TITLE: Unitary representation is strictly continuous
QUESTION [7 upvotes]: Let $G$ be a compact group and $u: G \to B(H)$ be a strongly continuous unitary representation on the Hilbert space $H$. Then is $u: G \to B(H)$ strictly continuous?
That is, give $B(H)$ the topology induced by the $*$-isomorphism $M(B_0(H))\cong B(H)$. Explicitely,  a net $(x_i)$ in $B(H)$ converges strictly to $x$ iff $\|x_i y -x y\|\to 0$ and $\|yx_i \to yx\| \to 0$ for all compact operators $y \in B_0(H)$. On bounded subsets this agrees with the $*$-strong topology.

REPLY [5 votes]: As you note, on bounded sets, the strict topology and the strong-$\ast$ topology agree on bounded sets.  As the set of unitary operators is bounded, we can just work with the strong-$\ast$ topology.  If $(u_i)$ is a net of unitary operators converging strongly to $u$ a unitary, then for $\xi\in H$,
$$ \| u_i^\ast(\xi) - u^\ast(\xi)\|^2 = \|u_i^\ast(\xi)\|^2 - (u_i^\ast(\xi)|u^\ast(\xi)) - (u^\ast(\xi)|u_i^\ast(\xi)) + \|u^\ast(\xi)\|^2 $$
Here I write $(\cdot|\cdot)$ for the inner product.  As $u_i,u$ are unitary, this is equal to
$$ 2\|\xi\|^2 - (\xi|u_iu^\ast(\xi)) - (u_iu^\ast(\xi)|\xi). $$
As $u_i(\eta)\rightarrow u(\eta)$ for any $\eta$, this converges to
$$ 2\|\xi\|^2 - (\xi|u u^\ast(\xi)) - (u u^\ast(\xi)|\xi) = 0. $$
Thus $u_i\rightarrow u$ strong-$\ast$.  This is just a proof that the strong and strong-$\ast$ topologies agree on the set of unitaries; this is surely in standard textbooks, if you look hard.