Transition probability of perturbative form for nu_mu -> nu_e oscillations in matter of constant density

We give a convenient expression for the appearance probability P(nu_mu -> nu_e) describing neutrino oscillations in matter of constant density, derived using textbook quantum mechanics stratagems. Our formulation retains the clarity of an expansion in alpha = Delta m_{21}^2/Delta m_{31}^2 exhibited by the popular Cervera et al. formula [Nucl. Phys. B 579, 17 (2000)] while enabling more accurate evaluation of oscillations over terrestrial baselines.