Apple Posts iMac (Late 2013) Memory Specifications
Install memory in an iMac - Apple Support
New iMac 21" Review: Thin, Immersive Display Stands Out (Part 1)
21.5 inch late 2013 iMac - RAM upgrade - Apple Community
A-Tech 32GB Kit (4x8GB) RAM for Apple iMac (Late 2012, Late 2013, Late 2014, Mid 2015) | DDR3 1600MHz SODIMM PC3-12800 204-Pin SO-DIMM Max Memory Upgrade at Amazon.com
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
Memory Upgrades for iMac 21.5-Inch 2012 - Late 2013
16GB KIT 2X 8GB for Apple iMac 2.7GHz 21.5" Late 2013 2.93GHz 27" Late
NEMIX RAM 32GB Kit (4x8GB Upgrade) DDR3-1600 SODIMM PC3-12800 for Apple MacBook Pro Mid 2012 / iMac Late 2012 / iMac 2013, Late 2014, Mid 2015, Mac Mini Late 2012
Memory Upgrades for iMac 21.5-Inch 2012 - Late 2013