Lecture notes for Numerik IVc - Numerics for Stochastic Processes, Wintersemester 2012/2013. Instructor: Prof. Carsten Hartmann

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1 Lecture notes for Numerik IVc - Numerics for Stochastic Processes, Wintersemester 22/23. Instructor: Prof. Carsten Hartmann Scribe: H. Lie Course outline. Probability theory (a) Some basics: stochastic processes, conditional probabilities and expectations, Markov chains References: [MS5, Kle6] 2. Stochastic differential equations (a) Brownian motion: properties of the paths, Strong Markov Property References: [MS5, Øks3, Arn73] (b) Stochastic integrals: Itô integrals, Itô calculus, Itô isometry References: [MS5, Øks3, Arn73] (c) SDEs: existence and uniqueness of solutions, numerical discretisation, applications from physics, biolopgy and finance References: [Øks3, Arn73, KP92] (d) Misc: Kolmogorov forward and backward PDEs, infinitesimal generators, semigroup theory, stopping times, invariant distributions, Markov Chain Monte Carlo methods for PDEs and SDEs References: [Øks3, Arn73, KP92] 3. Filtering theory (if time permits) (a) Linear filtering: conditional expectation, best approximation, Kalman- Bucy filter for SDEs Reference: [Jaz7, Øks3] 4. Approximation of stochastic processes (a) Spectral theory of Markov chains: infinitesimal generator, metastability, aggregation of Markov chains References: [HM5, Sar] (b) Markov jump processes: applications from biology, physiocs and finance, Markov decision processes, control theory References: [GHL9]

2 Figure : Simulation of a butane molecule and its approximation by a 3-state Markov chain (states in blue, green and yellow; solvent molecules not shown). Day, Different levels of modelling.. Time-discrete Markov chains Time index set I is discrete, e.g. I N and state space S is countable or finite, e.g. S = {s, s 2, s 3 } (see Figure ). Key objects are transition probabilities. For a state space S = {,..., n}, the transition probabilities p ij satisfy p ij = P (X t+ = j X t = i) and yield a row-stochastic matrix P = (p ij ) i,j S...2 Markov jump processes These are time-continuous, discrete state-space Markov chains. Time index set I R +, S discrete. For a fixed time step h >, the transition probabilities are given by (see Figure 2) P (X t+h = s j X t = s i ) = hl ij + o(h) where L = (l ij ) i,j S and P h are matrices satisfying P h = exp (hl). Note: the matrix L is row sum zero, i.e. j l ij =. The waiting times for the Markov chain in any state s i are exponentially distributed in the sense that P (X t+s = s i, s [, τ) X t = s i ) = exp (l ii τ) 2

3 Figure 2: Simulation of butane: typical time series of the central dihedral angle (blue: metastable diffusion process, red: Markov jump process) and the average waiting time is l ii (by definition of the exponential distribution). Note: the spectrum of the matrix P h is contained within the unit disk, i.e. for every eigenvalue λ of P h, λ. This property is a consequence of P h being row-stochastic, i.e. that j P h,ij =. Since P h = exp(hl) it follows that σ(p h ) D := { x R 2 x } σ(l) C = {y C Re(y) } Example.. Suppose one has a reversible reaction in which one has a large collection of N molecules of the same substance. The molecules can be either in state A or state B and the molecules can change between the two states. Let k + denote the rate of the reaction in which molecules change from state A to B and let k denote the rate at which molecules change from state B to A. For t >, consider the quantity µ A i (t) := P (number of molecules in state A at time t is i) where i = {,..., N}. One can define quantities µ B i (t) in a similar way, and one can construct balance laws for these quantities, e.g. dµ A i (t) dt = k + µ A i+(t) + k µ A i (t) (k + + k )µ A i (t). The above balance law can be written in vector notation using a tridiagonal matrix L. By adding an initial condition one can obtain an initial value problem dµ A (t) dt = L µ A (t), µ A () = µ. The solution of the initial value problem above is µ A (t) = µ exp ( tl ). 3

