f(x) is a four member South Korean girl group formed by SM Entertainment active from 2009 to 2016 (and a reunion in 2019 with a 3-night performance of 3 members on SMTown Live in Tokyo). The name is a play on the mathematical notation for function, with "f" signifying "flower" and "x" representing the female's double X chromosome. The group made their debut on September 1, 2009 with release of a digital single "라차타 (LA chA TA)". The quartet consists of 宋茜 (Victoria), 엠버 (Amber), 루나 (Luna), and 크리스탈 (Krystal).