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 and had the link: https://apps.dtic.mil/sti/tr/pdf/AD102328.pdf (which now appears to be broken) 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 I have left the cover on as a historical reference.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