An equivalence relation arising in formal language theory. If L is a language over alphabet Σ (see word) then its Myhill equivalence is the relation =M on Σ* defined as follows:
if, for all
w1,
w2 in Σ*,
Similarly (and more generally), if
f is a function from Σ* to any set, its Myhill equivalence is defined by:
if, for all
w1,
w2 in Σ*,
See also Nerode equivalence.
An important fact is that L is regular iff the equivalence relation =M is of finite index (i.e. there are finitely many equivalence classes). Indeed, L is regular iff it is a union of classes of any equivalence relation of finite index. In addition =M is a congruence on Σ*, i.e.
The equivalence classes therefore can be concatenated consistently and form a semigroup. This is in fact the semigroup of the minimal machine for
L (or
f).