  Homological mirror symmetry predicts that there is a relation between
autoequivalence groups of derived categories of coherent sheaves on Calabi-Yau
varieties, and the symplectic mapping class groups of symplectic manifolds. In
this paper, as an analogue of Dehn twists for closed oriented real surfaces, we
study spherical twists for dg-enhanced triangulated categories.
  We introduce the intersection number and relate it to group-theoretic
properties of spherical twists. We show an inequality analogous to a
fundamental inequality in the theory of mapping class groups about the behavior
of the intersection number via iterations of Dehn twists. We also classify the
subgroups generated by two spherical twists using the intersection number. In
passing, we prove a structure theorem for finite dimensional dg-modules over
the graded dual numbers and use this to describe the autoequivalence group.
  As an application, we compute the center of autoequivalence groups of derived
categories of K3 surfaces.
