 If the form formula_6 is equivalent to formula_7, then the products formula_5 and formula_9 differ by an element of the form formula_10 with formula_11 in "K".These elements form an additive subgroup "U" of "K".Hence the coset of formula_5 modulo U is an invariant of formula_13, which means that it is not changed when formula_13 is replaced by an equivalent form.