  In 1975, Ladner showed that under the hypothesis that P is not equal to NP,
there exists a language which is neither in P, nor NP-complete. This result was
latter generalized by Schoning and several authors to various polynomial-time
complexity classes. We show here that such results also apply to linear-time
reductions on RAMs (resp. Turing machines), and hence allow for separation
results in linear-time classes similar to Ladner's ones for polynomial time.
