Best uniform line for a fractional power ($p^{\ast }(x)=x+\frac{1}{2}(1-k)k^{k/(1-k)}$)

For $0<k<1$, $x^k-x$ is nonnegative on $[0,1]$ and has its maximum $(1-k)k^{k/(1-k)}$ at $x=k^{1/(1-k)}$. Shifting the chord $x$ upwards by half this maximum creates three alternating errors of equal magnitude. The equioscillation theorem proves that the displayed line is the unique best uniform linear approximation, with error equal to its constant shift.