f(x) (Korean: 에프엑스; pronounced /ˌɛf ˈɛks/) is a five member South Korean girl group formed by SM Entertainment in 2009. The name is a play on the mathematical notation for function, and when input with different 'x', there can be many different outcomes. f(x) strives to be the kind of girl group that can change and adapt to many situations. The group members have also been nicknamed "function girls". The quintet consists of Victoria, Amber, Luna, Sulli and Krystal. 

http://en.wikipedia.org/wiki/F(x)_(band))
f(x)