We introduce a new symmetry class of domino tilings of the Aztec diamond, called the off-diagonal symmetry class, which is motivated by the off-diagonally symmetric alternating sign matrices introduced by Kuperberg in 2002. We use the method of non-intersecting lattice paths and a modification of Stembridge's Pfaffian formula for families of non-intersecting lattice paths to enumerate our new symmetry class. The number of off-diagonally symmetric domino tilings of the Aztec diamond can be expressed as a Pfaffian of a matrix whose entries satisfy a nice and simple recurrence relation.
INTRODUCTION
Symmetry classesConsider a finite group G acting on a set of combinatorial objects X. Let H be a subgroup of G. A symmetry class is a collection of H-invariant objects of X. In enumerative combinatorics, it is quite challenging to enumerate each symmetry class because the structure of each class varies with different subgroups H. These usually require different methods to enumerate them.The study of symmetry classes of plane partitions goes back to MacMahon [21], but gained more attention in the 1970s and 80s. Stanley [31] identified ten symmetry classes of plane partitions, all the symmetry classes can be enumerated by nice product formulas. We refer the interested reader to the survey paper by Krattenthaler [15, Section 6] for a modern update to Stanley's paper.An alternating sign matrix (ASM) of order n is an n × n matrix with entries 0, 1 or −1 such that all row and column sums are equal to 1 and the non-zero entries alternate in sign in each row and column. They were introduced by Mills, Robbins and Rumsey [22] in the early 1980s. The symmetry classes of ASMs under the action of the dihedral group of order 8 were proposed and summarized by Stanley [30] and Robbins [27,28]. There are eight symmetry classes of ASMs; five of them were fully solved, one was partially solved, while for the remaining two there are no known or conjectured formula. We refer the interested reader to the detailed account written by Behrend, Fischer and Konvalinka [3, Section 1.2].In the 1990s, Kuperberg's seminal paper [17] brought the statistical mechanical six-vertex model into the study of ASMs. Later, in [18], he successfully provided a unified framework using the six-vertex model to solve some of the symmetry classes of ASMs. He also introduced several new types of ASMs, such as, off-diagonally symmetric, vertically and horizontally perverse, with U-turn sides, and combined them with the original eight symmetry classes. The enumerative results of these new types of ASMs were summarized in [3, Section 1.2].The Aztec diamond of order n, denoted by AD(n), is the union of all unit squares in the region x + y ≤ n + 1 which was introduced by Elkies, Larsen, Kuperberg and Propp [7,8] in the early 1990s. The symmetry classes of domino tilings of AD(n) 1 under the action of the 2020 Mathematics Subject Classification. 05A15, 05B20, 05B45. Key words and phrases. Aztec diamonds, domino tilings, method of non-intersecting lattice paths, Pfaffians,...