 
Summary: COUNTING 1324, 4231AVOIDING PERMUTATIONS
Michael H. Albert
Department of Computer Science
University of Otago
Dunedin, New Zealand
M. D. Atkinson
Department of Computer Science
University of Otago
Dunedin, New Zealand
Vincent Vatter
Department of Mathematics
Dartmouth College
Hanover, New Hampshire USA
A complete structural description and enumeration is found for
permutations that avoid both 1324 and 4231.
1. INTRODUCTION
Classes of permutations are sets of permutations that are closed downwards under taking
subpermutations. They are usually presented as sets C that avoid a given set B of per
mutations (i.e. the permutations of C have no subpermutation in the set B). We express
this by the notation C = Av(B). Much of the inspiration for elucidating the structure of