4 ..3 Stochastic differential equations (SDEs) These are time-continuous, continuous state space Markov chains. SDEs may be considered to be ordinary differential equations (ODEs) with an additional noise term (cf. Figure 2). Let b : R n R n be a smooth vector field and let x(t) be a deterministic dynamical system governed by the vector field b( ). Then x(t) evolves according to dx dt = b(x), x() = x. () Now let (B t ) t> be Brownian motion in R d, and let (X t ) t> be a dynamical system in R d which evolves according to the equation dx t dt = b(x t ) + db t dt. (2) The additional term dbt dt represents noise, or random perturbations from the environment, but is not well-defined because the paths of Brownian motion are nowhere differentiable. Therefore, one sometimes writes which is shorthand for X t = X + dx t = b(x t )dt + db t, t b(x t )dt + t db t. The most common numerical integration method for SDEs is the forward Euler method. If x is a C function of time t, then dx x(s + h) x(s) dt = lim. t=s h h The forward Euler method for ODEs of the form () is given by X t+h = X t + hb(x t ) and for SDEs of the form (2) it is given by X t+h = X t + hb(x t ) + ξ h where < h is the integration time step and the noise term ξ in the Euler method for SDEs is modeled by a mean-zero Gaussian random variable. For stochastic dynamical systems which evolve according to SDEs as in (2), one can consider the probability that a system at some point x R d will be in a set A R d after a short time h > : P (X t+h A X t = x). The associated transition probability density functions of these stochastic dynamical systems are Gaussian because the noise term in (2) is Gaussian. What has been the generator matrix L in case of a Markov jump process is an infinite-dimensional operator acting on a suitable Banach space. Specifically, E x [f(x t )] f(x ) Lf(x ) = lim, t t provided that the limit exists. Here f : R n R is any measurable function and E x [ ] denotes the expectation over all random paths of X t satisfying X = x. L is a second-order differential operator if f is twice differentiable. 4

5 2 Day 2, Preliminaries from probability theory Let (Ω, E, P) be a probability space, where Ω is a set and E 2 Ω is a σ-field or σ-algebra on Ω, and P is a probability measure (i.e., P is a nonnegative, countably additive measure on (Ω, E) with the property P(Ω) = ). 2. Conditioning Let A E be a set of nonzero measure, i.e. P(A) > and define E A to be the set of all subsets of A which are elements of E, i.e. E A := {E A E E}. Definition 2. (Conditional probability, part I). For an event A and an event E E A, the conditional probability of E given A is P(E A) := P(E A). P(A) Remark 2.2. Think of P A := P( A) as a probability measure on the measurable space (A, E A ). Given a set B E, the characteristic or indicator function χ B : Ω {, } satisfies { x B χ B (x) = x / B. Definition 2.3 (Conditional expectation, part I). Let X : Ω R be a random variable with finite expectation with respect to P. The conditional expectation of X given an event A is Remark 2.4. We have E(X A) = E [Xχ A] P(A) E(X A) = P(A) Remark 2.5. Observe that P(E A) = E [χ E A]. A. XdP = XdP A. Up to this point we have only considered the case where A satisfies P(A) >. We now consider the general case. Definition 2.6 (Conditional expectation, part II). Let X : Ω R be an integrable random variable with respect to P and let F E be any sub-sigma algebra of E. The conditional expectation of X given F is a random variable Y := E [X F] with the following properties: Y is measurable with respect to F: B B(R), Y (B) F. We have XdP = Y dp F F. F F 5

6 Remark 2.7. The second condition in the last definition amounts to the projection property as can be seen by noting that E [Xχ F ] = XdP = Y dp = E [Y χ F ] = E [E [X F] χ F ]. F F By the Radon-Nikodym theorem [MS5], the conditional expectation exists and is unique up to P-null sets. Definition 2.8 (Conditional probability, part II). Define the conditional probability of an event E E given A by P(E A) := E [χ E A] Exercise 2.9. Let X, Y : Ω R and scalars a, b R. Prove the following properties of the conditional expectation: (Linearity): (Law of total expectation): (Law of total probability): E [ax + by A] = ae [X A] + be [Y A]. E [X] = E [X A] + P(A) + E [X A c ] P(A c ) P(B) = P(B A)P(A) + P(B A c )P(A c ). Example 2.. The following is a collection of standard examples. Gaussian random variables: Let X, X 2 be jointly Gaussian with distribution N(µ, Σ), where ( ) ( ) E[X ] a b µ =, Σ = E[X 2 ] b c such that Σ is positive definite. The density of the distribution is [ ρ(x) = exp ] det(2πσ) 2 (x µ) Σ (x µ) (Ex.: Compute the distribution of X given that X 2 = a for some a R.) (Conditioning as coarse-graining): Let Z = {Z i } M i= i.e. Ω = M i= Z i with Z i Z j = and define be a partition of Ω, Y (ω) = M E [X Z i ] χ Zi (ω). Then Y = E [X Z] is a conditional expectation (cf. Figure 3) i= (Exponential waiting times): exponential waiting times are random variables T : Ω [, ) with the memoryless property: P (T > s + t T > s) = P (T > t). This property is equivalent to the statement that T has an exponential distribution, i.e. that P(T > t) = exp( λt) for a parameter value λ >. 6

