The matrix of the constructed instance #
What the matrix of the QSAT instance built by the Savitch reduction says, clause tag by clause tag.
A clause is satisfied when one of the literals of its list
(DescriptiveComplexity.qLits) is (DescriptiveComplexity.ClSat), and a literal
is satisfied when the variable its link points at gets the literal's sign
(DescriptiveComplexity.LitSat). Since every link pins the coordinates of that
variable, the existential quantifier of a literal collapses
(DescriptiveComplexity.litSat_atP and its companions, one per
DescriptiveComplexity.QLink), and each clause tag turns into a plain condition
on what the valuation reads (DescriptiveComplexity.clSat_cSrc and its
companions).
A literal of a clause sitting at (p, q) is satisfied by the valuation
σ: the variable its link points at gets its sign.
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A clause of the constructed instance is satisfied by the valuation σ.
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- DescriptiveComplexity.ClSat σ c p q = ∃ l ∈ DescriptiveComplexity.qLits c, DescriptiveComplexity.LitSat σ p q l
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The matrix of the constructed instance: every guarded clause tag is satisfied.
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Collapsing the quantifier of a literal #
Every link pins the coordinates of the variable it points at, except the two that range over the literals of a clause of the input.
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What each clause says #
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A copy of a source clause reads its literals on the source valuation.
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A copy of a target clause reads its literals on the target valuation.
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A copy of a transition clause reads its literals on the transition valuation, unless the base case is the equality branch.
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The source valuation reads the source endpoint.
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The target valuation reads the target endpoint.
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The transition valuation reads the first component of the bottom pair.
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The transition valuation writes the second component of the bottom pair.
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The equality branch of the base case.
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What one of the eight clauses of a level says: the branch of the universal bit, the component of the pair it constrains, and the value that component must copy – the midpoint, or the endpoint the level receives from above.
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A level clause says exactly what DescriptiveComplexity.LevSat says.