Form factors in the massless coset models su(2)_{k+1} otimes su(2)_k /su(2)_{2k+1} - Part I

Massless flows between the coset model su(2)_{k+1} \otimes su(2)_k /su(2)_{2k+1} and the minimal model M_{k+2} are studied from the viewpoint of form factors. These flows include in particular the flow between the Tricritical Ising model and the Ising model. Form factors of the trace operator with an arbitrary number of particles are constructed, and numerical checks on the central charge are performed with four particles contribution. Large discrepancies with respect to the exact results are observed in most cases.