7 Figure 3: Simulation of butane, coarse-grained into three states Z, Z 2, Z Stochastic processes Definition 2. (Stochastic process). A stochastic process X = {X t } t I is a collection of random variables on a probability space (Ω, E, P) indexed by a parameter t I [, ). We call X discrete in time if I N continuous in time if I = [, T ] for any T <. How does one define probabilities for X? We provide a basic argument to illustrate the possible difficulties in defining the probability of a stochastic process in an unambiguous way. By definition of a stochastic process, X t = X t (ω) is measurable for every fixed t I, but if one has an event of the form E = {ω Ω X t (ω) [a, b] t I} how does one define the probability of this event? If t is discrete, the σ-additivity of P saves us, together with the measurability of X t for every t. If, however, the process is time-continuous, X t is defined only almost surely (a.s.) and we are free to change X t on a set A t with P(A t ) =. By this method we can change X t on A = t I A t, The problem now is that P(A) need not be equal to zero even though P(A t ) = t I. Furthermore, P(E) may not be uniquely defined. So what can we do? The solution to the question of how to define probabilities for stochastic processes is to use finite-dimensional distributions or marginals. Definition 2.2. (Finite dimensional distributions): Fix d N, t,..., t d I. The finite-dimensional distributions of the stochastic process X for (t,..., t d ) are defined as µ t,...,t d (B) := P (Xtk ) k=,...,d (B) = P ({ω Ω (X t (ω),..., X td (ω)) B}) for B B(R d ). 7

8 Here and in the following we use the shorthand notation P Y := P Y to denote the push forward of P by the random variable Y. Theorem 2.3. (Kolmogorov Extension Theorem): Fix d N, t,..., t d I, and let µ t,...,t d be a consistent family of finite-dimensional distributions, i.e. for any permutation π of (,..., d), µ t,...,t d (B... B d ) = µ (tπ(),...,t π(d) (B π()... B π(d) ) For t,..., t d+ I, we have that µ t,...,t d+ (B... B d R) = µ t,...,t d (B... B d ). Then there exists a stochastic process X = (X t ) t I with µ t,...t d as its finitedimensional distribution. Remark 2.4. The Kolmogorov Extension Theorem does not guarantee uniqueness, not even P-a.s. uniqueness, and, as we will see later on, such a kind of uniqueness would not be a desirable property of a stochastic process. Definition 2.5. (Filtration generated by a stochastic process X): Let F = {F t } t I with F s F t for s < t be a filtration generated by F t = σ ({X s s t}) is called the filtration generated by X. 2.3 Markov processes Definition 2.6. A stochastic process X is a Markov process if where for some event E. P (X t+s A F s ) = P (X t+s A X s ) (3) P ( X s ) := P ( σ(x s )), P (E σ(x s )) := E [χ E σ(x s )] Remark 2.7. If I is discrete, then X is a Markov process if P (X n+ A X = x,..., X n = x n ) = P (X n+ A X n = x n ) Example 2.8. Consider a Markov Chain (X t ) t N on a continuous state space S R and let S be a σ-algebra on S. Let the evolution of (X t ) t N be described by the transition kernel p(, ) : S S [, ] which gives the single-step transition probabilities: p(x, A) := P (X t+ A X t = x) = q(x, y)dy. A In the above, A B(S) and q = dp dλ is the density of the transition kernel with respect to Lebesgue measure. The transition kernel has the property that 8

9 x S, p(x, ) is a probability measure on S, while for every A S, p(, A) is a measurable function on S. For a concrete example, consider the Euler-Maruyama discretization of an SDE for a fixed time step t, X n+ = X n + tξ n+, X =, where (ξ i ) i N are independent, identically distributed (i.i.d) Gaussian N (, ) random variables. The process (X i ) i N is a Markov Chain on R. The transition kernel p(x, A) has the Gaussian transition density q(x, y) = 2π t exp [ 2 y x 2 Thus, if X n = x, then the probability that X n+ A R is given by P (X n+ A X n = x) = q(x, y)dy. 3 Day 3, Recapitulation: A stochastic process X = (X t ) t I is a collection of random variables X t : Ω R indexed by t I (e.g. I = [, )) on some probability space (Ω, E, P). A filtration F := (F t ) t I is a collection of increasing sigma-algebras satisfying F t F s for t < s. A stochastic process X is said to be adapted to F if (X s ) s t is F t -measurable. For example, if we define F t := σ(x s : s t), then X is adapted to F. The probability distribution of a random variable X is given in terms of its finite dimensional distributions. Example 3. (Continued from last week). Let I = N and consider a sequence (X n ) n N of random variables X n = Xn t governed by the relation A X t n+ = X t n + tξ n+, X t = a.s. (4) where t >, and (ξ k ) k N are i.i.d random variables with E [ξ k ] = and E [ ξk] 2 = (not necessarily Gaussian). To obtain a continuous-time stochastic process, the values of the stochastic process on non-integer time values may be obtained by linear interpolation (cf. Figure 4 below). We want to consider the limiting behaviour of the stochastic process in the limit as t goes to zero. Set t = t/n for a fixed terminal time t < and let N ( t ). Then, by the central limit theorem, t N XN t = ξ k tz (5) N k= where Z N (, ), and means convergence in distribution, i.e., weak convergence of the induced probability measure; equivalently, the limiting random variable is distributed according to N (, t). In other words the limiting t ]. 9

