Slightly tweaked text and formatting of formulas at various places, for beautification.

]]>And he removed it in rev 12. Hereby.

Thanks for the heads-up. I forget why I went to put that condition in.

]]>This was added in Revision 8 by Urs Schreiber on September 24, 2018 at 11:38:27.

]]>slightly expanded the section on Generalizations and variants:

Made the pointer to Schur’s lemma for simple objects in abelian categories more recognizable, and added pointer to Schur’s lemma for Bridgeland-stable objects

]]>did some substantial edits on this entry:

gave it more of an Idea-section,

tried to streamline the statement of the lemma

spelled out the proof,

added a discussion explaining how this is about irreps forming a (de-)categorified orthogonal/orthonormal linear basis of the representation ring.