Department of Computer Science | Institute of Theoretical Computer Science | CADMO

Prof. Emo Welzl and Prof. Bernd Gärtner

Mittagsseminar Talk Information |

**Date and Time**: Thursday, November 17, 2011, 12:15 pm

**Duration**: 30 minutes

**Location**: OAT S15/S16/S17

**Speaker**: Thomas Rast

Assume a stack of pancakes can only be sorted (by size) by inserting a spatula between two pancakes, and flipping the entire upper part of the stack. This is formally known as sorting by prefix reversal (SBPR). The question for the number of reversals needed has been open since 1975.

There is a "natural" efficient sequence of flips in which some SBPR instances can be solved, giving rise to a lower bound. In a very recent arXiv paper, Bulteau, Fertin and Rusu show that deciding whether a given instance attains this lower bound is NP-hard. Thus more generally, determining the number of flips needed is NP-hard.

I will give an overview of the problem and known results, and then show some details of the (rather surprising at least for me) reduction from 3-SAT.

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