Season 3, Episode 9 - The Vengeance Formulation
Aired November 23, 2009
When Barry Kripke pulls a prank on Sheldon which embarrasses him on national radio, Sheldon vows revenge. Meanwhile, Howard tries to keep his new relationship with Bernadette going.
Season 4, Episode 6 - The Irish Pub Formulation
Aired October 28, 2010
Raj's sister Priya is in town and soon rekindles a past secret relationship with Leonard. Leonard keeps his new affair a secret from the rest of the gang, but Sheldon finds out about the relationship and helps Leonard by developing an elaborate lie.
Season 4, Episode 16 - The Cohabitation Formulation
Aired February 17, 2011
Raj's sister, Priya, is in town again and rekindles her relationship with Leonard, prompting Penny to reconsider her feelings for him. When Bernadette asks Howard to move in with her, he is forced to choose between his girlfriend and his mother.