Lyapunov exponents of the half-line SHE

We consider the half-line stochastic heat equation (SHE) with Robin boundary parameter $A = -\frac{1}{2}$. Under narrow wedge initial condition, we compute every positive (including non-integer) Lyapunov exponents of the half-line SHE. As a consequence, we prove a large deviation principle for the upper tail of the half-line KPZ equation under Neumann boundary parameter $A = -\frac{1}{2}$ with rate function $Φ_+^{\text{hf}} (s) = \frac{2}{3} s^{\frac{3}{2}}$. This confirms the prediction of [Krajenbrink and Le Doussal 2018] and [Meerson, Vilenkin 2018] for the upper tail exponent of the half-line KPZ equation.

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