Taking into account the truncated space, the norm is: n 2 = ∑ ∑ α α l = 1 χ χ c ( c α α l − − 1 α α l ) 2 ⋅ ⋅ ( λ λ ′ α α l [ l ] ) 2 = ∑ ∑ α α l = 1 χ χ c ( c α α l − − 1 α α l ) 2 ( λ λ α α l [ l ] ) 2 R = S 1 R