Improved uniform error bounds for the time-splitting methods for the long-time dynamics of the Schrödinger/nonlinear Schrödinger equation
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We establish improved uniform error bounds for the time-splitting methods for the long-time dynamics of the Schrödinger equation with small potential and the nonlinear Schrödinger equation (NLSE) with weak nonlinearity. For the Schrödinger equation with small potential characterized by a dimensionless parameter ε∈ (0, 1] representing the amplitude of the potential, we employ the unitary flow property of the (second-order) time-splitting Fourier pseudospectral (TSFP) method in L^2-norm to prove a uniform error bound at C(T)(h^m +τ^2) up to the long time T_ε= T/ε for any T>0 and uniformly for 0<ε≤1, while h is the mesh size, τ is the time step, m ≥ 2 depends on the regularity of the exact solution, and C(T) =C_0+C_1T grows at most linearly with respect to T with C_0 and C_1 two positive constants independent of T, ε, h and τ. Then by introducing a new technique of regularity compensation oscillation (RCO) in which the high frequency modes are controlled by regularity and the low frequency modes are analyzed by phase cancellation and energy method, an improved uniform error bound at O(h^m-1 + ετ^2) is established in H^1-norm for the long-time dynamics up to the time at O(1/ε) of the Schrödinger equation with O(ε)-potential with m ≥ 3, which is uniformly for ε∈(0,1]. Moreover, the RCO technique is extended to prove an improved uniform error bound at O(h^m-1 + ε^2τ^2) in H^1-norm for the long-time dynamics up to the time at O(1/ε^2) of the cubic NLSE with O(ε^2)-nonlinearity strength, uniformly for ε∈ (0, 1]. Extensions to the first-order and fourth-order time-splitting methods are discussed.
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