Macaulay posets and rings

Abstract Macaulay posets are posets in which an analog of the Kruskal–Katona Theorem holds. Macaulay rings (also called Macaulay–Lex rings) are rings in which an analog of Macaulay’s Theorem for lex ideals holds. The study of both of these objects started with Macaulay almost a century ago. Since then, these two branches have developed separately over the past century, with the last link being the Clements–Lindström Theorem. For every ring that is the quotient of a polynomial ring by a homogeneous ideal, we define the poset of monomials. Under certain conditions, we prove a Macaulay Correspondence Theorem; a ring is Macaulay if and only if its poset of monomials is Macaulay. Furthermore, the tensor product of rings corresponds to the Cartesian product of the posets of monomials. This allows us to transfer results between rings and posets. The Macaulay Correspondence Theorem generalizes a theorem by Shakin and allows ideals that are not monomial with orders that are different from the lexicographic one. By using this translation, we give several answers to a problem posed by Mermin and Peeva, a positive answer to Hoefel’s question about applying Macaulay poset theory to ring theory, and deduce several other results in both algebra and extremal combinatorics. A new proof of the Mermin–Murai Theorem on colored square-free rings is presented by using star posets. We extend the Mermin–Murai Theorem to rings that are not square-free by using the spider Macaulay Theorem of Bezrukov and Elsässer. Using the Mermin–Peeva and Shakin results about adding a variable to a ring such that it remains Macaulay, we give an answer to a question posed by Bezrukov and Leck about taking the product of a Macaulay poset with a chain. Some results of Chong also give answers to the Bezrukov–Leck problem. All of these results have a common feature. They involve the tensor product of rings whose Hasse graphs of the poset of monomials are trees. We call such rings, tree rings. We give a classification of Macaulay rings that are the tensor product of a tree ring. Finally, we show that there are Macaulay rings that are not the tensor product of tree rings, and present the first examples of Macaulay rings that are not quotients by a monomial ideal and not quotients by a toric ideal.