CHAPTER 5

Extended One-Factor Short-Rate Models

5.1. Ho–Le Model

Definition 5.1 (Ho–Le model). In the Ho–Le model, the short rate is assumed to satisfy the stochastic differential equation

dr(t)=θ(t)dt + σdW (t), where σ>0, θ is deterministic, and W is a Brownian motion under the risk-neutral measure.

Theorem 5.2 (Ho–Le model). In the Ho–Le model, we have the following for- mulas: t r(t)=r(s)+ θ(u)du + σ(W (t) − W (s)), s t E(r(t)|F(s)) = r(s)+ θ(u)du and V(r(t)|F(s)) = σ2(t − s), s P (t, T )=A(t, T )e−r(t)(T −t), σ2 T where A(t, T )=exp (T − t)3 − (T − u)θ(u)du , 6 t

dP (t, T )=r(t)P (t, T )dt − σ(T − t)P (t, T )dW (t), 1 σ2(T − t)2 − r(t) σ(T − t) d = dt + dW (t), P (t, T ) P (t, T ) P (t, T ) dW T (t)=dW (t)+σ(T − t)dt, dr(t)= θ(t) − σ2(T − t) dt + σdW T (t), σ2 T f(t, T )=r(t) − (T − t)2 + θ(u)du and df(t, T )=σdW T (t), 2 t 1 dF (t; T,S)=σ F (t; T,S)+ (S − T )dW S(t), τ(T,S) ZBC(t, T, S, K)=P (t, S)Φ(h) − KP(t, T )Φ(h − σ˜),

25 26 5. EXTENDED ONE-FACTOR SHORT-RATE MODELS

ZBP(t, T, S, K)=KP(t, T )Φ(−h +˜σ) − P (t, S)Φ(−h), √ 1 P (t, S) σ˜ where σ˜ = σ(S − T ) T − t and h = ln + , σ˜ P (t, T )K 2 β Cap(t, T ,N,K)=N [P (t, Ti−1)Φ(−hi +˜σi) − (1 + τiK)P (t, Ti)Φ(−hi)] , i=α+1 β Flr(t, T ,N,K)=N [(1 + τiK)P (t, Ti)Φ(hi) − P (t, Ti−1)Φ(hi − σ˜i)] , i=α+1 1 P (t, Ti) σ˜i where σ˜i = σ(Ti − Ti−1) Ti−1 − t and hi = ln + . σ˜i P (t, Ti−1)K 2

Theorem 5.3 (Calibration in the Ho–Le model). IftheHo–Lemodeliscali- brated to a given interest rate structure {f M(0,t):t ≥ 0}, i.e.,

f(0,t)=f M(0,t) for all t ≥ 0, then ∂fM ,t θ t (0 ) σ2t t ≥ . ( )= ∂t + for all 0

Theorem 5.4 (Zero-coupon bond price in the calibrated Ho–Le model). If the Ho–Le model is calibrated to a given interest rate structure {f M(0,t):t ≥ 0},then P M(0,T) σ2 P (t, T )=e−r(t)(T −t) exp (T − t)f M(0,t) − t(T − t)2 , P M(0,t) 2 where t P M(0,t)=exp − f M(0,u)du for all t ≥ 0. 0

5.2. Hull–White Model (Extended Vasicek Model)

Definition 5.5 (Short-rate dynamics in the Hull–White model). In the Hull– White model, the short rate is assumed to satisfy the stochastic differential equation

dr(t)=k(θ(t) − r(t))dt + σdW (t), where k, σ > 0, θ is deterministic, and W is a Brownian motion under the risk- neutral measure. 5.2. HULL–WHITE MODEL (EXTENDED VASICEK MODEL) 27

Remark 5.6 (Hull–White model). The Hull–White model is also called the extended Vasicek model or the G++ model and can be considered, more generally, with the constants k and σ replaced by deterministic functions.

Theorem 5.7 (Short rate in the Hull–White model). Let 0 ≤ s ≤ t ≤ T .The short rate in the Hull–White model is given by t t r(t)=r(s)e−k(t−s) + k θ(u)e−k(t−u)du + σ e−k(t−u)dW (u) s s and is, conditionally on F(s), normally distributed with t E(r(t)|F(s)) = r(s)e−k(t−s) + k θ(u)e−k(t−u)du s and σ2 V(r(t)|F(s)) = 1 − e−2k(t−s) . 2k

Remark 5.8 (Short rate in the Hull–White model). As in the Vasicek model, the short rate r(t) in the extended Vasicek model, for each time t, can be negative with positive probability, namely, with probability ⎛ ⎞  t r(0)e−kt + k θ(u)e−k(t−u)du Φ ⎝−  0 ⎠ , σ2 −2kt 2k (1 − e ) which is often “negligible in practice”. On the other hand, the short rate in the Vasicek model is mean reverting provided t ϕ∗ = lim k θ(u)e−k(t−u)du t→∞ 0 exists, and then E(r(t)) → ϕ∗ as t →∞.

