\(E_n\) computable groups
Let \(G\) be a group with an admissible index \(i\) and corresponding multiplication function \(m\). If \(i(G)\) is \(E_n\) decidable (( there is an \(E_n\) computable function which returns \(1\) on numbers which are indices of elements of \(G\), and \(0\) otherwise )) and \(m\) is \(E_n\) computable, then \(G\) is said to be \(E_n\) computable.
We can also add the characteristic function (( \(c_A\) such that \(c_A(x) = 1\) if \(x in A\), and \(0\) otherwise )) of a particular set to the primitive functions in the Grzegorczyk hierarchy to make some arguments about group computability easier. If we have \(c_A\), the characteristic function of the set \(A\), to our hierarchy, then we say a function (or group) is \(E_n(A)\) computable ((or “\(E_n\) computable relative to \(A\)”)) if it can be obtained from composition and bounded recursion on \(E_n\) functions and \(c_A\).