In the Cee Lo Green song “Forget you”, the chord progression is C - D - F - C, the D has an F# which is out of key.
Within the limited music theory I learned so far, the closest thing I can relate this to is secondary dominant? Like C - D - G - C, the second D major chord is out of key and if you read it literally, it’s a D major chord, but really it functions like V/V, five of the five chord, which is G, which resolves to the next G chord, then back to C.
Is this C - D - F - C progression similar to C - D - G - C?
After reading answers and other resources I don't think the D is functioning as a secondary dominant in this particular case