Past exam of the mathematics course of the University of Cambridge 2017 iii Paper 201 4 b Solution Created 2026-10-03 Updated 2026-10-05
Fix an exponent in the range from (a), and work on the single probability-one event where and all dyadic values are finite. We give the dyadic increment chaining argument. For dyadic , put and choose with . Let and . The level- approximations are at most two grid steps apart, so their difference is bounded by . At each subsequent level an approximation either stays fixed or moves by one adjacent level increment. Because are dyadic, these approximations eventually equal . ThusThe dyadic rationals are dense in , so this uniform bound gives a unique continuous extension to the entire interval, withAll these equalities hold on that one event, not merely one event per dyadic time. Define to be the zero path on its null complement. Each is measurable as the limit of the random variables at deterministic left dyadic approximations. Alternatively, their measurable linear interpolations converge uniformly to , which also shows measurability as a random element of . This is the continuous extension version of a continuous modification.