Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 37 / 1 / b / Solution

As with quadratic variation, the increasing process must be normalized by $A_0=0$. Suppose $A,A^{\prime }$ have this initial value and both $M^2-A$ and $M^2-A^{\prime }$ are continuous local martingales. Since $M^2$ and these differences are continuous, both $A$ and $A^{\prime }$ are continuous. Their difference is a finite-variation process, andis a continuous local martingale. The result proved in part (a) makes this difference constant. Its initial value is zero, so $A$ and $A^{\prime }$ are indistinguishable. This is the uniqueness of an increasing square compensator; it uses no existence argument.

If no initial-value normalization is imposed, the literal uniqueness assertion is false: whenever $A$ works, $A+c$ works for every deterministic constant $c$, since subtracting a constant preserves the local martingale property. The exact unnormalized conclusion is $A_t-A_t^{\prime }=A_0-A_0^{\prime }$ for all $t$, up to indistinguishability of stochastic processes.