= Initial integrability under a density change
{title2=$X_0\in L^1(Q)$}
The density-product proof of the <Girsanov theorem> makes drift-corrected zero-starting increments a <local martingale> under the new measure. Adding the initial random variable requires its integrability under that measure. This is automatic if the density is one at time zero, because the measures agree on the initial <sigma-algebra>. Without this normalization it can fail: take $P(n)=2^{-n}$, $Q(n)=c/n^2$, constant $M_t=\log(Q(n)/P(n))$ and constant $X_t=n$. These are P-martingales and the prescribed exponential density is valid, but $X_0$ is not Q-integrable.
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