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We introduce the notion of Karamata regular operators, which is a notion of regularity
that is suitable for obtaining concrete convergence rates for common fixed point problems.
This provides a broad framework that includes, but goes beyond, Hölderian error bounds and
Hölder regular operators. By concrete, we mean that the rates we obtain are explicitly expressed in terms of a function of the iteration number \(k\) instead, of say, a function of the iterate
\(x^k\). While it is well-known that under Hölderian-like assumptions many algorithms converge
linearly/sublinearly (depending on the exponent), little it is known when the underlying prob-
lem data does not satisfy Hölderian assumptions, which may happen if a problem involves
exponentials and logarithms. Our main innovation is the usage of the theory of regularly
varying functions which we showcase by obtaining concrete convergence rates for quasi-cylic
algorithms in non-Hölderian settings. This includes certain rates that are neither sublinear
nor linear but sit somewhere in-between, including a case where the rate is expressed via the
Lambert W function. Finally, we connect our discussion to o-minimal geometry and show that
definable operators in any o-minimal structure are always Karamata regular.
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