title: Comment on "Structure of positive decompositions of exponential operators".
abstract: An elementary proof is shown on the necessary existence of negative coefficients in splitting methods of order p > or = 3.
authors.affiliation: Instituto de Matemática Multidisciplinar, Universitat Politécnica de Valencia, 46022-Valencia, Spain. serblaza@imm.upv.es
authors.forename: Sergio
authors.initials: S
authors.lastname: Blanes
authors.forename: Fernando
authors.initials: F
authors.lastname: Casas