What was found
LANDING GEAR WOULD NOT RETRACT AFTER TAKEOFF. GEAR DOOR OPEN LIGHT AND ALL UNSAFE LIGHTS WERE ILLUMINATED. GEAR HANDLE WAS PLACED DOWN AND DOOR OPEN LIGHT REMAINED ON. CREW DECLARED AN EMERGENCY AND FLIGHT WAS RETURNED TO MEM. AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED BROKEN RIVET AT MAIN LANDING GEAR DOOR BYPASS LEVER. ALSO, REPLACED FLAG ON LEVER. OPERATIONAL CHECK NORMAL. AIRCRAFT RETURNED TO SERVICE.