10 distribution of the random variable X t N for fixed t = N t is the same as the distribution of a centered Gaussian random variable with variance t. As this is true for any t >, we can think of the limiting process as a continuous-time Markov process B = (B t ) t> with Gaussian transition probabilities, P (B t+s A B s = x) = = A q s,t (x, y)dy 2π t s A exp ( y x 2 2 t s ) dy. The stochastic process B is homogeneous or time-homogeneous because the transition probability density q s,t (, ) does not depend on the actual values of t and s, but only on their difference, i.e., q s,t (, ) = q s t (, ) (6) X t X t t t 2 x 4.4 X t t 5 5 x Figure 4: Sample paths of (Xn t ) n for t =.5,.2,. over the unit time interval [, ], with piecewise constant interpolation. The lower right plot shows the histogram (i.e., the unnormalized empirical distribution) of (X) t at time t =, averaged over independent realizations. Remark 3.2. The choice of exponent /2 in t = ( t) /2 in (5) is unique. For ( t) α with α (, 2 ), the limit of X t n explodes in the sense that the variance of the process blows up, i.e., E[(XN t)2 ] as N. On the other hand, for ( t) α with α > /2, XN t in probability as N.

11 3. Brownian motion Brownian motion is named after the British botanist, Robert Brown ( ), who first observed the random motion of pollen particles suspended in water. Einstein called the Brownian process Zitterbewegung in his 95 paper, Über die von der molekularkinetischen Theorie der Wärme gefordete Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen. The Brownian motion is a continuous-time stochastic process which is nowhere differentiable. It is also a martingale in the sense that on average, the particle stays in the same location at which it was first observed. In other words, the best estimate of where the particle will be after a time t > is its initial location. Definition 3.3. (Brownian motion) The stochastic process B = (B t ) t> with B t R is called the -dimensional Brownian motion or the -dimensional Wiener process if it has the following properties: (i) B = P-a.s. (ii) B has independent increments, i.e., for all s < t, (B t B s ) is a random variable which is independent of B r for r s. (iii) B has stationary, Gaussian increments, i.e., for t > s we have B t B s D = Bt s (7a) D = N (, t s). (7b) (iv) Trajectories of Brownian motion are continuous functions of time. We now make precise some important notions: Definition 3.4. (Filtered probability space) A filtered probability space is a probability space (Ω, F, P) with a filtration (F t ) t such that t, F t F. Remark 3.5. One may write (Ω, F, F t, P) to refer to a filtered probability space. However, if one is working with a particular stochastic process X, one may consider the sigma-algebra F on Ω to simply be the smallest sigma-algebra which contains the union of the F X t, where F X t := σ(x s : s t). In symbols, we define the sigma-algebra in the probability space to be F := t F t := σ ( t F t ). Definition 3.6. (Martingale) A stochastic process X = (X t ) t> is a martingale with respect to a filtered probability space (Ω, F, F t, P) if X satisfies the following properties: (i) X is adapted to F, i.e. X t is measurable with respect to F t for every t (ii) X is integrable: X L (Ω, F, P), i.e. E [ X ] = X(ω) dp(ω) < The notation X D = Y means X has the same distribution as Y. Ω

