  We study a frustrated mixed spin chain with side chains, where the spin
species and the exchange interactions are spatially varied. A nonlinear sigma
model method is formulated for this model, and a phase diagram with two
disordered spin-gap phases is obtained for typical cases. Among them we examine
the case with a main chain consisting of an alternating array of spin-1 and
spin-1/2 sites and side chains each of a single spin-1/2 site in great detail.
Based on numerical, perturbational, and variational approaches, we propose a
singlet cluster solid picture for each phase, where the ground state is
expressed as a tensor product of local singlet states.
