It's always annoying when one finds in books (written by (theoretical) physicists for (theoretical) physics students) statements like those below without a mere cross-reference for a mathematically-rigurous proof. And that's what i'm searching for right now: either point me to some books, or post the proofs right here (that would be perfect). So here's what's been bugging me lately: 1. A noncompact Lie group has no nontrivial finite dimensional unitary linear irreps. 2. The linear representations of a simply connected Lie group are fully reducible. 3. The unitary linear irreps of a simply connected Lie group can be put in bijective correspondence with (essentially) selfadjoint irreps of the corresponding Lie algebra. 4. The Theorem of Nelson. The only reference for a proof that i found is the original article by Nelson, but, unfortunately, it's not within my reach. The relevance of this thread: these mathematical results are fundamental in understanding the concept of implementing space-time symmetries in the Hilbert space language of QM through the so-called "Wigner-Weyl method". Daniel.