Past exam of the mathematics course of the University of Cambridge 2013 iii Paper 24 6 a Solution Created 2026-10-03 Updated 2026-10-07
Sample Brownian motion at integer times and put . Every has normal distribution , although the are correlated. LetFor every fixed integer ,because almost surely. The right-hand expression depends only on the future independent increments with . Thus is, up to a null set, a tail event of those independent increments, and its probability is zero or one by the Kolmogorov zero-one law.
For any finite real , every exceeds with the same positive probability . For each ,Taking the decreasing intersection over shows that infinitely often with probability at least . This event implies , so . The zero-one law makes it one. Intersecting over positive integers gives almost surely. The continuous-time limit superior is at least the one along integers, henceThis proves that Brownian fluctuations exceed the square-root scale without needing the lower bound in the law of the iterated logarithm.