Directed Isoperimetric Theorems for Boolean Functions on the Hypergrid and an O(n√(d)) Monotonicity Tester
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The problem of testing monotonicity for Boolean functions on the hypergrid, f:[n]^d →{0,1} is a classic topic in property testing. When n=2, the domain is the hypercube. For the hypercube case, a breakthrough result of Khot-Minzer-Safra (FOCS 2015) gave a non-adaptive, one-sided tester making O(ε^-2√(d)) queries. Up to polylog d and ε factors, this bound matches the Ω(√(d))-query non-adaptive lower bound (Chen-De-Servedio-Tan (STOC 2015), Chen-Waingarten-Xie (STOC 2017)). For any n > 2, the optimal non-adaptive complexity was unknown. A previous result of the authors achieves a O(d^5/6)-query upper bound (SODA 2020), quite far from the √(d) bound for the hypercube. In this paper, we resolve the non-adaptive complexity of monotonicity testing for all constant n, up to poly(ε^-1log d) factors. Specifically, we give a non-adaptive, one-sided monotonicity tester making O(ε^-2n√(d)) queries. From a technical standpoint, we prove new directed isoperimetric theorems over the hypergrid [n]^d. These results generalize the celebrated directed Talagrand inequalities that were only known for the hypercube.
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