In the last years convergence rates results for Tikhonov regularization of nonlinear ill-posed problems in Banach spaces
have been published, where the classical concept of source conditions was replaced with
variational inequalities holding on some level sets. Also this advanced essentially the analysis of non-smooth situations
with respect to forward operators and solutions. In fact, such variational inequalities combine both structural conditions
on the nonlinearity of the operator and smoothness properties of the solution. Varying exponents in
the variational inequalities correspond to different levels of convergence rates. In this paper, we discuss
the range of occurring exponents in the Banach space setting. To lighten the cross-connections between
generalized source conditions, degree of nonlinearity of the forward operator and associated variational inequalities
we study the Hilbert space situation and even prove some converse result for linear operators.