Aggregates are a computational model similar to circuits, but the
underlying graph is not necessarily acyclic. Logspace-uniform
polynomial-size aggregates decide exactly the languages in PSPACE;
without uniformity condition they decide the languages in
PSPACE/poly. As a measure of similarity to boolean circuits we
introduce the parameter component size. We prove that
already aggregates of component size 1
are powerful enough to capture polynomial space.
The only type of cyclic components needed to make polynomial-size
circuits as powerful as polynomial-size aggregates are binary
xor-gates whose output is fed back to the gate as one of the inputs.