Definition of Rn: [mDtDtue]n+β|[Dtue]n−12|[Dtue]n+12+s(uen)−Fn=Rn.
Truncation error of the geometric mean, see (21)-(22), |[Dtue]n−12|[Dtue]n+12=[|Dtue|Dtue]n−14u′(tn)2Δt2+14u(tn)u″(tn)Δt2+O(Δt4). Using (3)-(4) for the Dtue factors results in [|Dtue|Dtue]n=|ue′+124ue‴(tn)Δt2+O(Δt4)|×(ue′+124ue‴(tn)Δt2+O(Δt4))