12 (iii) X has the martingale property: t > s E [X t F s ] = X s. Definition 3.7. (Gaussian process) A -dimensional process G = (G t ) t> is called a Gaussian process if for any collection (t,..., t m ) I for arbitrary m N, the random variable (G t,..., G tm ) has a Gaussian distribution, i.e. it has a density [ f(g) = exp ] det(2πσ) 2 (g µ) Σ (g µ) (8) where g = (g,..., g m ), µ R m is a constant vector of means and Σ = Σ R m m is a symmetric positive semi-definite matrix. Remark 3.8. The Brownian motion process is a Gaussian process with the vector of means µ = and covariance matrix t.... Σ = t 2 t (9)..... t m t m The covariance matrix is diagonal due to the independence of the increments of Brownian motion. Remark 3.9. Some further remarks are in order. (a) Conditions (i)-(iii) define a consistent family of finite-dimensional distributions. Hence, the existence of the process B is guaranteed by the Kolmogorov Extension Theorem. (b) Conditions (i)-(iii) imply that E [B t ] = and E [B t B s ] = min(t, s) s, t R. The proof is left as an exercise. ) t> (which is obtained by linear interpolation between the XN t ) and B as random variables on the space of continuous trajectories (C(R + ) and B(C(R + ))), then the process (Xt t ) t> converges in distribution to B. (c) The discrete process (Xn t ) n N converges in distribution to a Brownian motion (B t ) t if the time discrete is linearly interpolated between two successive points. In other words, if we consider the continuous-time stochastic processes (X t t (d) We have that E [ (B t B s ) 2] = E [ (B t s ) 2] by (7a) in Definition 3.3 = t s by (7b) in Definition 3.3. (e) Brownian motion enjoys the following scaling invariance, also known as self-similarity of Brownian motion: for every t > and α >, B t D = α /2 B αt. 2

13 An alternative construction of Brownian motion Observe that we have constructed Brownian motion by starting with the scaled random walk process and using the Kolmogorov Extension Theorem. Now we present an alternative method for constructing Brownian motion that is useful for numerics, called the Karhunen-Loève expansion of Brownian motion. We will consider this expansion for Brownian motion on the unit time interval [, ]. Let {η k } k N be a collection of independent, identically distributed (i.i.d) Gaussian random variables distributed according to N (, ), and let {φ k (t)} k N be an orthonormal basis of L 2 ([, ]) = { u : [, ] R : By construction, the basis functions satisfy φ i, φ j = φ i (t)φ j (t)dt = δ ij, and we can represent any function f L 2 ([, ]) by f(t) = k N α k φ k (t) } u(t) 2 dt <. () for α k = f, φ k. We have the following result. Theorem 3.. (Karhunen-Loève): The process (W t ) t defined by is a Brownian motion. W t = k N t η k φ k (s)ds () Proof. We give only a sketch of the proof. For details, see the Appendix in [MS5], or [KS9]). The key components of the proof are to show the following: (i) The infinte sum which defines the Karhunen-Loève expansion is absolutely convergent, uniformly on [, ]. (ii) It holds that E [W t ] = and E [W t W s ] = min(s, t). 4 Day 4, Brownian motion From last week, we saw that the Brownian motion (B t ) t is a continuous-time stochastic process on R with stationary, independent, Gaussian increments a.s. continuous paths. That is, for fixed ω, each (B t ) t (ω) is a continuous trajectory in R. 3

14 Moreover the scaled random walk defined by X t n+ = X t n + tξ n+ with linear interpolation converges weakly (i.e. converges in distribution) to the Brownian motion process. Above, the (ξ n ) n N are independent, identically distributed (i.i.d) normalized Gaussian random variables (i.e. ξ n is Gaussian with mean zero and variance ). Remark 4.. Two remarks are in order. Continuity can be understood using the Lévy construction of Brownian motion on the set of dyadic rationals, D := { } k D n, D n := : k =...., 2n. 2n n N The construction of Brownian motion on the unit time interval is as follows. Let {Z t } t D be a collection of independent, normalized random variables defined on a probability space. Define the collection of functions (F n ) n N, where F n : [, ] R are given by t D n F n (t) := 2 (j+)/2 Z t t D j \ D j lin. interp. in between. Then the process B(t) = F n (t). n= is indeed a Brownian motion on [, ]. The Gaussianity of the {Z t } t D leads to the stationary, independent Gaussian increments of the process (B t ) t [,]. The continuity of the process follows from an application of the Borel-Cantelli Lemma, which states that there exists a random and almost surely finite number N N such that for all n N and d D n, Z d < c n holds. This boundedness condition implies that n N we have a decay condition for the F n : F n < c n2 n/2. Therefore the sum j F j( ) converges uniformly on [, ]. As each F j is continuous and the uniform limit of continuous functions is continuous, the process (B t ) t [,] is continuous. For more details, see [MP]. The Hausdorff dimension dim H of Brownian motion paths depends on the dimension of the space R d in which the Brownian motion paths live. 2 Let B [,] = {B t R d : t [, ]} be the graph of B t over I = [, ]. Then { 3/2 d = dim H B [,] = 2 d 2. 2 If you do not know what this is, just think of the box counting dimension that is an upper limit of the Hausdorff dimension. 4

