for which functions

$\displaystyle f$ $\displaystyle :$ $\displaystyle R$ $\displaystyle \rightarrow$ $\displaystyle R$

$\displaystyle \forall$ $\displaystyle x,y$ $\displaystyle \in$ $\displaystyle R$

does

| $\displaystyle f(y)$ - $\displaystyle f(x)$ | $\displaystyle \mid$ $\displaystyle \leq$ $\displaystyle (y-x)^2$

hold