Is there a general process for conditioning a stochastic process above a boundary? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-21T15:41:12Z http://mathoverflow.net/feeds/question/118339 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/118339/is-there-a-general-process-for-conditioning-a-stochastic-process-above-a-boundary Is there a general process for conditioning a stochastic process above a boundary? GB 2013-01-08T07:59:17Z 2013-02-01T16:00:10Z <p>$(X_t, Y_t)$ is a two-dimensional Markov stochastic process that runs on time interval $[0, t_f]$. Given its transition function $a(x, y | x', y')$, I would like to condition the process on $\inf_{s \in [0, t_f]} X_s \ge k$ and find the new transition function.</p> <p>Can the problem be solved at this level of generality? Or must we dig into the specifics of $a$ to find a solution on a case-by-case basis?</p> http://mathoverflow.net/questions/118339/is-there-a-general-process-for-conditioning-a-stochastic-process-above-a-boundary/119244#119244 Answer by Didier Piau for Is there a general process for conditioning a stochastic process above a boundary? Didier Piau 2013-01-18T09:07:54Z 2013-01-18T09:07:54Z <p>More generally, let $A$ denote some subset of the state space of a homogenous Markov process $(Z_t)_{0\leqslant t\leqslant t_0}$ with transition kernel $(a_{t-s})_{0\leqslant s\leqslant t\leqslant t_0}$, $T=\inf\{0\leqslant t\leqslant t_0\mid Z_t\in A\}$, $\mathbb Q=\mathbb P(\ \mid T=+\infty)$, and $h_t(z)=\mathbb P(T\gt t\mid Z_0=z)$. By an elementary conditioning, with respect to $\mathbb Q$, the process $(Z_t)_{0\leqslant t\leqslant t_0}$ has inhomogenous transition kernel $$\mathbb Q(Z_t=z\mid Z_s=z')=a_{t-s}(z\mid z')h_{t_0-t}(z)h_{t_0-s}(z')^{-1},$$ for every $0\leqslant s\leqslant t\leqslant t_0$ and every $z$ and $z'$ not in $A$.</p> <p>Apply this to $Z=(X,Y)$ and $A=(-\infty,k)\times\mathfrak Y$, where $\mathfrak Y$ is the state space of $(Y_t)_t$.</p> http://mathoverflow.net/questions/118339/is-there-a-general-process-for-conditioning-a-stochastic-process-above-a-boundary/120527#120527 Answer by Yuri Bakhtin for Is there a general process for conditioning a stochastic process above a boundary? Yuri Bakhtin 2013-02-01T16:00:10Z 2013-02-01T16:00:10Z <p>www.math.harvard.edu/~alexb/rm/Doob.pdf</p>