"Fantaisie-Impromptu in C-Sharp Minor", by Frédéric Chopin, is a solo piano composition featured in "The Variable" in which it was played on the piano by Daniel Faraday, both as a boy and again in 2004.
In the flash-sideways timeline, David Shephard performed this piece and had a copy in his room. ("Lighthouse") Also in the flash-sideways timeline, Daniel played the piece at Eloise Widmore's charity fundraiser. ("Happily Ever After")
Daniel plays the middle section of this piano piece. The classic song "I'm Always Chasing Rainbows" is an adaptation of the Chopin melody with lyrics about a person's failure to find happiness in his life.
- Wikipedia - "Fantaisie-Impromptu in C-Sharp Minor"