iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Amazon.com: Timetec 16GB KIT(2x8GB) Compatible for Apple DDR3L 1600MHz for Mac Book Pro(Early/Late 2011,Mid 2012), iMac(Mid 2011,Late 2012,Early/Late 2013,Late 2014,Mid 2015), Mac Mini(Mid 2011,Late 2012) MAC RAM : Electronics
Install memory in an iMac - Apple Support
Memory Upgrades for Apple iMac 2011 Up to 32GB
Installation de mémoire dans un iMac - Assistance Apple (FR)