Abstract. We study the statistical properties of geodesics, i.e. paths of minimal length, in large random planar quadrangulations. We extend Schaeffer's well-labeled tree bijection to the case of quadrangulations with a marked geodesic, leading to the notion of 'spine trees', amenable to a direct enumeration. We obtain the generating functions for quadrangulations with a marked geodesic of fixed length, as well as with a set of 'confluent geodesics', i.e. a collection of non-intersecting minimal paths connecting two given points. In the limit of quadrangulations with a large area n, we find in particular an average number 3 × 2ⁱ of geodesics between two fixed points at distance i 1 from each other. We show that, for generic endpoints, two confluent geodesics remain close to each other and have an extensive number of contacts. This property fails for a few 'exceptional' endpoints which can be linked by truly distinct geodesics. Results are presented both in the case of finite length i and in the scaling limit i ∝ n^1/4. In particular, we give the scaling distribution of the exceptional points.

PACS numbers: 02.10.Ox, 05.50.+q, 04.60.Nc, 04.60.−m

Print publication: Issue 14 (11 April 2008)
Received 14 December 2007, in final form 25 February 2008
Published 26 March 2008