# Mittagsseminar (in cooperation with M. Ghaffari, A. Steger and B. Sudakov)

__Mittagsseminar Talk Information__ | |

**Date and Time**: Tuesday, August 07, 2012, 12:15 pm

**Duration**: 30 minutes

**Location**: CAB G51

**Speaker**: Micha Sharir (Tel Aviv University)

## Waiting for the ceiling to fall: On the longest path in the randomized incremental point-location DAG

A simple and classical solution for point location amid n (nonintersecting) segments in the plane is via a randomized incremental construction: One insertes the segments one by one in a random order, and maintains a vertical trapezoidal decomposition of the plane for the segments inserted so far. In addition, one maintains a History DAG, where each nonfinal trapezoid points to the trapezoids that have ``stepped'' on it at the insertion step that destroyed it. A point location query is implemented simply by searching with the query point q through the path in the DAG of the trapezoids that contain q. The expected length of the path, for any query point q, is O(\log n). Suppose one runs the algorithm and wants to decide whether the produced DAG is ``good'', in the sense that every query has a search path of logarithmic depth? The simplest solution is to check that the depth of the DAG is O(\log n), but unfortunately, as recently observed by Hemmer, Kleinbort, and Halperin, the depth may be much larger, even though the DAG is good, due to the existence of paths that are unrealizable by any query point. The main open question here is whether the expected maximum depth of the DAG is O(\log n). In a work in progress (joint with Haim Kaplan and others) we examine this question, and make a few observations concerning the expected maximum depth of the DAG. One goal of this talk is to solicit ideas from the audience, on this innocently looking but seemingly difficult problem.

Upcoming talks | All previous talks | Talks by speaker | Upcoming talks in iCal format (beta version!)

Previous talks by year: 2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996

Information for students and suggested topics for student talks

Automatic MiSe System Software Version 1.4803M | admin login