  We develop algorithms for online linear regression which achieve optimal
static and dynamic regret guarantees \emph{even in the complete absence of
prior knowledge}. We present a novel analysis showing that a discounted variant
of the Vovk-Azoury-Warmuth forecaster achieves dynamic regret of the form
$R_{T}(\vec{u})\le O\left(d\log(T)\vee
\sqrt{dP_{T}^{\gamma}(\vec{u})T}\right)$, where $P_{T}^{\gamma}(\vec{u})$ is a
measure of variability of the comparator sequence, and show that the discount
factor achieving this result can be learned on-the-fly. We show that this
result is optimal by providing a matching lower bound. We also extend our
results to \emph{strongly-adaptive} guarantees which hold over every
sub-interval $[a,b]\subseteq[1,T]$ simultaneously.