15 The significance of this is as follows: if you consider Brownian motion paths confined to a smooth and compact two-dimensional domain and impose reflecting boundary conditions, then the Brownian motion paths will fill the domain in the limit as t. 4.2 Brownian bridge (Karhunen-Loève expansion of Brownian motion) Theorem 4.2. Let {η k } k N be i.i.d normalized random variables and {φ k } k N form a real orthonormal basis of L 2 ([, ]). Then W t = k N t η k φ k (s)ds is a Brownian motion on the interval I = [, ]. Exercise 4.3. Show that, for the definition of (W t ) t [,] above, it holds that E [W t W s ] = min(s, t) W t.2 W t t t W t t x Figure 5: Sample paths of the Karhunen-Loève expansion of (W t for M = 2, 64, 248 basis functions (you can guess which one is which). The lower right plot shows the unnormalized histogram of W t at time t =, using M = 64 basis functions and averaged over independent realizations. Remark 4.4. Unlike the scaled random walk construction of Brownian motion, no forward iterations are required here. This helps for the consideration of round-off errors in the construction of (W t ) t [,]. Furthermore: 5

16 Standard choices for the orthonormal basis {φ k } k N are Haar wavelets or trigonometric functions. Hence the numerical error can be controlled by truncating the series and by the choice of the basis. To obtain a Brownian motion on any general time interval [, T ], it suffices to use the scaling property, e.g. W [,T ] D = T W[,]/T = T k N t/t η k φ k (s)ds. 4.3 Application: filtering of Brownian motion Suppose we know that W = and W is equal to some constant ω. Without loss of generality, let ω =. Suppose we wanted to generate a Brownian motion path which interpolated between the values W = and W =. Definition 4.5. A continuous, mean-zero Gaussian process (BB t ) t is called a Brownian bridge to ω if it has the same distribution as (W t ) t [,] conditional on the terminal value W = ω. Equivalently, (BB t ) t is a Brownian bridge if Cov [BB t BB s ] = min(s, t) st. Lemma 4.6. If (W t ) t [,] is a Brownian motion, then BB t = W t tw is a Brownian bridge. Proof. Observe that E [BB t ] = E [W t tw ] = t =, so that (BB t ) t [,] is indeed mean-zero. The process (BB t ) t [,] inherits continuity from the process (W t ) t [,]. The covariance process is given by Cov(BB t BB s ) = E [BB t BB s ] = E [(W t tw ) (W s sw )] = E [W t W s ] t E [W W s ] s E [W W t ] +tse [W W ] }{{}}{{} =min(s,) =min(t,) = min(t, s) ts st + ts How does one simulate a Brownian bridge? First approach: forward iteration, using Euler s method. The time interval is [, ] and we have a time step of t := /N, so we have (N + ) discretized time nodes (t n = n t) n=,...,n and (N +) values (Yn t ) n=,...,n. Let {ξ n } n=,...,n be a collection of i.i.d normalized random variables. Forward iteration gives ( Yn+ t = Yn t t ) + tξ n+. t n It holds that t N = t by definition of t = /N. Therefore from the formula above we have YN t = tξ N+. 6

17 Therefore YN t is a mean zero Gaussian random variable with variance t. While this implies that YN t should converge in probability to the value as the step size t, the forward iteration approach is not optimal because the random variable ξ N+ is continuous, so P ( Y t N = ) =. Therefore this construction of the Brownian bridge to the value ω = will in general not yield processes which are at at time t =. As a matter of fact, YN t is unbounded and can be arbitrarily far away from zero. Second approach: Recall the Karhunen-Loéve construction of Brownian motion and choose trigonometric functions as an orthonormal basis. Then the process (W t ) t [,] given by W t (ω) = 2 M k= η k (ω) sin((k 2 )πt (k 2 )π is a Brownian motion and we can define the Brownian bridge to ω at t = by Remark 4.7. It holds that BB t = W t t(w ω). BB t = 2 k N η k sin(kπt) kπ = k N η k λk ψ k (t), where {λ k, ψ k } k N = { 2/kπ, sin(kπt) } is the eigensystem of the covariance k N operator T : L ([, ]) L ([, ]) of the process (BB t ) t [,], defined by i.e., (T u)(t) = Cov(BB t BB s ) u(s)ds, }{{} =min(t,s) st T ψ k ( ) = λ k ψ k ( ). The second approach works for any stochastic process which has finite variance over a finite time interval. For details, see [Xiu]. 7

