  A (v,b,r,k) combinatorial configuration is a (r,k)-biregular bipartite graph
with v vertices on the left and b vertices on the right and with no cycle of
length 4. Combinatorial configurations have become very important for some
cryptographic applications to sensor networks and to peer-to-peer communities.
Configurable tuples are those tuples (v,b,r,k) for which a (v,b,r,k)
combinatorial configuration exists. It is proved in this work that the set of
configurable tuples with fixed r and k has the structure of a numerical
semigroup. The numerical semigroup is completely described for r=2 and r=3.
