Posets are easily testable

Abstract: Alon and Shapira proved that every monotone class (closed under taking subgraphs) of undirected graphs is strongly testable, that is, under the promise that a given graph is either in the class or $\varepsilon$-far from it, there is a test using a constant number of samples (depending on $\varepsilon$ only) that rejects every graph not in the class with probability at least one half, and always accepts a graph in the class. However, their bound on the number of samples is quite large since they heavily rely on Szemer\'edi's regularity lemma. We study the case of posets and show that every monotone class of posets is easily testable, that is, a polynomial (of $\varepsilon^{-1}$) number of samples is sufficient. We achieve this via proving a polynomial removal lemma for posets. We give a simple classification: for every monotone class of posets, there is an $h$ such that the class is indistinguishable (every large enough poset in one class is $\varepsilon$-close to a poset in the other class) from the class of $C_h$-free posets, where $C_h$ denotes the chain with $h$ elements. This allows us to test every monotone class of posets using $O(\varepsilon^{-1})$ samples. The test has a two-sided error, but it is almost complete: the probability of refuting a poset in the class is polynomially small in the size of the poset. The analogous results hold for comparability graphs, too.