The paper is devoted to the analysis of ill-posed operator
equations $Ax=y$ with injective linear operator $A$ and solution $x_0$
in a Hilbert space setting.
We present some new ideas and results for finding convergence rates in Tikhonov
regularization based on the concept of approximate source conditions
by means of using distance functions with a general benchmark.
For the case of compact operator $A$ and benchmark functions of power-type
we can show that there is a one-to-one correspondence between the maximal
power-type decay rate of the distance function and the best possible
H\"older exponent for the noise-free convergence rate in Tikhonov
regularization. As is well-known this exponent coincides
with the supremum of exponents in power-type source conditions.
The main theorem of this paper is devoted to the impact of range inclusions
under the smoothness assumption that $x_0$ is in the range of some positive
self-adjoint operator $G$. It generalizes a convergence rate result
proven for compact $G$ in [12] to the case of general operators $G$ with non-closed range.
Keywords:
linear ill-posed problems, Tikhonov regularization, distance functions, convergence rates, Hölder exponent, index function, qualification, source condition, range inclusion