Episode Titles: Oscillation

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  The Separation Oscillation
  Season 9, Episode 2 - Aired September 28, 2015

  As Leonard and Penny struggle with the revelation he kissed another woman, Leonard goes to meet the woman in question, Mandy, in a bid to show Penny there's nothing to worry about. Elsewhere, Sheldon films an episode of Fun with Flags alone following his break-up with Amy, who is angered by Sheldon's thinly veiled comments about their recent split.

  Meanwhile, Bernadette is uncomfortable with the fact she's known about Leonard's kiss for two years after Howard told her, while Raj is unhappy that he did not share the gossip with him.
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  The Solo Oscillation
  Season 11, Episode 13 - Aired January 11, 2018

  After Sheldon decides to work solo without Amy, she turns to Leonard to work together on a series of science experiments. Meanwhile, geologist Bert replaces Howard in Footprints on the Moon, while Sheldon gets an unexpected source of scientific inspiration from Penny.
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  The Donation Oscillation
  Season 12, Episode 15 - Aired February 7, 2019

  When Penny's father, Wyatt, comes to town, he reopens the debate over Penny not wanting kids and Leonard donating his sperm to Zack and Marissa.

  Meanwhile, Howard decides to keep the reservation for the "vomit comet" he originally made for Raj's planned bachelor party. Raj invites Anu and wants to turn it into a couples' trip, but Howard doesn't think Bernadette would enjoy it.