We are interested in new proofs, connections with others theories and variations of the following theorem.
Th K (Karamata). If $f$ is monotone on some interval $[A,\infty)$,
$$ \lim{x\rightarrow \infty}(f(x+t)- f(x))= \psi(t) . \leqno (1)$$
for $t_1,t_2 \in \mathbb{R}$ such that $t_2/t_1$ is irrational and $\psi(t_1),\psi(t_2)$ finite, then (1) holds for all $ t \in \mathbb{R}$ and
$\psi(t)= c t$ for some real constant $c$.