Infinite mean first passage of a simple symmetric random walk
ID: infinite-mean-first-passage-of-a-simple-symmetric-random-walk
For a simple symmetric random walk starting from zero, its first hitting time of is almost surely finite but has infinite expectation. Stopping at exit from , the optional stopping theorem for gives upper-exit probability , while stopping gives mean exit time . The first probabilities tend to one, and the exit times are bounded above by the first passage to , proving both conclusions. Finite-time stopping and bounded stopped positions justify passage to the exit time.
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