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Apple iMac 2011 Memory 4GB DDR3 1333MHz PC3-10600 204-Pin
Install memory in an iMac - Apple Support
Memory Upgrades for Apple iMac 2011 Up to 32GB
New 16GB 4x4GB Memory Apple iMac (21.5 and 27-inch, Mid 2011) PC3-10600 at 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 at Amazon.com
32.0GB OWC Memory Upgrade Kit For 2010-2011 Apple iMac
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET