 The Kochen operator can be thought of as a "p"-adic (or "v"-adic) analogue of the square function in the real case.An extension field "F" of "K" is formally "v"-adic if and only if formula_19 does not belong to the subring generated over the value ring of "K" by the image of the Kochen operator on "F".This is an analogue of the statement (or definition) that a field is formally real when formula_20 is not a sum of squares.