String equation

Given a Gromov-Witten invariant involving $n+1$ marked points but with no constraints on the last point, the string equation expresses this in terms of invariants with fewer marked points. It is equivalent to annihilation by the differential operator $L_{-1}$ in the Virasoro conjecture. (The string equation is a known fact; this part of the Virasoro conjecture is a proved theorem.)