On finite width questionable representations of orders (1903.02028v7)

Abstract: In this article, we study "questionable representations" of (partial or total) orders, introduced in our previous article "A class of orders with linear? time sorting algorithm". (Later, we consider arbitrary binary functional/relational structures instead of orders.) A "question" is the first difference between two sequences (with ordinal index) of elements of orders/sets. In finite width "questionable representations" of an order O, comparison can be solved by looking at the "question" that compares elements of a finite order O'. A corollary of a theorem by Cantor (1895)is that all countable total orders have a binary (width 2) questionable representation. We find new classes of orders on which testing isomorphism or counting the number of linear extensions can be done in polynomial time. We also present a generalization of questionable-width, called balanced tree-questionable-width, and show that if a class of binary structures has bounded tree-width or clique-width, then it has bounded balanced tree-questionable-width. But there are classes of graphs of bounded balanced tree-questionable-width and unbounded tree-width or clique-width.