Theorem 5.9 (Zero-coupon bond in the Hull–White model). In the Hull–White model, the price of a zero-coupon bond with maturity T at time t ∈ [0,T] is given by P (t, T )=A¯(t, T )e−r(t)B(t,T ), where T A¯(t, T )=A(t, T )exp −k θ(u)B(u, T )du t and A and B are as in the Vasicek model, Theorem 4.4 with θ =0. 28 5. EXTENDED ONE-FACTOR SHORT-RATE MODELS

Theorem 5.10 (Forward rate in the Hull–White model). In the Hull–White model, the instantaneous forward interest rate with maturity T is given by T σ2 f(t, T )=k θ(u)e−k(T −u)du − B2(t, T )+r(t)e−k(T −t). t 2

Theorem 5.11 (Calibration in the Hull–White model). IftheHull–Whitemodel is calibrated to a given interest rate structure {f M(0,t):t ≥ 0},then

1 ∂fM(0,t) σ2 θ(t)=f M(0,t)+ + 1 − e−2kt for all t ≥ 0. k ∂t 2k2

Theorem 5.12 (Zero-coupon bond in the calibrated Hull–White model). If the Hull–White model is calibrated to a given interest rate structure, then P M(0,T) σ2 P (t, T )=e−r(t)B(t,T ) exp B(t, T )f M(0,t) − 1 − e−2kt B2(t, T ) . P M(0,t) 4k

Theorem 5.13 (Option on a zero-coupon bond in the Hull–White model). In the Hull–White model, the price of a European call option with strike K and maturity T and written on a zero-coupon bond with maturity S at time t ∈ [0,T] is given by

ZBC(t, T, S, K)=P (t, S)Φ(h) − KP(t, T )Φ(h − σ˜), where σ˜ and h are as in the Vasicek model, Theorem 4.9.

ZBP(t, T, S, K)=KP(t, T )Φ(−h +˜σ) − P (t, S)Φ(−h).

Theorem 5.14 (Caps and floors in the Hull–White model). In the Hull–White model, the price of a cap with notional value N,caprateK, and the set of times T , is given by

β Cap(t, T ,N,K)=N [P (t, Ti−1)Φ(−hi +˜σi) − (1 + τiK)P (t, Ti)Φ(−hi)] , i=α+1 while the price of a floor with notional value N,floorrateK, and the set of times T , is given by

β Flr(t, T ,N,K)=N [(1 + τiK)P (t, Ti)Φ(hi) − P (t, Ti−1)Φ(hi − σ˜i)] , i=α+1 where σ˜i and hi are as in the Vasicek model, Theorem 4.10. 5.3. BLACK–KARASINSKI MODEL 29

5.3. Black–Karasinski Model

Definition 5.15 (Black–Karasinski model). In the Black–Karasinski model,the shortrateisgivenby

r(t)=ey(t) with dy(t)=k(θ(t) − y(t))dt + σdW (t), where k, σ > 0, θ is deterministic, and W is a Brownian motion under the risk- neutral measure.

Remark 5.16 (Black–Karasinski model). The Black–Karasinski model is also called the extended exponential Vasicek model and can be considered, more gener- ally, with the constants k and σ replaced by deterministic functions.

Theorem 5.17 (Short rate in the Black–Karasinski model). The short rate in the Black–Karasinski model satisfies the stochastic differential equation σ2 dr(t)= kθ(t)+ − k ln(r(t)) r(t)dt + σr(t)dW (t). 2 Let 0 ≤ s ≤ t ≤ T .Thenr is given by t t r(t)=exp ln(r(s))e−k(t−s) + k e−k(t−u)θ(u)du + σ e−k(t−u)dW (u) s s and is, conditionally on F(s), lognormally distributed with

E(r(t)|F(s)) t σ2 =exp ln(r(s))e−k(t−s) + k e−k(t−u)θ(u)du + 1 − e−2k(t−s) s 4k and

t V(r(t)|F(s)) = exp 2ln(r(s))e−k(t−s) +2k e−k(t−u)θ(u)du × s σ2 σ2 × exp 1 − e−2k(t−s) exp 1 − e−2k(t−s) − 1 . 2k 2k

Remark 5.18 (Short rate in the Black–Karasinski model). Since the short rate r in the Black–Karasinski model is lognormally distributed, it is always positive. A disadvantage is that P (t, T ) cannot be calculated explicitly. An advantage of the Black–Karasinski model is that r is always mean reverting provided t ϕ∗ = lim k θ(u)e−k(t−u)du t→∞ 0 30 5. EXTENDED ONE-FACTOR SHORT-RATE MODELS exists, and then σ2 E(r(t)|F(s)) → exp ϕ∗ + as t →∞ 4k and σ2 σ2 V(r(t)|F(s)) → exp 2ϕ∗ + exp − 1 as t →∞. 2k 2k

5.4. Deterministic-Shift Extended Models

Definition 5.19 (Short rate in a deterministic-shift extended model). In a deterministic-shift extended model, the short rate is given by

r(t)=x(t)+ϕ(t)withdx(t)=μ(t, x(t))dt + σ(t, x(t))dW (t), where ϕ, μ, σ are deterministic functions and W is a Brownian motion under the risk-neutral measure. The stochastic differential equation for x is called the ref- erence model, and prices of zero-coupon bonds and forward interest rates in the REF REF reference model are denoted by Px (t, T )andfx (t, T ), respectively.

