The synthesis problem of concurrent systems is the problem of synthesi
zing a concurrent system model from sequential observations. The paper
studies the synthesis problem for elementary Petri nets and transitio
n systems. A characterization of the class of transition systems which
correspond to elementary Petri nets is proven. It is shown how to gen
erate all elementary Petri nets corresponding to a given transition sy
stem. If there is any such elementary Petri net, it is proven that the
re always exists a small one which has only polynomially many elements
in the size of the transition system.