Provedvariational-cost-lawproposalreview-response
Excess-cost factorisation: with B = (Σ√A)² and r* ∝ √A, Σ_j A_j/r_j = B·Σ_j r*_j²/r_j = B·(1 + χ²(r*‖r)) for every proposal r
For all and , if , and (given and ), then
What the notation means · or point at any symbol
- Type
- Type: a definition from the library, not special notation. Its name is its meaning; open it in Lean to see how it is defined. (
Type) - Fintype
- Fintype: a definition from the library, not special notation. Its name is its meaning; open it in Lean to see how it is defined. (
Fintype) - f(x)Function application
- Fintype applied to J. Lean writes f x; papers write f(x). (
Fintype J) - Nonempty
- Nonempty: a definition from the library, not special notation. Its name is its meaning; open it in Lean to see how it is defined. (
Nonempty) - ⟹Implies
- If J, then ℝ. (Between types, A → B is the type of functions from A to B.) (
J → ℝ) - x : TVariables
- A, r range over J → ℝ: the statement must hold for every choice. (
(A r : J → ℝ)) - ∀For all
- The statement after the comma holds for every j. (
∀ j, 0 < A j) - ∑Summation
- Add up r(j) for every j. (
∑ j, r j) - =Equality
- ∑ j, r(j) and 1 are the same. (
∑ j, r j = 1) - ∧And
- Both hold: (∑ j, A(j)/r(j)) = (∑ k, √(A(k)))^2 · (∑ j, (√(A(j))/(∑ k,…, and (∑ j, (√(A(j))/(∑ k, √(A(k))))^2/r(j)) = 1 + (∑ j, (√(A(j)…. (
∑ j, A j / r j = (∑ k, Real.sqrt (A k))…) - RReal numbers
- The real numbers: every point on the number line. (
ℝ) - <Less than
- 0 is strictly smaller than A(j). (
0 < A j) - a/bDivision
- A(j) divided by r(j). In Lean, dividing by 0 gives 0. (
A j / r j) - aᵇPower
- ∑ k, √(A(k)) raised to the power 2. (
(∑ k, Real.sqrt (A k)) ^ 2) - ·Multiplication
- (∑ k, √(A(k)))^2 times ∑ j, (√(A(j))/(∑ k, √(A(k))))^2/r(j). (
(∑ k, Real.sqrt (A k)) ^ 2 * ∑ j, (Real…) - √Square root
- The non-negative number whose square is A(k). (
Real.sqrt (A k)) - +Addition
- 1 plus ∑ j, (√(A(j))/(∑ k, √(A(k))) − r(j))^2/r(j). (
1 + ∑ j, (Real.sqrt (A j) / ∑ k, Real.s…) - -Subtraction
- √(A(j))/(∑ k, √(A(k))) minus r(j). On ℕ, Lean's subtraction stops at 0: 2 − 5 = 0. (
Real.sqrt (A j) / ∑ k, Real.sqrt (A k) …)
theorem vcl-excess-cost-factorisation {J : Type} [Fintype J] [Nonempty J] (A r : J → ℝ) (hA : ∀ j, 0 < A j) (hr : ∀ j, 0 < r j) (hrs : ∑ j, r j = 1) :
∑ j, A j / r j = (∑ k, Real.sqrt (A k)) ^ 2 * ∑ j, (Real.sqrt (A j) / ∑ k, Real.sqrt (A k)) ^ 2 / r j ∧ ∑ j, (Real.sqrt (A j) / ∑ k, Real.sqrt (A k)) ^ 2 / r j = 1 + ∑ j, (Real.sqrt (A j) / ∑ k, Real.sqrt (A k) - r j) ^ 2 / r jAn exact multiplicative decomposition of a proposal's expected cost into what remains difficult given the features (B, the variational optimum) and the failure to learn the best proposal available from them (1 + χ²(r*‖r)). Suggested by an external reviewer (2026-10-05) as the precise form of a distinction the paper made informally.
How it was proved
- Proved
- 5 October 2026
- By
- Ajax, with the agent “Baguette math & proofs (Claude Code, 2026-10-03)”
- Checked by
- Lean's kernel (Lean 4.33.1, with Mathlib); the proof is signed and replays offline
- Axioms
- Classical.choice, Quot.sound, propext
- Proof
- 27 lines of Lean
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