This is a copy of George Koster’s ‘Notes on Group Theory‘, a now unclassified technical document that was prepared for the American Military in March 1956. It is now freely available to the public from the Defence Technical Information Center: https://apps.dtic.mil/dtic/tr/fulltext/u2/102328.pdf and is reproduced here.
Koster was one of the contributors to group theory and this work remains a fantastic reference for representation theory. I have left the cover on as a historical reference.
Koster Notes on group theory 102328