IEOR 4706: Foundations of financial engineering.
Foundations of financial engineering.
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IEOR 4706: Foundations of financial engineering.
Instructions:
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1
Exercice 1: Super–replication under interest rate and volatility uncertainty
We consider a one–period binomial, that is to say we work on the space Ω := {ωu, ωd}, which we endow with the
σ−algebra F := {∅,Ω, {ωu}, {ωd}}. The filtration considered is F := (F0,F1), with F0 := {∅,Ω}, and F1 := F . The
market contains one risky asset whose price evolves as follows
S0
dS0
P[{ω d}] = 1− p
uS0
P[{ω
u}] = p
where p ∈ (0, 1) and 0 < d < u. There is also one non–risky asset, whose price at time t is given by S0t := (1 + r)t, t =
0, 1. However, unlike the model considered in the lectures, we assume that the true values of the parameters r, u and
d are not known perfectly, and that the only information that we have is that they lie in intervals with known bounds.
More precisely, we have
0 ≤ r ≤ r ≤ r, 0 < u ≤ u ≤ u, and 0 < d ≤ d ≤ d,
where the bounds are known explicitly. We moreover assume that
u > d.
To take this uncertainty into account, we consider a family of probability measures on (Ω,F), denoted by P, such
that
P := {P : ∃(r, d, u) ∈ [r, r]× [d, d]× [u, u], P[{S1 = uS0}] = p, P[{S1 = dS0}] = 1− p, P[{S01 = 1 + r}] = 1}.
Every P ∈ P thus represents one possible binomial model with parameters (r, d, u) ∈ [r, r]× [d, d]× [u, u].
1) Prove that if d < 1 + r, and 1 + r < u, then we have that for any P ∈ P, the usual Condition (NA) holds under the
measure P.1 We will assume that these inequalities hold throughout the rest of the exercise.
2) Let us consider an option with payoff h(S1). Our goal will be to compute its super–replication price, defined in this
context by
p
(
h(S1)
)
:= inf
{
x ∈ R : ∃∆ ∈ R,P[{Xx,∆1 ≥ h(S1)}] = 1, ∀P ∈ P}.
Show that if the self–financing portfolio Xx,∆ super–replicates the option, then we necessarily have that for any
(r, d, u) ∈ [r, r]× [d, d]× [u, u] {
∆uS0 + (1 + r)(x−∆S0) ≥ h(uS0),
∆dS0 + (1 + r)(x−∆S0) ≥ h(dS0).
3) Deduce that if the self–financing portfolio Xx,∆ super–replicates the option, then we necessarily have that for any
(r, d, u) ∈ [r, r]× [d, d]× [u, u]
h(uS0)− (1 + r)x
S0(u− 1− r) ≤ ∆ ≤
(1 + r)x− h(dS0)
S0(1 + r − d) ,
and then that x ≥ max
(r,d,u)∈[r,r]×[d,d]×[u,u]
f(r, d, u), where we defined
f(r, d, u) := 11 + r
(
1 + r − d
u− d h(uS0) +
u− 1− r
u− d h(dS0)
)
.
1We recall that Condition (NA) under P stipulates that for any ∆ ∈ R, P[X0,∆1 ≥ 0] = 1 =⇒ P[X0,∆1 = 0] = 1.
2
4) Prove that for any (d, u) ∈ [d, d]× [u, u]
max
r∈[r,r]
f(r, d, u) =
{
f(r, d, u), if dh(uS0) ≥ uh(dS0),
f(r, d, u), if dh(uS0) < uh(dS0).
We denote the corresponding point where the maximum is attained by r?(d, u), with
r?(d, u) :=
{
r, if dh(uS0) ≥ uh(dS0),
r, if dh(uS0) < uh(dS0).
5)(?) Assume now that there exists a pair (u?, d?) ∈ [u, u]× [d, d] such that
x = f
(
r?(d?, u?), d?, u?
)
.
Prove that the strategy with initial capital x and with a number of risky assets held at time 0 given by
∆ := h(u
?S0)− h(d?S0)
(u? − d?)S0 ,
super–replicates the option.
6) Conclude that p
(
h(S1)
)
= x.
7) For any probability measure Pr,d,u ∈ P associated to some given (r, d, u) ∈ [r, r]× [d, d]× [u, u], we can associate an
equivalent probability measure Qr,d,u such that
Qr,d,u[{ωu}] = 1−Qr,d,u[{ωd}] := 1 + r − d
u− d .
Check, and comment, that for any (r, d, u) ∈ [r, r] × [d, d] × [u, u], we have that (St/(1 + r))t=0,1 is an (F,Qr,d,u)–
martingale, and that the following super–hedging duality holds
p