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The unitary group of unitary operators on an infinite-dimensional separable complex Hilbert space is traditionally denoted . It does not quite have an established prose name, but is often referred to by some combination of the words “unitary group” and “strong topology”.
Its quotient by the circle subgroup U(1) is the corresponding projective unitary group PU(ℋ).
Throughout, let be any separably infinite-dimensional complex Hilbert space.
The set of bounded operators carries the following topologies, characterized by the conditions under which a sequence converges to a fixed operator :
The limit of this sequence is
…in the norm topology or topology of uniform convergence if
…in the strong operator topology or topology of pointwise convergence if
…in the weak operator topology if
…in the compact-open topology if
uniform convergence above holds for the restrictions and to any compact subset .
The norm topology makes a topological group, in fact a Banach Lie group. Further, it is metrizable, hence paracompact.
On the weak operator topology and strong operator topology agree and make it a topological group.
The compact-open topology coincides with the strong operator topology
The norm topology on (from Prop. ) is strictly finer than the (weak or strong) operator topology (from Prop. ).
In summary:
(also Espinoza & Uribe 2014, Thm. 1.2)
Equipped with the strong topology (Prop. ), is completely metrizable.
Equipped with the strong topology (Prop. ), is not locally compact.
(Kuiper’s theorem)
The topological unitary group in either the
or the
is contractible in that there is a left homotopy between the identity and the constant function .
The U(1)-quotient space coprojection of U(ℋ) over PU(ℋ) – both in their strong operator topology – is a circle-principal bundle:
Prop. means in particular that is locally trivial, hence that the coset space coprojection admits local sections. See also at coset space coprojection admitting local sections.
Early discussion of motivated from quantum mechanics and in relation to PU(ℋ):
David John Simms, pp. 5 in: Lie Groups and Quantum Mechanics, Lecture Notes in Mathematics 52, Springer (1968) [doi:10.1007/BFb0069914]
David John Simms: Topological aspects of the projective unitary group, Math. Proc. Camb. Phil. Soc. 68 1 (1970) 57-60 [doi:10.1017/S0305004100001043]
Beware of the false claim in Simms 1968 p. 10 that U(ℋ) is not a topological group in the strong operator topology, corrected by Schottenloher 2013, Espinoza & Uribe 2014, going back to Schottenloher 1995, but repeated by Atiyah & Segal 2004.
The mistake arises because multiplication in (bounded linear operators) is not generally continuous in the strong operator topology, according to
whence is indeed not a topological group in the strong operator topology, which however does not imply that its subgroup cannot have that property (cf. Schottenloher 2013 p 2-3).
Proof that is a topological group in the strong operator topology:
Martin Schottenloher, Thm. 3.2 in: Geometrie und Symmetrie in der Physik – Leitmotiv der Mathematischen Physik, Springer (1995) [doi:10.1007/978-3-322-89928-6, pdf]
Martin Schottenloher, Prop. 3.11 in: A Mathematical Introduction to Conformal Field Theory, Lecture Notes in Physics 759, Springer (2008) [doi:10.1007/978-3-540-68628-6, web]
Proof that the weak- and strong- operator topologies agree on :
Proof that weak-, strong- as well as the compact-open topology all agree on :
Martin Schottenloher: The Unitary Group In Its Strong Topology, Advances in Pure Mathematics 08 05 (2018) [doi:10.4236/apm.2018.85029, arXiv:1309.5891]
Jesus Espinoza, Bernardo Uribe: Topological properties of the unitary group, JP Journal of Geometry and Topology 16 1 (2014) 45-55 [arXiv:1407.1869, journal]
Previous influential but wrong claim that they do not and that the compact open topology does not even make a topological group (cf. above, concerning Simms 1968 p 10):
Further properties:
Proof of complete metrizability for the strong (and hence weak) operator topology:
Proof of failure of local compactness:
Proof of Kazhdan's property (T) for :
Bachir Bekka, Kazhdan’s Property (T) for the unitary group of a separable Hilbert space, Geom. funct. anal. 13, 509–520 (2003) [doi:10.1007/s00039-003-0420-0]
Thomas Nikolaus, Christoph Sachse, Christoph Wockel, A Smooth Model for the String Group, Int. Math. Res. Not. IMRN 16 (2013) 3678-3721 [doi:10.1093/imrn/rns154, arXiv:1104.4288]
Last revised on June 24, 2025 at 08:57:15. See the history of this page for a list of all contributions to it.