S04E18 The Prestidigitation Approximation

Priya forces Leonard to choose between a relationship with her or a friendship with Penny. A magic trick by Howard has Sheldon fooled.

S04E17 The Toast Derivation

Sheldon becomes concerned when everybody begins to hang out at Raj's apartment and discovers Leonard is the center of the group.

S04E16 The Cohabitation Formulation

Bernadette makes Howard choose between herself and his mother, and how will Penny react when Leonard begins seeing Raj's sister again when she's back in town?