# [FOM] Question about Congruence

Vaughan Pratt pratt at cs.stanford.edu
Mon Jan 28 16:50:48 EST 2008

```
hdeutsch at ilstu.edu wrote:
> Is the following fact well known?  It is of some philosophical
> interest, as explained below.
>
> (*) Let R be an equivalence relation defined on a set A and let B be a
> set. Then B is closed under R iff each element E of the partition P of
> A induced by R is such that if some element of E is in B, then they
> all are.

("let B be a set" should have been "let B be a subset of A")

The union of a subset of a partition P is a straightforward notion, for
which the above is easily seen to provide two equivalent
characterizations.  If either of them are documented somewhere (I've
never run across either) you can cite that, otherwise "evident" might be
more appropriate than "well-known."

One generalization would be to the notion of compatibility of a
subalgebra of an algebra A with a quotient of A, the above being the
case of an algebra with no operations.

Vaughan Pratt
```