Back-and-forth method

In mathematical logic, especially set theory and model theory, the back-and-forth method is a method for showing isomorphism between countably infinite structures satisfying specified conditions. In particular:

• It can be used to prove that any two countably infinite atomless Boolean algebras are isomorphic to each other.
• It can be used to prove that any two equivalent countable atomic models of a theory are isomorphic.

Application to densely ordered sets

Suppose that

• (A, ≤A) and (B, ≤B) are linearly ordered sets;
• They are both unbounded, in other words neither A nor B has either a maximum or a minimum;
• They are densely ordered, i.e. between any two members there is another;
• They are countably infinite.

Fix enumerations (without repetition) of the underlying sets:

A = { a1, a2, a3, … },
B = { b1, b2, b3, … }.

Now we construct a one-to-one correspondence between A and B that is strictly increasing. Initially no member of A is paired with any member of B.

(1) Let i be the smallest index such that ai is not yet paired with any member of B. Let j be some index such that bj is not yet paired with any member of A and ai can be paired with bj consistently with the requirement that the pairing be strictly increasing. Pair ai with bj.
(2) Let j be the smallest index such that bj is not yet paired with any member of A. Let i be some index such that ai is not yet paired with any member of B and bj can be paired with ai consistently with the requirement that the pairing be strictly increasing. Pair bj with ai.
(3) Go back to step (1).

It still has to be checked that the choice required in step (1) and (2) can actually be made in accordance to the requirements. Using step (1) as an example:

If there are already ap and aq in A corresponding to bp and bq in B respectively such that ap < ai < aq and bp < bq, we choose bj in between bp and bq using density. Otherwise, we choose a suitable large or small element of B using the fact that B has neither a maximum nor a minimum. Choices made in step (2) are dually possible. Finally, the construction ends after countably many steps because A and B are countably infinite. Note that we had to use all the prerequisites.

History

According to Hodges (1993):

Back-and-forth methods are often ascribed to Cantor, Bertrand Russell and C. H. Langford […], but there is no evidence to support any of these attributions.

While the theorem on countable densely ordered sets is due to Cantor (1895), the back-and-forth method with which it is now proved was developed by Huntington (1904) and Hausdorff (1914). Later it was applied in other situations, most notably by Roland Fraïssé in model theory.