EN
Suppose f = (fₙ), g = (gₙ) are martingales with respect to the same filtration, satisfying
$|fₙ-f_{n-1}| ≤ |gₙ-g_{n-1}|$, n = 1,2,...,
with probability 1. Under some assumptions on f₀, g₀ and an additional condition that one of the processes is nonnegative, some sharp inequalities between the pth norms of f and g, 0 < p < ∞, are established. As an application, related sharp inequalities for stochastic integrals and harmonic functions are obtained.