18 .4.6 Y n BB 2 t t n t Figure 6: Sample paths of the Brownian bridge approximation, using the Euler scheme with t =.5 (left panel) and Karhunen-Loève expansion with M = 2 basis functions (right panel). 5 Day 5, (Lecturer: Stefanie W.) 5. Stochastic Integration (Itô integral) Recall that Brownian motion (B t ) t> is a stochastic process with the following properties: B = P-a.s. t < t < t 2 <... < t n, the increments B ti B ti are independent for i =,..., n and Gaussian with mean and variance t i t i. t B t (ω) is continuous P-a.s. but is P-a.s. nowhere differentiable. One of the motivations for the development of the stochastic integral lies in financial mathematics, where one wishes to determine the price of an asset that evolves randomly. The French mathematician Louis Bachelier is generally considered one of the first people to model random asset prices. In his PhD thesis, Bachelier considered the following problem. Let the value S t of an asset at time t > be modeled by S t = σb t where σ > is a scalar that describes the volatility of the stock price. Let f(t) be the amount of money an individual invests in the asset in some infinitesimal time interval [t, t + dt]. Then the wealth of the individual at the end of a time interval [, T ] is given by T T f(t)ds t = σ f(t)db t. However, it is not clear what the expression db t means. In this section, we will consider what an integral with respect to db t means, and we will also consider the case when the function f depends not only on time but on the random element ω. 8

19 The first idea is to rewrite f(t)db t = f(t) db t dt dt but as Brownian motion is almost surely nowhere differentiable, we cannot write db t dt Ṫhe second idea is to proceed as in the definition of the Lebesgue integral: start with simple step functions and later extend the definition to more general functions by the Itô Isometry. Step : Consider simple functions f(t) = n a i χ (ti,t i+](t) i= where χ A is the indicator function of a set A satisfying { x A χ A (x) = x / A. Observe that f takes a finite number n of values. By the theory of Lebesgue integration, we know that the set of these simple functions is dense in L 2 ([, )). We also know that the usual Riemann integral of such a function f corresponds to the area under the graph of f, with f(t)dt = i a i (t i+ t i ) Step 2: We now extend the method above to stochastic integral with respect to Brownian motion: f(t)db t = a i (B ti+ B ti ). Remark 5.. By the equation above, it follows that the integral f(t)db t is a random variable, since the B ti are random variables. Since increments of Brownian motion are independent and Gaussian, the integral f(t)db t is normally distributed with zero mean. What about its variance? Lemma 5.2. (Itô Isometry for simple functions) For a simple function f(t) = i a iχ (ti,t i+](t), it holds that [ ( ) ] 2 E f(t)db t = (f(t)) 2 dt. 9

20 Proof: ( ) ( ( ) ) var f(t)db t = var a i Bti+ B ti i n = a 2 i var ( ) B ti+ B ti = = = i= n a 2 i (t i+ t i ) i= n a 2 i χ (ti,t i+]dt i= n i= a 2 i χ (ti,t i+]dt = (f(t)) 2 dt. On the other hand it holds ( ) [ ( ) ] 2 [ ] var f(t)db t = E f(t)db t E f(t)db t }{{} = [ ( ) ] 2 f(t)db t = E 2 Step 3: Now we extend the definition of the integral to L 2 ([, )). result is the following The main Theorem 5.3. (Itô integral for L 2 ([, )) functions) The definition of the Itô integral can be extended to elements f L 2 ([, )) by setting f(t)db t := lim n f n (t)db t where the sequence (f n ) n N is a sequence of a simple functions satisfying f n f in L 2 ([, )), i.e. ( f n f L 2 ([, )) = (f n f) 2 (t)dt) /2 n. Proof: By the Itô isometry, we can show that ( f n (t)db t ) n N is a Cauchy sequence in the L 2 space } L 2 (Ω, F, P) := {F : Ω R : F (B(R)) F, F 2 L 2 (Ω,F,P) < where F 2 L 2 (Ω,F,P) := F 2 (ω)dp(ω). 2

