Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2013/iii/paper-59/3/c/solution

The AND-NOT circuit value problem belongs to P: validate the Boolean circuit and evaluate its gates in a topological ordering. Evaluation takes polynomial time even if its description is not already in a topological ordering.
For P-completeness we use logspace many-one reductions. Start from the supplied circuit value problem over the usual AND (logical conjunction), OR (logical disjunction) and NOT (negation) basis. Retain AND and NOT gates, and replace every OR gate by the De Morgan's laws gadget
This adds only a constant number of gates per old gate, preserves its truth value for all inputs and keeps the Boolean circuit acyclic. If the format includes constant source nodes, replace them by additional input nodes assigned fixed bits zero and one; these are inputs, not disallowed gates. Larger fan-in gates can first be replaced by binary trees of their inputs.
To output the new description, keep the old gate index and a constant-size gadget position, rescan old references when necessary, and assign consistent new indices to each gadget's terminal output. These counters and references occupy bits; the input assignment is copied with any constant-source bits appended. Thus the construction is a logspace many-one reduction preserving acceptance. Since Boolean circuit Value is P-complete under such reductions,

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