A model is proposed for the spectrum of $Lambda$ hypernuclei based on the $u(3)times u(2)$ Lie algebra, in which the internal degrees of freedom of the spin-1/2 $Lambda$ particle are treated in the Fermionic $u(2)$ scheme, while the motion of the hyperon inside a nucleus is described in the Bosonic $u(3)$ harmonic oscillator scheme. Within this model, a simple formula for single-particle energies of the $Lambda$ particle is obtained from the natural dynamical symmetry. The formula is applied to the experimental data on the reaction spectroscopy for the $^{89}_Lambda$Y and $^{51}_Lambda$V hypernuclei, providing a clear theoretical interpretation of the observed structures.
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