eng Science and Education Publishing Turkish Journal of Analysis and Number Theory 2333-1232 2017-11-03 5 6 210 225 10.12691/tjant-5-6-3 TJANT2017563 article David Callan 1 Toufik Mansour tmansour@univ.haifa.ac.il 2 Department of Statistics, University of Wisconsin, Madison, WI Department of Mathematics, University of Haifa, Haifa, Israel Let Sn be the symmetric group of all permutations of n letters. We show that there are precisely 64 Wilf classes consisting of exactly 2 symmetry classes of subsets of four 4-letter patterns. http://pubs.sciepub.com/tjant/5/6/3/tjant-5-6-3.pdf pattern avoidance Wilf-equivalence