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EconCSLib.MechanismDesign.Auction.OptimalSingleItem

EconCSLib.MechanismDesign.Auction.OptimalSingleItem #

Regular Myerson optimal single-item auctions.

This file defines virtual values, the virtual-surplus-maximizing allocation rule, its Myerson payment rule, and the IC/IR revenue-optimality interface for MSZ 12.59. The main public result is virtualSurplusMaximizingAuction_regularMyersonOptimalICIR_of_isRegular.

Structure hierarchy #

BayesianSingleItemAuction I                       -- direct Bayesian single-item layer
  ├─ virtualSurplusMaximizingMechanism            -- SingleParameterMechanism view
  └─ virtualSurplusMaximizingAuction              -- BayesianSingleItemAuction view

SingleParameterMechanism.withMyersonPayment       -- payment formula used by the optimizer

Main definitions #

Main proofs #

References:

Virtual values and regularity #

noncomputable def BayesianSingleItemAuction.virtualValue {I : Type u_1} (A : BayesianSingleItemAuction I) (i : I) (v : ) :

Myerson virtual value v - (1 - F_i(v)) / f_i(v) for bidder i.

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    Regularity: every bidder's virtual value is monotone.

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      Regular virtual values are measurable by Mathlib's measurability theorem for monotone real functions.

      A reserve threshold separating nonpositive and nonnegative virtual values.

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        theorem BayesianSingleItemAuction.virtualValue_nonneg_of_isReserveThreshold {I : Type u_1} (A : BayesianSingleItemAuction I) {i : I} {reserve v : } (hreserve : A.IsReserveThreshold i reserve) (hv : reserve v) :

        At a reserve threshold, the virtual value is nonnegative.

        theorem BayesianSingleItemAuction.virtualValue_nonpos_of_lt_isReserveThreshold {I : Type u_1} (A : BayesianSingleItemAuction I) {i : I} {reserve v : } (hreserve : A.IsReserveThreshold i reserve) (hv : v < reserve) :

        Below a reserve threshold, the virtual value is nonpositive.

        A zero of a regular virtual value is a reserve threshold.

        noncomputable def BayesianSingleItemAuction.virtualSurplus {I : Type u_1} [Fintype I] (A : BayesianSingleItemAuction I) (x : (I)I) (b : I) :

        Virtual surplus of an allocation rule at a reported type profile.

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          noncomputable def BayesianSingleItemAuction.expectedVirtualSurplus {I : Type u_1} [Fintype I] (A : BayesianSingleItemAuction I) (x : (I)I) :

          Ex-ante expected virtual surplus under the auction prior.

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            Integrability condition for ex-ante virtual surplus under the auction prior.

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              Pointwise virtual-surplus dominance lifts to ex-ante dominance.

              A nonnegative fractional allocation rule with total mass at most 1.

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                theorem BayesianSingleItemAuction.IsSingleItemAllocationRule.le_one {I : Type u_1} [Fintype I] [DecidableEq I] {x : (I)I} (hx : IsSingleItemAllocationRule x) (b : I) (i : I) :
                x b i 1

                A feasible single-item allocation gives each bidder probability at most 1.

                theorem BayesianSingleItemAuction.withPayment_isAllocFeasible_of_isSingleItemAllocationRule {I : Type u_1} [Fintype I] [DecidableEq I] {x p : (I)I} (hx : IsSingleItemAllocationRule x) :
                { allocationRule := x, paymentRule := p }.IsAllocFeasible

                A feasible single-item allocation rule satisfies IsAllocFeasible with any payment rule.

                Feasible allocation rules that pointwise maximize virtual surplus.

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                  Deterministic virtual-surplus maximization #

                  noncomputable def BayesianSingleItemAuction.virtualScore {I : Type u_1} [LinearOrder I] (A : BayesianSingleItemAuction I) (b : I) (i : I) :
                  Lex ( × I)

                  Lexicographic score used to break ties among virtual values.

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                    The virtual-value winner with deterministic tie-breaking.

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                      The selected winner is exactly the bidder whose lexicographic virtual score dominates every other bidder's score.

                      The virtual value attained by the selected virtual-value winner.

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                        Allocates to the highest positive virtual value, otherwise withholds the item.

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                          The virtual-surplus allocation rule paired with its Myerson payment.

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                            The Bayesian auction using the virtual-surplus mechanism and the original priors.

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                              Allocation-rule facts and virtual-surplus maximality #

                              The selected winner has maximal virtual value.

                              theorem BayesianSingleItemAuction.virtualSurplus_le_virtualSurplusMaximizingAllocationRule {I : Type u_1} [Fintype I] [Nontrivial I] [DecidableEq I] [LinearOrder I] (A : BayesianSingleItemAuction I) {x : (I)I} {b : I} (hx_nonneg : ∀ (i : I), 0 x b i) (hx_capacity : i : I, x b i 1) :

                              Pointwise virtual-surplus maximality.

