  Cohomological genus-0 Gromov-Witten invariants of a given target space can be
encoded by the "descendant potential," a generating function defined on the
space of power series in one variable with coefficients in the cohomology space
of the target. Replacing the coefficient space with the subspace
multiplicatively generated by degree-2 classes, we explicitly reconstruct the
graph of the differential of the restricted generating function from one point
on it. Using the Quantum Hirzebruch--Riemann--Roch Theorem from our joint work
with Valentin Tonita, we derive a similar reconstruction formula in genus-0
quantum K-theory. The results amplify the role of the divisor equations, and
the structures of $D$-modules and $D_q$-modules in quantum cohomology and
quantum K-theory with respect to Novikov's variables.
