Gauge Fields and Singletons of $AdS_{2p+1}$
  We show that $p$-forms on $AdS_{2p+1}$ describe both singletons and massless particles. On the $2p$-dimensional boundary the singleton $p$-form Lagrangian reduces to the conformally invariant functional $\int F^2$. All the representations, singletons as well as massless, are zero center modules and involve a vacuum mode. Two- and three-form singleton fields are required by supersymmetry in AdS$_5$ and AdS$_7$ supergravity respectively. 
S. Ferrara and C. Fronsdal