Lie group homomorphisms and the adjoint representation

[10/10%]
Show
LaTeX

Construct an isomorphism of the groups U(1)U(1) and SO(2,R)S O(2, \mathbb {R}).

?

Show, without appealing to the complete description of the irreducible representations of the group SO(3)\mathrm{SO}(3), that a homomorphism τ:SO(3)⟶SU(2)\tau : \mathrm{SO}(3) \longrightarrow \mathrm{SU}(2) can only be trivial.

?

References

[Кос00] Кострикин А.И. Линейная алгебра. ФИЗМАТЛИТ, 2000.
[Кос04] Кострикин А.И. Основные структуры алгебры. ФИЗМАТЛИТ, 2004.
[Вин14] Винберг Э.Б. Курс алгебры. 2-е изд., МЦНМО, 2014.