21 To show that the sequence ( f n (t)db t ) n N is a Cauchy sequence, let (f l ) l N be a sequence of functions converging to f in L 2 ([, )) and consider for m, n N f n (t)db t f m (t)db t L2 (Ω,F,P) ( [ ( ) ]) 2 /2 = E f n (t)db t f m (t)db t ( [ ( ) ]) 2 /2 = E f n (t) f m (t)db t ( = (f n (t) f m (t)) 2 dt) /2 (Itô isometry) = f n f m L 2 ([, )) f n f L 2 ([, )) + f m f L 2 ([, )) and using that f n f L 2 ([, )) and f m f L 2 ([, )) as m, n, the result follows. We use the fact that L 2 (Ω, F, P) is complete (see...) to see that the limit exists and is in the same space. Moreover, by the Itô isometry the limit is independent of the sequence (f n ) n N used to approximate f in L 2 ([, )). Example 5.4. Consider the random variable exp( t)db t. How is it distributed? Using the Itô Isometry, the random variable is Gaussian with mean zero and variance 2 = exp( 2t)dt. Corollary 5.5. The Itô Isometry holds as well for f L 2 ([, )), not just simple functions. Step 4 : Now we consider functions f which depend both on the random element ω as well as time t. That is, we consider stochastic integrals of stochastic processes f : [, ) Ω R with the following properties: (i) f is B F-measurable, where B is the Borel sigma-algebra on [, ) and F is a given sigma-algebra on Ω. (ii) f(t, ω) is adapted with respect to F t, where F t := σ (B s : s t) (iii) E [ f(t, ω) 2 dt ] <. Consider simple stochastic processes of the form f(t, ω) = n a i (ω)χ (ti,t i+](t). i= Then n f(t, ω)db t = a i (ω) ( ) B ti+ B ti. i= 2

22 Example 5.6. Fix n N, fix a time step t := 2 n and define the time nodes t i := i t for i =,, 2,.... Let (B t ) t> be the standard Brownian motion. Define the following processes on [, ): f (t, ω) = i N B ti (ω)χ [ti,t i+)(t) f 2 (t, ω) = i N B ti+ (ω)χ [ti,t i+)(t). Now fix T > and N such that T = t N = N t = N2 n and compute the expected values of the integrals of f and f 2 over [, T ]. By the independent increments property of Brownian motion (or the martingale property of Brownian motion), we have [ ] T E f (t, ω)db t = N i= E [ B ti ( Bti+ B ti )] =. Using the fact above with linearity of expectation, we also have [ ] T N E f 2 (t, ω)db t = E [ ( )] B ti+ Bti+ B ti = = = i= N i= N i= N i= ( [ ( )] [ ( )]) E Bti+ Bti+ B ti E Bti Bti+ B ti [ ) ] 2 E (Bti+ B ti t i+ t i = T. In the case of Riemann integration of deterministic integrals, letting n would lead to the result that both integrals above are equal. We see that for stochastic integration, this is not the case; even if we let n, the expectations of the Itô integrals would not be equal. This is because the choice of endpoint of the interval matters in stochastic integration. Choosing the left endpoint (i.e. choosing B ti ) for f and the right endpoint (i.e. B ti+ for f 2 leads to different expectations. Note also that taking the right endpoint in f 2 leads to f 2 not being adapted, since B ti+ is not measurable with respect to F t for t < t i+. Therefore, by property (ii) above, we may not integrate f 2 with respect to db t in the way we have just described. References [Arn73] Ludwig Arnold. Stochastische Differentialgleichungen: Theorie und Anwendung. Wiley & Sons, 973. [GHL9] Xianping Guo and Onésimo Hernández-Lerma. Continuous-Time Markov Decision Processes: Theory and Applications. Springer, Heidelberg,

23 [HM5] Wilhelm Huisinga and Eike Meerbach. Markov chains for everybody. Lecture Notes, Fachbereich Mathematik und Informatik, Freie Universität Berlin, 25. [Jaz7] Andrew H. Jazwinski. Stochastic Processes and Filtering Theory. Dover, New York, 27. [Kle6] Achim Klenke. Wahrscheinlichkeitstheorie. Springer, Berlin, 26. [KP92] [KS9] Peter E. Kloeden and Eckhard Platen. Numerical Solution of Stochastic Differential Equations. Springer, Berlin, 992. Ioannis Karatzas and Steven Shreve. Brownian Motion and Stochastic Calculus. Springer, New York, 99. [MP] Peter Morters and Yuval Peres. Brownian Motion (Cambridge Series in Statistical and Probabilistic Mechanics). Cambridge University Press, 2. [MS5] David Meintrup and Stefan Schäffler. Stochastik. Springer, Berlin, 25. [Øks3] [Sar] Bernt K. Øksendal. Stochastic Differential Equations: An Introduction With Applications. Springer, Berlin, 23. Marco Sarich. Multiscale Stochastic Processes. Lecture Notes, Fachbereich Mathematik und Informatik, Freie Universität Berlin, 2. [Xiu] Dongbin Xiu. Numerical Methods for Stochastic Computations: A Spectral Method Approach. Princeton University Press, 2. 23