                              Reserve-price interpretation of the allocation rule #

                              The winning virtual value is positive iff some virtual value is positive.

                              The winning virtual value is nonpositive iff all virtual values are.

                              Allocation probability 1 characterizes the positive virtual-value winner.

                              A bidder allocated probability 1 has positive virtual value.

                              An allocated bidder reports at least her reserve threshold.

                              A sale occurs exactly when some bidder has positive virtual value.

                              theorem BayesianSingleItemAuction.exists_reserveThreshold_le_bid_of_exists_virtualSurplusMaximizingAllocationRule_eq_one {I : Type u_1} [Fintype I] [Nontrivial I] [DecidableEq I] [LinearOrder I] (A : BayesianSingleItemAuction I) {reserve b : I} (hreserve : ∀ (i : I), A.IsReserveThreshold i (reserve i)) (hsale : ∃ (i : I), A.virtualSurplusMaximizingAllocationRule b i = 1) :
                              ∃ (i : I), reserve i b i

                              If a sale occurs, some bidder reaches her reserve threshold.

                              A positive virtual-value winner reaches her reserve threshold.

                              theorem BayesianSingleItemAuction.exists_reserveThreshold_le_bid_of_winningVirtualValue_pos {I : Type u_1} [Fintype I] [Nontrivial I] [DecidableEq I] [LinearOrder I] (A : BayesianSingleItemAuction I) {reserve b : I} (hreserve : ∀ (i : I), A.IsReserveThreshold i (reserve i)) (hpos : 0 < A.winningVirtualValue b) :
                              ∃ (i : I), reserve i b i

                              Positive winning virtual value implies some report reaches reserve.

                              theorem BayesianSingleItemAuction.winningVirtualValue_nonpos_of_forall_lt_reserveThreshold {I : Type u_1} [Fintype I] [Nontrivial I] [LinearOrder I] (A : BayesianSingleItemAuction I) {reserve b : I} (hreserve : ∀ (i : I), A.IsReserveThreshold i (reserve i)) (hb : ∀ (i : I), b i < reserve i) :

                              If all reports are below reserve, the winning virtual value is nonpositive.

                              theorem BayesianSingleItemAuction.not_forall_lt_reserveThreshold_of_winningVirtualValue_pos {I : Type u_1} [Fintype I] [Nontrivial I] [LinearOrder I] (A : BayesianSingleItemAuction I) {reserve b : I} (hreserve : ∀ (i : I), A.IsReserveThreshold i (reserve i)) (hpos : 0 < A.winningVirtualValue b) :
                              ¬∀ (i : I), b i < reserve i

                              Positive winning virtual value rules out all reports below reserve.

                              If all virtual values are nonpositive, withhold the item.

                              theorem BayesianSingleItemAuction.virtualSurplusMaximizingAllocationRule_eq_zero_of_forall_lt_reserveThreshold {I : Type u_1} [Fintype I] [Nontrivial I] [DecidableEq I] [LinearOrder I] (A : BayesianSingleItemAuction I) {reserve b : I} (hreserve : ∀ (i : I), A.IsReserveThreshold i (reserve i)) (hb : ∀ (i : I), b i < reserve i) :

                              If all reports are below reserve thresholds, withhold the item.

                              Withholding is equivalent to all virtual values being nonpositive.

                              Feasibility #

                              Measurability helpers #

                              Allocation-rule optimality #

                              The deterministic virtual-surplus-maximizing allocation rule is feasible and pointwise maximizes virtual surplus among all feasible fractional single-item allocation rules.

                              The allocation rule of the Myerson-payment mechanism is virtual-surplus optimal. This exposes the optimal-allocation content at the mechanism layer, while payments continue to be supplied by MechanismDesign.Auction.Myerson.

                              Ex-ante virtual-surplus optimality of the deterministic rule, conditional on the explicit integrability assumptions needed by the Bochner integral.

                              The allocation rule of the constructed Bayesian auction is ex-ante virtual-surplus optimal under the same explicit integrability assumptions.

                              Zero normalization for a Bayesian single-item auction's payment rule: reporting 0 gives payment 0, holding the other reports fixed.

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                                A zero-normalized payment rule has zero interim expected payment at type 0.

                                Under the interim envelope formula and nonnegative envelope increments, zero normalization implies support-restricted interim IR.

                                Monotonicity and Myerson implementation #

                                Under regularity, the deterministic virtual-surplus-maximizing allocation rule is monotone in the sense required by SingleParameterMechanism.

                                The regular deterministic virtual-surplus-maximizing mechanism is DSIC by the existing Myerson payment theorem.

                                The constructed Bayesian single-item auction is DSIC after forgetting to the underlying single-parameter mechanism.

                                Myerson payment formula and uniqueness #

                                The constructed Myerson payment is bounded by twice the absolute report.

                                This uses only the pointwise allocation bounds 0x1; it is the boundedness half of the interim-integrability proof route.

