Duality in the non-relativistic harmonic oscillator quark model in the   Shifman-Voloshin limit : a pedagogical example
  The detailed way in which duality between sum of exclusive states and the free quark model description operates in semileptonic total decay widths, is analysed. It is made very explicit by the use of the non relativistic harmonic oscillator quark model in the SV limit, and a simple interaction current with the lepton pair. In particular, the Voloshin sum rule is found to eliminate the mismatches of order $\delta m/m_b^2$. 
A. Le Yaouanc, D. Melikhov, V. Morenas, L. Oliver, O. P\`ene, J.-C.   Raynal