Deterministic quasipolynomial-time approximation schemes
Lax263307.QPTAS · concepts/Lax263307/QPTAS.lean · lax-263307
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Definition
A deterministic QPTAS for maximum-weight independent set on a class uses one finite program, depending only on , which returns a -approximation on every member with rational weights and rational . Its running time has the form
where is the binary length of the weights and accuracy. Neither class recognition nor a separator is supplied as input.
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| 1 | import Lax263307.GraphClasses |
| 2 | import Lax263307.WeightedIndependentSet |
| 3 | import Lax808846.Ram |
| 4 | import Mathlib.Data.Nat.Log |
| 5 | |
| 6 | /-! |
| 7 | --- |
| 8 | title: Deterministic quasipolynomial-time approximation schemes |
| 9 | type: definition |
| 10 | --- |
| 11 | A deterministic QPTAS for maximum-weight independent set on a class |
| 12 | uses one finite program, depending only on , |
| 13 | which returns a -approximation on every member with |
| 14 | rational weights and rational . |
| 15 | Its running time has the form |
| 16 | |
| 17 | |
| 18 | where is the binary length of the weights and accuracy. |
| 19 | Neither class recognition nor a separator is supplied as input. |
| 20 | |
| 21 | # Formalization notes |
| 22 | |
| 23 | Execution is `Lax808846.Ram.RunsTo` on the registered word RAM. |
| 24 | The supplied input is exactly `encodeInput`, with no implicit length prefix. |
| 25 | The constants and program precede the input and word width in the quantifiers. |
| 26 | The natural-number time bound uses and |
| 27 | to cover small inputs. |
| 28 | |
| 29 | The same bound is a sufficient word width, and the program must work at |
| 30 | every larger width. In particular it works at that explicitly prescribed |
| 31 | width; there is no advice hidden in a choice of width. Allowing this many |
| 32 | bits accommodates exact rational subroutines. Simulating these bounded-word |
| 33 | operations with bit operations costs a polynomial in the time bound, |
| 34 | which remains of the displayed form after increasing its constants and |
| 35 | exponent: . Thus the additive polynomial |
| 36 | dependence on is preserved. Correctness and time are promised only |
| 37 | on well-formed inputs whose graphs belong to . |
| 38 | -/ |
| 39 | |
| 40 | namespace Lax263307.QPTAS |
| 41 | |
| 42 | open GraphClasses WeightedIndependentSet Lax808846.Ram |
| 43 | |
| 44 | /-- An explicit bound representing the paper's two-parameter running time. -/ |
| 45 | def timeBound (c d n ℓ : ℕ) (ε : ℚ) : ℕ := |
| 46 | 2 ^ (c * (⌈ε⁻¹⌉₊ * (Nat.log2 (n + 2) + 1)) ^ d) + c * (ℓ + 1) ^ d |
| 47 | |
| 48 | /-- One uniform deterministic word-RAM program realizes the approximation scheme. -/ |
| 49 | def HasQPTAS (C : GraphClass) : Prop := |
| 50 | ∃ (program : Program) (c d : ℕ), 0 < c ∧ 0 < d ∧ |
| 51 | ∀ (n : ℕ) (G : SimpleGraph (Fin n)) [DecidableRel G.Adj], |
| 52 | G ∈ C n → ∀ (weights : Fin n → ℚ) (ε : ℚ), 0 < ε → ε < 1 → |
| 53 | let T := timeBound c d n (rationalBitSize weights ε) ε |
| 54 | ∀ wordLength : ℕ, T ≤ wordLength → |
| 55 | ∃ I : Finset (Fin n), IsApproximation G weights ε I ∧ |
| 56 | ∃ t ≤ T, RunsTo wordLength program |
| 57 | (encodeInput G weights ε) (encodeOutput I) t |
| 58 | |
| 59 | end Lax263307.QPTAS |
| 60 |
Formalization notes
Execution is on the registered word RAM. The supplied input is exactly , with no implicit length prefix. The constants and program precede the input and word width in the quantifiers. The natural-number time bound uses and to cover small inputs.
The same bound is a sufficient word width, and the program must work at every larger width. In particular it works at that explicitly prescribed width; there is no advice hidden in a choice of width. Allowing this many bits accommodates exact rational subroutines. Simulating these bounded-word operations with bit operations costs a polynomial in the time bound, which remains of the displayed form after increasing its constants and exponent: . Thus the additive polynomial dependence on is preserved. Correctness and time are promised only on well-formed inputs whose graphs belong to .
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