Theorem 5.20 (Zero-coupon bond in a deterministic-shift extended model). In a deterministic-shift extended model, the price of a zero-coupon bond with maturity T at time t ∈ [0,T] is given by T REF P (t, T )=exp − ϕ(u)du Pr−ϕ (t, T ). t

Theorem 5.21 (Forward rate in a deterministic-shift extended model). In a deterministic-shift extended model, the instantaneous forward interest rate with ma- turity T is given by

REF f(t, T )=ϕ(T )+fr−ϕ (t, T ).

Theorem 5.22 (Calibration in a deterministic-shift extended model). If a deterministic-shift extended model is calibrated to a given interest rate structure {f M(0,t):t ≥ 0},then

M REF ϕ(t)=f (0,t) − fr−ϕ (0,t) for all t ≥ 0. 5.5. EXTENDED CIR MODEL 31

Theorem 5.23 (Zero-coupon bond in a calibrated deterministic-shift extended model). If a deterministic-shift extended model is calibrated to a given interest rate structure, then P M ,T P REF(0,t) P t, T (0 ) r−ϕ P REF t, T . ( )= M REF r−ϕ ( ) P (0,t) Pr−ϕ (0,T)

Theorem 5.24(Optiononazero-couponbondinadeterministic-shiftextended model). In a deterministic-shift extended model, the price of a European call option with strike K and maturity T and written on a zero-coupon bond with maturity S at time t ∈ [0,T] is given by S REF  ZBC(t, T, S, K)=exp − ϕ(u)du ZBCr−ϕ (t, T, S, K ), t where S K = K exp ϕ(u)du . T

5.5. Extended CIR Model

Definition 5.25 (Short rate in the extended CIR model). In the extended CIR model, the short rate is given by r(t)=x(t)+ϕ(t)withdx(t)=k(θ − x(t))dt + σ x(t)dW (t), where k, σ, θ > 0andW is a Brownian motion under the risk-neutral measure.

Remark 5.26 (Extended CIR model). The extended CIR model is also called the CIR++ model and can be considered, more generally, with the constants k and σ replaced by deterministic functions.

Theorem 5.27 (Zero-coupon bond in the CIR++ model). In the CIR++ model, the price of a zero-coupon bond with maturity T at time t ∈ [0,T] is given by

P (t, T )=A¯(t, T )e−r(t)B(t,T ), where T A¯(t, T )=A(t, T )exp ϕ(t)B(t, T ) − ϕ(u)du t and A and B are as in the CIR model, Theorem 4.20. 32 5. EXTENDED ONE-FACTOR SHORT-RATE MODELS

Theorem 5.28 (Forward rate in the CIR++ model). In the CIR++ model, the instantaneous forward interest rate with maturity T is given by

f(t, T )=ϕ(T ) − ϕ(t)BT (t, T )+kθB(t, T )+r(t)BT (t, T ), where B is as in the CIR model, Theorem 4.20.

Theorem 5.29 (Calibration in the CIR++ model). If the CIR++ model is calibrated to a given interest rate structure {f M(0,t):t ≥ 0},then

M ϕ(t)=f (0,t)+ϕ(0)BT (0,T) − kθB(0,T) − r(0)BT (0,T) for all t ≥ 0, where B is as in the CIR model, Theorem 4.20.

Theorem 5.30 (Zero-coupon bond in the CIR++ model). If the CIR++ model is calibrated to a given interest rate structure, then P M(0,T) A(0,t)A(t, T ) P (t, T )= e(r(0)−ϕ(0))(B(0,T )−B(0,t))+ϕ(t)B(t,T )e−r(t)B(t,T ), P M(0,t) A(0,T) where A and B are as in the CIR model, Theorem 4.20.

5.6. Extended Affine Term-Structure Models

Theorem 5.31 (Extended affine term-structure models). Assume the reference model is an affine term-structure model, i.e.,

REF −r(t)B(t,T ) Pr (t, T )=A(t, T )e .

If this model is extended according to Definition 5.19 by using the deterministic shift ϕ, then we have the following formulas:

P (t, T )=A¯(t, T )e−r(t)B(t,T ), T where A¯(t, T )=A(t, T )exp ϕ(t)B(t, T ) − ϕ(u)du , t

AT (t, T ) f(t, T )=ϕ(T ) − ϕ(t)BT (t, T ) − + r(t)BT (t, T ), A(t, T ) and if the extended model is calibrated to a given interest rate structure, then

M AT (0,t) ϕ(t)=f (0,t)+(ϕ(0) − r(0))BT (0,t)+ , A(0,t) P M(0,T) A(0,t)A(t, T ) P (t, T )= e(r(0)−ϕ(0))(B(0,T )−B(0,t))+ϕ(t)B(t,T )e−r(t)B(t,T ). P M(0,t) A(0,T)