It is best not to use......
The best one could hope for is K≥ F.
This result is best possible. [Or: the best possible]
In Theorem 2 we show that the bound is not far from best possible.
The above bound on an is close to best possible.
The best known of these is the Knaster continuum.
It can be inferred from known results that these series at best converge conditionally in Lp.