Speaker: Peter Danziger (Ryerson University)
Time and Place: 10 February 2006, 3:40
pm, HP4369, Herzberg building,
Carleton University
Title: Hill Climbing to Pasch Valleys
Abstract:
Exhaustive enumeration of Steiner Triple Systems is not feasible, due to the
the combinatorial explosion of instances. The next-best hope is to quickly
find a sample that is representative over isomorphism classes.
Stinson's Hill-Climbing algorithm is widely used to produce `random' Steiner
Triple Systems. It certainly finds a sample quickly, but how close is the
sample thus produced to the `uniform' distribution? In this talk I will
present statistical arguments that the sample is far from uniformly
distributed with respect to the isomorphism classes of the STSs, at least
for v =< 19. In particular, we find that isomorphism classes with a large
number of Pasch configurations are under-represented.
I will also discuss a modification to the standard hill-climbing algorithm
that appears to make the sample found somewhat closer to the uniform
distribution over isomorphism classes in return for a modest increase in
running time.
This is joint work with D. Heap and E. Mendelsohn.