  Anisotropic flow coefficients beyond triangular flow receive important
contributions from lower-order harmonics through nonlinear coupling. We present
a theoretical framework which allows one to quantify the contribution induced
by such nonlinear couplings to any flow harmonic of any order. We show the
effectiveness of this formalism through an application to hexagonal flow,
$V_6$. We study, in particular, the coupling of $V_6$ to triangular flow,
$V_3$, in Pb+Pb collisions at $~\sqrt[]{s}=2.76$ TeV, using both Large Hadron
Collider data and event-by-event hydrodynamic calculations.
