  We present an account of the construction by S. Sekiguchi and N. Suwa of a
cyclic isogeny of affine smooth group schemes unifying the Kummer and
Artin-Schreier-Witt isogenies. We complete the construction over an arbitrary
base ring. We extend the statements of some results in a form adapted to a
further investigation of the models of the group schemes of roots of unity.
