Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems The Fundamental Theorem of Asset Pricing under Transaction Costs Paolo Guasoni (joint work with Miklós Rásonyi) Boston University Department of Mathematics and Statistics
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Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
The Fundamental Theorem of Asset Pricingunder Transaction Costs
Paolo Guasoni(joint work with Miklós Rásonyi)
Boston UniversityDepartment of Mathematics and Statistics
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Overview
ModelBid and Ask Prices in continuous time. Jumps allowed.Theorem
(Robust No Free Lunch with Vanishing Risk)m
(Exists Strictly Consistent Price System)
Getting there: what is an admissible strategy?Consequences
(RNFLVR)⇒ Finite variation strategies.No stochastic integrals.Do we need a probability?
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Model
One risky and one risk-free asset.Risk-free asset as numeraire.Risky asset: Bid price St − κt , Ask price St + κt .Prices may become negative.Numeraire does matter.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Simple Strategies
Definition
Simple strategy: θ predictable, θ0 = θT = 0, and:
θ =∞∑
n=1
(θσn1JσnK + θσ+
n1Kσn,σn+1J
)
(σn)n≥1 strictly increasing stopping times.supn≥1 σn > T a.s., that is P(∪n≥1σn > T) = 1.
Finite number of transactions. May depend on ω.Doubling Strategies?Left and Right Transactions.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Left and Right Transactions
Right transaction at a stopping time σ and price (S ± κ)σ.Trade “when market opens”.
qaLeft transaction at a predictable time σ and price (S±κ)σ− .Trade “before market closes”.
aqIn general two transactions:
aqaBoth right and left transactions considered simple.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Cost
Definition
Cost of a simple strategy θ:
C(θ) =∞∑
n=1
[(S + κ)σ−n
(θσn − θσ+n−1
)+ + (S + κ)σn (θσ+n− θσn )+
]−∞∑
n=1
[(S − κ)σ−n
(θσn − θσ+n−1
)− + (S − κ)σn (θσ+n− θσn )−
]
Purchases minus sales, for left and right transactions.Terminal value V (θ) = −C(θ).
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
What is an Admissible Strategy?
Numeraire-free version. For some c > 0:
V (θ) ≥ −c(1 + ST )
Too loose:
Not the usual definition. Martingales vs. Local martingales.Leverage without collateral. c(ST − S0) admissible.Many banks still alive...
Naïve definition. For some c > 0:
V (θ1[0,t]) ≥ −c for all t ∈ [0,T ]
Too strict:
Payoff space not closed. Forget separation arguments.No leverage with markets closed.All banks dead.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Freeze, Wait, Close
You cannot trade your way out of losses.Anytime, the broker can freeze the account, and wait for agood time to close risky positions, for a bounded loss.A simple strategy θ is admissible if and only if, after everytransaction, there exists a liquidation time.Continuous prices (or totally inaccessible jumps):for all t , there exists a stopping time t ≤ τ ≤ T such thatV (θ1[0,t] + θt1Kt ,τK) + x ≥ 0 for some x > 0.Accessible jumps allowed:Both freeze and liquidation either left or right. Four cases.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Four Cases
Right Freeze and Right Close.
aqa qaRight Freeze and Left Close.
aqa aqLeft Freeze and Right Close.
aq qaLeft Freeze and Left Close.
aq aq
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Definitionθ simple x-admissible if, for all k ≥ 0, there exists a liquidationstrategy kθ, such that:
i) kθ = θ·∧k1·<λk for some F-stopping time λk > k a.s.(liquidation time).
ii) x + V (kθ) ≥ 0.
Reduces to frictionless definition for κ = 0.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
No Simple Arbitrage
DefinitionSimple arbitrage:θ ∈ As such that P(V (θ) ≥ 0) = 1 and P(V (θ) > 0) > 0.(NA-S):θ ∈ As and P(V (θ) ≥ 0) = 1 implies that V (θ) = 0.
Proposition
If (NA-S) holds, then Asx = θ ∈ As : x + V (θ) ≥ 0 a.s..
Admissibility of θ depends on final payoff only.Key property to obtain closedness of admissible payoffs.⊂ easy. ⊃ far less so.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
The Frictionless Story
Frictionless markets: κ = 0.(1) (NFLVR) for Simple Strategies
⇓S is a semimartingale
⇓Payoffs of general strategies as stochastic integrals
∫θdS
(2) (NFLVR) for General Strategies⇓
Equivalent Local Martingale Measure“The use of general integrands however seems moredifficult to interpret and their use can be questioned ineconomic models” (Delbaen and Schachemayer, 1994)
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Payoffs as Integrals
Frictionless payoffs:∫θdS stochastic integrals.
Approximations.θ is x-admissible. (x + ε)-admissible θn with |θ − θn| < ε?No, in general.Model misspecifications.If S and S′ are close, are
∫θ dS and
∫θ dS′ close?
No, again.Needs underlying probability. Why?Troubling properties.Only simple strategies concrete.No probability in accounting.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
(Robust) No Free Lunch with Vanishing Risk
Definition(S, κ) satisfies
i) (NFLVR) if, for any sequence (θn)n≥1 such that θn ∈ As1/n
and V (θn) converges a.s. to some limit V , then V = 0 a.s.
ii) (RNFLVR) if, there exists (S′, κ′) satisfying (NFLVR), andthe bid-ask spread of (S′, κ′) is within that of (S, κ):
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
A Path Downhill
Understanding admissibility and value as main problems.Kreps-Yan theorem: separating measure.Sandwich martingale within bid and ask.Well-known path(Jouini-Kallal, Cherny, Choulli-Stricker)New admissibility: supermartingale property?
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Consistent Price Systems
DefinitionStrictly Consistent Price System (SCPS): pair (M,Q) ofprobability Q equivalent to P andQ-local martingale M within bid-ask spread:
inft∈[0,T ]
(κt − |St −Mt |) > 0 a.s.
Consistent Price System (CPS) if inequality not strict.
Proposition
EQ[V (M,0)(θ)] ≤ 0 for any CPS (M,Q) and θ ∈ A.
Analogue of supermartingale property.(SCPS)⇒ (RNFLVR) clear.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
From Separating Measure to CPS
Lemma
(Xt )t∈[0,T ] and (Yt )t∈[0,T ] be two càdlàg processes.The following conditions are equivalent:
i) There exists a càdlàg martingale (Mt )t∈[0,T ] such that:
X ≤ M ≤ Y a.s.
ii) For all stopping times σ, τ such that 0 ≤ σ ≤ τ ≤ T a.s.:
E [Xτ | Fσ] ≤ Yσ and E [Yτ | Fσ] ≥ Xσ a.s.
ii)⇒ i) delivers CPS from separating measure.
Outline Simple Strategies (RNFLVR) Predictable Stieltjes Integrals Consistent Price Systems
Conclusion
Bid and ask prices moving freely.Value? Admissibility? Arbitrage? Finite Variation?The Fundamental Theorem as a tool to understand.Left and Right Transactions.Admissibility: freeze, wait and close. Anytime.Robust no free lunches and finite variation.