In property testing, the goal is to distinguish structures that have some desired property from those that are far from having the property, based on only a small, random sample of the structure. We focus on the classification of first-order sentences according to their testability. This classification was initiated by Alon et al. [2], who showed that graph properties expressible with prefix there exists*for all* are testable but that there is an untestable graph property expressible with quantifier prefix for all*there exists*. The main results of the present paper are as follows. We prove that all (relational) properties expressible with quantifier prefix there exists*for all there exists* (Ackermann's class with equality) are testable and also extend the positive result of Alon et al. [2] to relational structures using a recent result by Austin and Tao [8]. Finally, we simplify the untestable property of Alon et al. [2] and show that prefixes for all(3)there exists, for all(2)there exists for all, for all there exists for all(2) and for all there exists V there exists can express untestable graph properties when equality is allowed.
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