Interpolate degree ge of no data
KZG.CommitmentScheme.interpolate_degree_ge_of_no_data
Plain-language statement
If no degree-n coefficient vector fits the data, interpolation has degree at least n + 1.
Exact Lean statement
lemma interpolate_degree_ge_of_no_data {n L : ℕ} (S : Finset (Fin L))
{queryOf responseOf : Fin L → ZMod p}
(hquery : Set.InjOn queryOf ↑S)
(hNoData : ¬ ∃ d : Fin (n + 1) → ZMod p,
∀ i ∈ S, (CPolynomial.ofFn d).eval (queryOf i) = responseOf i) :
(↑(n + 1) : WithBot ℕ) ≤
(CLagrange.interpolate S queryOf responseOf).degreeFormal artifact
Lean source
lemma interpolate_degree_ge_of_no_data {n L : ℕ} (S : Finset (Fin L)) {queryOf responseOf : Fin L → ZMod p} (hquery : Set.InjOn queryOf ↑S) (hNoData : ¬ ∃ d : Fin (n + 1) → ZMod p, ∀ i ∈ S, (CPolynomial.ofFn d).eval (queryOf i) = responseOf i) : (↑(n + 1) : WithBot ℕ) ≤ (CLagrange.interpolate S queryOf responseOf).degree := by by_contra hlt push Not at hlt set Q : Polynomial (ZMod p) := Lagrange.interpolate S queryOf responseOf with hQ_def have hQdeg_lt : Q.degree < (↑(n + 1) : WithBot ℕ) := by have h := hlt rw [show (CLagrange.interpolate S queryOf responseOf).degree = Q.degree from by rw [hQ_def, ← CLagrange.cinterpolate_eq_interpolate, ← degree_toPoly]] at h exact h have hQ_mem : Q ∈ Polynomial.degreeLT (ZMod p) (n + 1) := Polynomial.mem_degreeLT.mpr hQdeg_lt apply hNoData refine ⟨Polynomial.degreeLTEquiv (ZMod p) (n + 1) ⟨Q, hQ_mem⟩, ?_⟩ intro i hi have hQ_eval : Q.eval (queryOf i) = responseOf i := by rw [hQ_def] exact Lagrange.eval_interpolate_at_node responseOf hquery hi have hQ_sum : Q.eval (queryOf i) = ∑ k : Fin (n + 1), Polynomial.degreeLTEquiv (ZMod p) (n + 1) ⟨Q, hQ_mem⟩ k * (queryOf i) ^ (k : ℕ) := Polynomial.eval_eq_sum_degreeLTEquiv hQ_mem (queryOf i) set d : Fin (n + 1) → ZMod p := Polynomial.degreeLTEquiv (ZMod p) (n + 1) ⟨Q, hQ_mem⟩ with hd_def let P_C : CPolynomial (ZMod p) := ⟨(CompPoly.CPolynomial.Raw.mk (Array.ofFn d)).trim, CompPoly.CPolynomial.Raw.Trim.isCanonical_trim _⟩ change CPolynomial.eval (queryOf i) P_C = responseOf i rw [eval_toPoly] have hPC_eq : P_C.toPoly = Q := by apply Polynomial.ext intro k rw [← coeff_toPoly] change ((CompPoly.CPolynomial.Raw.mk (Array.ofFn d)).trim).coeff k = Q.coeff k rw [CompPoly.CPolynomial.Raw.Trim.coeff_eq_coeff] change (Array.ofFn d).getD k 0 = Q.coeff k rw [Array.getD_eq_getD_getElem?, Array.getElem?_ofFn] by_cases hk : k < n + 1 · simp [hk, hd_def, Polynomial.degreeLTEquiv] · push Not at hk simp only [hk.not_gt, dite_false, Option.getD_none] symm exact Polynomial.coeff_eq_zero_of_degree_lt (lt_of_lt_of_le hQdeg_lt (by exact_mod_cast hk)) rw [hPC_eq] exact hQ_eval- Project
- ArkLib
- License
- Apache-2.0
- Commit
- fad5cbf80877
- Source
- ArkLib/Commitments/Functional/KZG/FunctionBinding/DegreeConflict.lean:87-144
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This lemma proves the final algebraic step in the DG25 Theorem 3.1 proof. It shows that if R > e + 1, then e * (R / (R - 1)) < e + 1. The intuition is that the fraction R / (R - 1) is always greater than 1, but as R gets larger, it gets closer to 1. The hypothesis R > e + 1 provides a strong enough bound to ensure the product e * (fraction) do...
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Person-level attribution pending.
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Person-level attribution pending.
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Plain-language statement
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