                                For each fixed report, the constructed Myerson payment integrand is bounded uniformly over opponent profiles.

                                Once the constructed allocation and payment integrands are measurable, boundedness gives interim integrability for the constructed auction.

                                Main mechanism-level package theorem for the regular case: the virtual-surplus-maximizing mechanism uses a virtual-surplus-optimal allocation rule and is DSIC with the canonical Myerson payment rule.

                                Main auction-level package theorem for the regular case: the constructed Bayesian single-item auction has a virtual-surplus-optimal allocation rule, is feasible, and is DSIC after forgetting to the underlying single-parameter mechanism.

                                Support and density assumptions for the analytic identity layer #

                                Agent i's type lies in [0, ωᵢ].

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                                  A type profile lies in the product support.

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                                    Densities are positive on support interiors.

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                                      Densities are nonnegative on supports.

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                                        The joint density is positive at a profile.

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                                          A profile lies in the interior of the product support.

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                                            theorem BayesianSingleItemAuction.survivalIntegral_eq_intervalIntegral_mul_deriv {F Q : } {ω : } ( : 0 ω) (hF : AbsolutelyContinuousOnInterval F 0 ω) (hF0 : F 0 = 0) (hFω : F ω = 1) (hQ : IntervalIntegrable Q MeasureTheory.volume 0 ω) :
                                            (v : ) in 0..ω, Q v * (1 - F v) = (v : ) in 0..ω, ( (z : ) in 0..v, Q z) * deriv F v

                                            Integration by parts for the survival term.

                                            CDF assumptions for the interim virtual-surplus comparison.

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                                              Environment-level analytic assumptions for the envelope/virtual-surplus comparison.

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                                                Abstract expected-revenue optimality interface #

                                                Ex-ante virtual surplus through interim allocation probabilities.

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                                                  Fubini hypotheses connecting ex-ante and interim expressions.

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                                                    Type-measure Fubini hypotheses.

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                                                      theorem BayesianSingleItemAuction.typeMeasureInterimFubiniAnalyticAssumptions_of_typeMeasure_fubini {I : Type u_1} [Fintype I] {A B : BayesianSingleItemAuction I} (hdens_ae : ∀ (i : I), ∀ᵐ (v : ) MeasureTheory.volume.restrict (Set.Ioc 0 (A.typeData.omega i)), 0 A.typeDensity i v) (hpay_int : ∀ (i : I), MeasureTheory.Integrable (fun (t : I) => B.paymentRule t i) A.prior) (hvs_int : ∀ (i : I), MeasureTheory.Integrable (fun (t : I) => B.allocationRule t i * A.virtualValue i (t i)) A.prior) (hpay_fubini : ∀ (i : I), (t : I), B.paymentRule t i A.prior = (v : ), B.interimExpectedPayment i v A.typeMeasure i) (hvs_fubini : ∀ (i : I), (t : I), B.allocationRule t i * A.virtualValue i (t i) A.prior = (v : ), B.interimAllocProb i v * A.virtualValue i v A.typeMeasure i) :

                                                      Interim-payment revenue is bounded by interim virtual surplus.

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                                                        The envelope-term upper bound for interim expected payments.

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                                                          Envelope-term revenue is bounded by interim virtual surplus.

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                                                            Product-side virtual-surplus integrability gives interval integrability of the density-weighted interim virtual surplus.

                                                            Product-side virtual-surplus integrability gives ex-ante virtual-surplus integrability under independent priors.

                                                            Build type-measure Fubini hypotheses from independent priors and product-side integrability. This is the main bridge from the full prior to interim one-dimensional expressions.

                                                            Analytic assumptions for MSZ 12.59.

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                                                              Expected seller-revenue optimality among candidates.

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                                                                Regular Myerson optimality among feasible IC/IR mechanisms.

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                                                                  theorem BayesianSingleItemAuction.hasInterimPaymentEnvelopeUpperBound_of_pointwise {I : Type u_1} [Fintype I] (A B : BayesianSingleItemAuction I) (hdens_ae : ∀ (i : I), ∀ᵐ (v : ) MeasureTheory.volume.restrict (Set.Ioc 0 (A.typeData.omega i)), 0 A.typeDensity i v) (hint_pay : ∀ (i : I), IntervalIntegrable (fun (v : ) => B.interimExpectedPayment i v * A.typeDensity i v) MeasureTheory.volume 0 (A.typeData.omega i)) (hint_env : ∀ (i : I), IntervalIntegrable (fun (v : ) => (B.interimAllocProb i v * v - (z : ) in 0..v, B.interimAllocProb i z) * A.typeDensity i v) MeasureTheory.volume 0 (A.typeData.omega i)) (hpoint : ∀ (i : I) (v : ), 0 vv A.typeData.omega iB.interimExpectedPayment i v B.interimAllocProb i v * v - (z : ) in 0..v, B.interimAllocProb i z) :