Dr. Wolcott asks Sheldon is he is familiar with non-abelian group theory. In The Higgs Boson Observation (2012), Penny found a paper written by a five-year-old Sheldon dealing with abelian groups.
Title reference: Sheldon sees potential in becoming a reclusive scientist, thinking that the isolation and lack of distraction will help him make his big scientific breakthrough.
In The Precious Fragmentation (2010), the guys purchased a box at a garage sale. One of the contents is a script for Ghostbusters II (1989). Guest star Peter MacNicol played Dr. Janosz Poha in that movie, the minion to Vigo the Carpathian.