“…and C is a positive constant depending only on the norms of u in the spaces mentioned. [41,45] We know that, the J N (Λ)×J N (0, T ) is a finite subspace of H r (Λ)×L 2 (0, T ), according to the assumption u ∈ H r (Λ) × L 2 (0, T ), the ∥u∥ = ∫ T 0 ∥u∥ 2 r,ω dt < ∞, therefore, there is an N 0 ∈ N that for any N > N 0 , N r be bigger than ∥u∥ or equivalently C. So, if N be large enough, we can say: if N −→ ∞ then ∥u − u N ∥ H r (Λ)×L 2 (0,T ) −→ 0, hence, the error of this approximation by increasing N will be decreased. In numerical examples this principle will be shown.…”