphs that exclude a fixed minor. If all demands are polynomially-bounded integers, this is somewhat easy using dynamic programming in bounded-treewidth graphs. Our main technical contribution is a sparsification lemma allowing us to scale the demands to be used in a more intricate dynamic programming algorithm, followed by randomized rounding to filter our scaled-demand solution to a feasible solution.

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arXiv2017

Better Guarantees for k-Means and Euclidean k-Median by Primal-Dual Algorithms

Clustering is a classic topic in optimization with $k$-means being one of the most fundamental such problems. In the absence of any restrictions on the input, the best known algorithm for $k$-means with a provable guarantee is a simple local search heuristic yielding an approximation guarantee of $9+ε$, a ratio that is known to be tight with respect to such methods. We overcome this barrier by presenting a new primal-dual approach that allows us to (1) exploit the geometric structure of $k$-means and (2) to satisfy the hard constraint that at most $k$ clusters are selected without deteriorating the approximation guarantee. Our main result is a $6.357$-approximation algorithm with respect to the standard LP relaxation. Our techniques are quite general and we also show improved guarantees for the general version of $k$-means where the underlying metric is not required to be Euclidean and for $k$-median in Euclidean metrics.

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arXiv2016

Approximation Algorithms for Clustering Problems with Lower Bounds and Outliers

We consider clustering problems with {\em non-uniform lower bounds and outliers}, and obtain the {\em first approximation guarantees} for these problems. We have a set $\F$ of facilities with lower bounds $\{L_i\}_{i\in\F}$ and a set $\D$ of clients located in a common metric space $\{c(i,j)\}_{i,j\in\F\cup\D}$, and bounds $k$, $m$. A feasible solution is a pair $\bigl(S\sse\F,σ:\D\mapsto S\cup\{\mathsf{out}\}\bigr)$, where $σ$ specifies the client assignments, such that $|S|\leq k$, $|σ^{-1}(i)|\geq L_i$ for all $i\in S$, and $|σ^{-1}(\mathsf{out})|\leq m$. In the {\em lower-bounded min-sum-of-radii with outliers} (\lbksro) problem, the objective is to minimize $\sum_{i\in S}\max_{j\inσ^{-1}(i)}c(i,j)$, and in the {\em lower-bounded $k$-supplier with outliers} (\lbkso) problem, the objective is to minimize $\max_{i\in S}\max_{j\inσ^{-1}(i)}c(i,j)$. We obtain an approximation factor of $12.365$ for \lbksro, which improves to $3.83$ for the non-outlier version (i.e., $m=0$). These also constitute the {\em first} approximation bounds for the min-sum-of-radii objective when we consider lower bounds and outliers {\em separately}. We apply the primal-dual method to the relaxation where we Lagrangify the $|S|\leq k$ constraint. The chief technical contribution and novelty of our algorithm is that, departing from the standard paradigm used for such constrained problems, we obtain an $O(1)$-approximation {\em despite the fact that we do not obtain a Lagrangian-multiplier-preserving algorithm for the Lagrangian relaxation}. We believe that our ideas have {broader applicability to other clustering problems with outliers as well.} We obtain approximation factors of $5$ and $3$ respectively for \lbkso and its non-outlier version. These are the {\em first} approximation results for $k$-supplier with {\em non-uniform} lower bounds.

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arXiv2013

Local-Search based Approximation Algorithms for Mobile Facility Location Problems

We consider the {\em mobile facility location} (\mfl) problem. We are given a set of facilities and clients located in a common metric space. The goal is to move each facility from its initial location to a destination and assign each client to the destination of some facility so as to minimize the sum of the movement-costs of the facilities and the client-assignment costs. This abstracts facility-location settings where one has the flexibility of moving facilities from their current locations to other destinations so as to serve clients more efficiently by reducing their assignment costs. We give the first {\em local-search based} approximation algorithm for this problem and achieve the best-known approximation guarantee. Our main result is $(3+ε)$-approximation for this problem for any constant $ε>0$ using local search. The previous best guarantee was an 8-approximation algorithm based on LP-rounding. Our guarantee {\em matches} the best-known approximation guarantee for the $k$-median problem. Since there is an approximation-preserving reduction from the $k$-median problem to \mfl, any improvement of our result would imply an analogous improvement for the $k$-median problem. Furthermore, {\em our analysis is tight} (up to $o(1)$ factors) since the tight example for the local-search based 3-approximation algorithm for $k$-median can be easily adapted to show that our local-search algorithm has a tight approximation ratio of 3. One of the chief novelties of the analysis is that in order to generate a suitable collection of local-search moves whose resulting inequalities yield the desired bound on the cost of a local-optimum, we define a tree-like structure that (loosely speaking) functions as a "recursion tree", using which we spawn off local-search moves by exploring this tree to a constant depth.

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arXiv2012

Improved Approximation Guarantees for Lower-Bounded Facility Location

We consider the {\em lower-bounded facility location} (\lbfl) problem (also sometimes called {\em load-balanced facility location}), which is a generalization of {\em uncapacitated facility location} (\ufl), where each open facility is required to serve a certain {\em minimum} amount of demand. More formally, an instance $\I$ of \lbfl is specified by a set $\F$ of facilities with facility-opening costs $\{f_i\}$, a set $\D$ of clients, and connection costs $\{c_{ij}\}$ specifying the cost of assigning a client $j$ to a facility $i$, where the $c_{ij}$s form a metric. A feasible solution specifies a subset $F$ of facilities to open, and assigns each client $j$ to an open facility $i(j)\in F$ so that each open facility serves {\em at least $M$ clients}, where $M$ is an input parameter. The cost of such a solution is $\sum_{i\in F}f_i+\sum_j c_{i(j)j}$, and the goal is to find a feasible solution of minimum cost. The current best approximation ratio for \lbfl is 448 \cite{Svitkina08}. We substantially advance the state-of-the-art for \lbfl by devising an approximation algorithm for \lbfl that achieves a significantly-improved approximation guarantee of 82.6. Our improvement comes from a variety of ideas in algorithm design and analysis, which also yield new insights into \lbfl.

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