Systematic Approach to Gauge-Invariant Relations between Lepton Flavor   Violating Processes
  We analyze four-lepton contact interactions that lead to lepton flavor violating processes, with violation of individual family lepton number but total lepton number conserved. In an effective Lagrangian framework, the assumption of gauge invariance leads to relations among branching ratios and cross sections of lepton flavor violating processes. In this paper, we work out how to use these relations systematically. We also study the consequences of loop-induced processes. 
Alejandro Ibarra, Eduard Masso and Javier Redondo