"Lobachevsky" is a humorous song by Tom Lehrer, referring to the mathematician Nikolai Lobachevsky. According to Lehrer, the song is "not intended as a slur on [Lobachevsky's] character" and the name was chosen "solely for prosodic reasons".
In the introduction, Lehrer describes the song as an adaptation of a routine that Danny Kaye did to honor the Russian actor Constantin Stanislavski. Lehrer sings the song from the point of a view of a preeminent Russian mathematician who learns, from Lobachevsky, that plagiarism is the secret of success in mathematics. The narrator later uses this strategy to get a paper published ahead of a rival, then to write a book and earn a fortune selling the movie rights. Lehrer wrote that he did not know Russian. In the song he quotes two book reviews in Russian, the actual text of which bears no relation: the first phrase quotes Mussorgsky's Song of the Flea: "Once there was a king who had a pet flea." The second references a Russian joke: "Now I must go where even the Tsar goes on foot" [the bathroom].
The song was first performed as part of The Physical Revue, a 1951–1952 musical revue by Lehrer and a few other professors. It is track 6 on Songs by Tom Lehrer, which was re-released as part of Songs & More Songs by Tom Lehrer and The Remains of Tom Lehrer. In this early version, Ingrid Bergman is named to star in the role of "Hypotenuse" in The Eternal Triangle, a film purportedly based on the narrator's book. It was recorded again for Revisited (Tom Lehrer album), with Brigitte Bardot as Hypotenuse. A third recording is included in Tom Lehrer Discovers Australia (And Vice Versa), a live album recorded in Australia, featuring Marilyn Monroe as Hypotenuse. A fourth recording was made in 1966 when Songs by Tom Lehrer was reissued in stereo, with Doris Day playing Hypotenuse.
The song is frequently quoted, especially in works about plagiarism. Writing about it in Billboard, Jim Bessman calls the song "dazzlingly inventive in its shameless promotion of plagiarism", calling out in particular a sequence in which Lehrer strings together rhymes from the names of ten Russian cities. Mathematician Jordan Ellenberg has called it "surely the greatest comic musical number of all time about mathematical publishing".