  We give a pedagogical introduction to dynamical invariant formalism of
shortcuts to adiabaticity. For a given operator form of the Hamiltonian with
undetermined coefficients, the dynamical invariant is introduced to design the
coefficients. We discuss how the method allows us to realize adiabatic dynamics
and describe a relation to the counterdiabatic formalism. The equation for the
dynamical invariant takes a familiar form and is often used in various fields
of physics. We introduce examples of Lax pair, quantum brachistochrone, and
flow equation.
