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Unformatted text preview: Math 236, Stochastic Differential Equations Solutions to problem set 3 February 17, 2009 Problem 1 Let X t be the onedimensional diffusion process satisfying the Ito SDE dX t = b ( X t ) dt + σ ( X t ) dB t , X = x where b ( x ) and σ ( x ) satisfy the Ito conditions (linear growth bound and Lipschitz condition). Let x ∈ ( α,β ) and let τ = τ x = inf { t > 0 : X t ∈ [ α,β ] c } be the exit time from the interval [ α,β ] starting from x in its interior, and let L be the generator of the process L = 1 2 σ 2 ( x ) ∂ 2 ∂x 2 + b ( x ) ∂ ∂x Show using Ito’s formula and the optional stopping theorem that if there is a nonnegative and bounded function u ( x ) for x ∈ [ α,β ] such that L u ( x ) ≤ − 1, then E x { τ } < ∞ and so τ x < ∞ with probability one. Solution First we note that t ∧ τ are stopping times that are bounded since t ∧ τ ≤ t . From Ito’s formula we have du ( X s ) = L u ( X s ) + σ ( X s ) u ( X s ) dB s and integrating u ( X t ∧ τ ) = u ( x ) + integraldisplay t ∧ τ L u ( X s ) ds + integraldisplay t ∧ τ σ ( X s ) u x ( X s ) dB s Assuming that u x is bounded (not explicitly stated in the statement), taking expectations and using the optional stopping theorem we have E x { u ( X t ∧ τ ) } = u ( x ) + E x { integraldisplay t ∧ τ L...
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This note was uploaded on 11/08/2009 for the course MATH 236 taught by Professor Papanicolaou during the Winter '08 term at Stanford.
 Winter '08
 Papanicolaou
 Differential Equations, Equations

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