Keccak follows a sponge construction. Can we say that Keccak employs a compression function? Generally speaking, for sponge constructions, can we say that there is an underlying compression function?
The sponge construction does not have a compression function in the sense of traditional hash constructions like Merkle–Damgård. Instead, it operates using a permutation function $f$ which "mixes" or "absorbs" the input into the state of the algorithm. Strictly speaking, it does take an input larger than the output it produces, but this function is actually reversible and so is not comparable to one-way compression functions like in MD5 or SHA-1.