Sepideh Mahabadi: Composable Core-sets for Diversity and Coverage Maximization, and Its Application in Diverse Near Neighbor Problem

Wednesday, May 7, 2014 - 4:00pm to 5:00pm
Sepideh Mahabadi
This talk consists of two parts.

In the first part, we consider efficient construction of ``composable
core-sets" for basic diversity and coverage maximization problems. A
core-set for a point-set in a metric space is a subset of the point-set
with the property that an approximate solution to the whole point-set can
be obtained given the core-set alone. A composable core-set has the
property that for a collection of sets, the approximate solution to the
union of the sets in the collection can be obtained given the union of the
composable core-sets for the point sets in the collection. Using composable
core-sets one can obtain efficient solutions to a wide variety of massive
data processing applications, including nearest neighbor search, streaming
algorithms and map-reduce computation.

 Our main results are algorithms for constructing composable core-sets for
several notions of ``diversity objective functions", a topic that attracted
a significant amount of research over the last few years. The composable
core-sets we construct are small and accurate: their approximation factor
almost matches that of the best ``off-line" algorithms for the relevant
optimization problems (up to a constant factor). Moreover, we also show
applications of our results to diverse nearest neighbor search, streaming
algorithms and map-reduce computation. Finally, we show that for an
alternative notion of diversity maximization based on the maximum coverage
problem small composable core-sets do not exist.

In the second part, motivated by the recent research on diversity-aware
search, we investigate the k-diverse near neighbor reporting problem. The
problem is defined as follows: given a query point q, report the "maximum
diversity" set S of k points in the ball of radius r around q. The
diversity of a set S is measured by the minimum distance between any pair
of points in S (the higher, the better). We present two approximation
algorithms for the case where the points live in a d-dimensional Hamming
space. Our algorithms guarantee query times that are sub-linear in n and
only polynomial in the diversity parameter k, as well as the dimension d.

For low values of k, our algorithms achieve sub-linear query times even if
the number of points within distance r from a query q is linear in n. To
the best of our knowledge, these are the first known algorithms of this
type that offer provable guarantees.

The first part is a joint work with Piotr Indyk, Mohammad Mahdian, and
Vahab S. Mirrokni; and the second part is a joint work with Sofiane Abbar,
Sihem Amer-Yahia, Piotr Indyk, and Kasturi Varadarajan.