  We show that all languages accepted in time f(n) >= n^2 can be accepted in
space O(f(n)^{1/2})_and_ in time O(f(n)). The proof is carried out by
simulation, based on the idea of guessing the sequences of internal states of
the simulated TM when entering certain critical cells, whose location is also
guessed. Our method cannot be generalised easily to many-tapes TMs, and in no
case can it be relativised.
