Several other notations are commonly used for groups whose elements are not numbers. For a group whose elements are functions, the operation is often function composition ⁠ f ∘ ∘ g ⁠ ; then the identity may be denoted id. In the more specific cases of geometric transformation groups, symmetry groups, permutation groups, and automorphism groups, the symbol ∘ ∘ is often omitted, as for multiplicative groups. Many other variants of notation may be encountered.