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It is not the case that FO equivalence to AC^0 depends critically on the presence of a linear order predicate; over unordered structures, FO captures only a strict subset of AC^0.
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Reasons For
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Reason for
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1.
The distinction conflates expressiveness with definability; AC^0 itself is invariant to presentation order, not order-dependent.
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2.
Many AC^0-complete problems (like parity on unordered sets) have straightforward polynomial-time solutions without lexical ordering.
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3.
The claim assumes FO inadequacy on unordered structures without distinguishing between natural query properties and their representations.
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Reasons Against
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Reason against
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1.
Linear order enables FO to encode binary counters and threshold predicates, matching AC^0's parallel counting capabilities.
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2.
Unordered structures prevent FO from expressing majority functions and symmetric predicates requiring global comparisons.
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3.
This aligns with proven results: FO+LO captures exactly AC^0, while FO alone on unordered domains has strictly lower expressiveness.
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