  We report results of a study of the Bs oscillation frequency using a large
sample of Bs semileptonic decays corresponding to approximately 1 fb-1 of
integrated luminosity collected by the D0 experiment at the Fermilab Tevatron
Collider in 2002-2006. The amplitude method gives a lower limit on the Bs
oscillation frequency at 14.8 ps-1 at the 95% C.L. At \Delta m_s = 19 ps-1, the
amplitude deviates from the hypothesis A=0 (A=1) by 2.5 (1.6) standard
deviations, corresponding to a two-sided C.L. of 1% (10%). A likelihood scan
over the oscillation frequency, \Delta m_s, gives a most probable value of 19
ps-1 and a range of 17 < \Delta m_s < 21 ps-1 at the 90% C.L., assuming
Gaussian uncertainties. This is the first direct two-sided bound measured by a
single experiment. If \Delta m_s lies above 22 ps-1, then the probability that
it would produce a likelihood minimum similar to the one observed in the
interval 16 < \Delta m_s < 22 ps-1 is (5.0+-0.3)%.
