On a reverse Heinz–Kato–Furuta inequality

In the set up of Minkowski spaces, the Schwarz inequality holds with the reverse inequality sign. As a consequence, the same occurs with the triangle inequality. In this note, extensions of this indefinite version of the Schwarz inequality are presented. Namely, a reverse Heinz–Kato–Furuta inequality valid for timelike vectors is included and related inequalities that also hold with the reverse sign are investigated.