We present a simple and direct proof of the equivalence of various fun
ctional inequalities such as Sobolev or Nash inequalities. This proof
applies in the context of Riemannian or sub-elliptic geometry, as well
as on graphs and to certain non-local Sobolev norms. It only uses ele
mentary cut-off arguments. This method has interesting consequences co
ncerning Trudinger type inequalities.