A play by David Aubern, first produced in 2000. It won the 2001 Pulitzer Prize for Drama and the 2001 Tony Award for Best Play. [[Series/{{Weeds}} Mary Louise Parker]] won a Tony for Best Actress for her portrayal of Catherine.

Catherine is the daughter of a brilliant but mentally ill mathematician, Robert, and has acted as his caretaker up until his recent death, putting off her own education to do so. Now in the wake of his death she has to deal with her well-meaning but frustrating older sister Claire, Robert's former grad student Hal's discovery of a paradigm-shifting proof he attributes to Robert despite Catherine's insistence that she wrote it, and with the ghost of her father's genius and insanity, and how much of each she has inherited.

Was made into a film in 2005. The film starred Creator/GwynethPaltrow as Catherine, Creator/AnthonyHopkins as Robert, Creator/JakeGyllenhaal as Hal and Hope Davis as Claire.
!!Tropes featured include:
* {{Adorkable}}: Hal.
* BlackSheep: Claire. She doesn't share her father and sister's genius, nor, as a result, their bond.
* BrokenBird: Catherine has a lot of emotional baggage. A lot.
* TheCaretaker: Catherine, to her father, and later Claire to Catherine for a short period.
* UsefulNotes/{{Chicago}}: Set in the Windy City
* CloudCuckooLander: A heartbreaking example, Robert's writings in the scene in which Catherine discovers he's relapsed.
* CoolOldGuy: Robert, when he's coherent.
* CoolTeacher: Before his illness Robert was a professor, and well loved by his students.
* DeadAllAlong The play opens with a conversation between Catherine and Robert, until Catherine remembers that wait a second, Robert's dead. Makes his reassurance that she's not crazy ring kind of hollow.
* DeadPersonConversation: Opening scene between Catherine and Robert
* DefrostingTheIceQueen: Catherine goes through a bit of this in the first act, going from angry and sullen to actually trusting Hal enough to show him the proof.
* TheDutifulSon: Played with; both sisters invoke it on the other. Catherine stays home to care for her father and resents her older sister Claire for going to college and living her life in New York. But Claire is the more practical and stable one, and points out that she made a great deal of sacrifices in order to be able financially support her father and sister.
* ItRunsInTheFamily: Catherine - and her sister - worry the Robert's mathematical genius isn't the only thing Catherine inherited from him.
* GeekyTurnOn: Hal and Catherine's first kiss is while they are discussing mathematicians.
* TheGloriousWarOfSisterlyRivalry: Catherine and Claire to minor extent, but it isn't played up very much. Both are very smart and both are attractive but Claire plays up her looks more while Catherine has a natural genius. As a result they fill the roles of the pretty and the smart sister, respectively.
* GoodWithNumbers: Applies to all four characters, though Claire explicitly says that she's always been good with numbers, but doesn't come close to her father's (or her sister's) genius.
* [[LikeFatherLikeSon Like Father Like Daughter]]
* MadMathematician: Robert has schizophrenia, as seen in the flashback scenes, and his work devolves into bizarre ramblings (see below). Possibly Catherine.
* MadnessMantra: Robert's writings degrade into this.
-->"Let X equal the quantity of all quantities of X. Let X equal the cold. It is cold in December. The months of cold equal November through February. There are four months of cold, and four of heat, leaving four months of indeterminate temperature. In February it snows. In March the Lake is a lake of ice. In September the students come back and the bookstores are full. Let X equal the month of full bookstores. The number of books approaches infinity as the number of months of cold approaches four. I will never be as cold now as I will in the future. The future of cold is infinite. The future of heat is the future of cold. The bookstores are infinite and so are never full except in September..."
* MaybeEverAfter: At the end of the play it seems that Catherine has forgiven Hal, as she's discussing the proof with him, but the nature of their future relationship is left open ended.
* MaybeMagicMaybeMundane: The opening scene where Catherine talks to her (dead) father. Maybe she's losing it, or maybe she was just dozing off, or maybe it was ghost (generally thought to be one of the first two).
* TheMentallyDisturbed: Robert
* MissingMom: The mom gets barely a passing mention, having died years before the action of the play.
* [[NiceJobBreakingItHero Nice job betraying her trust, Hal.]]
* PosthumousCharacter: Robert
* ThePublicDomainChannel: Robert watches ''Film/NightOfTheLivingDead1968''.
* SecondActBreakup: Though the romance isn't the main focus of the play, it pretty much follows this. After finally starting to trust Hal, that all goes away when he doesn't believe her about the proof. By the end of the play she's tentatively forgiven him and we're left with a MaybeEverAfter.
* SheCleansUpNicely: Catherine in the dress Claire buys her.