We now consider circuits of depth d ≥ 3. Out of attempts to prove lower bounds for such circuits, two powerful methods emerged. The first is a “depth reduction” argument: One tries to reduce the depth one layer at a time, until a circuit of depth 2 (or depth 1) remains. The key is the so-called Switching Lemma,which allows us to replace CNFs on the first two layers by DNFs, thus reducing the depth by 1.1
KeywordsBoolean Function Parity Function Boolean Matrix Majority Gate Symmetric Boolean Function
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