# Reducing a system of two boolean algebra assertions to a single one

Given a system of two Boolean Algebra equalities

a = b.
c = d.


one can exhibit a single equation

F(a,b,c,d) = 0.


which is equivalent to the former system. (Symmetric difference is pivotal for constructing such quaternary operation F(a,b,c,d)).

Questions:

• Is there binary operation x ? y (infix notation) such that

a ? c = b ? d.

is equivalent to the original system?

• Is there binary operation x ? y such that

a ? b = c ? d.

is equivalent to the original system?

Edit (June 13): The question is more subtle than I managed to convey. Boolean algebra is a finitely definable variety. I was wondering if we can get a single axiom system by combining identities. Padmanabhan proved in 1968 that every definable class of lattices (such as BA) can be defined by a single identity within the class of lattices. The key observation in his method is equipping the identities

a = b.
c = d.


with disjoint sets of variables. Then, simple disjunction

a v c = b v d.


would do.

• You could write a small program to generate all possibilities to find out! – Dave Clarke Jun 6 '14 at 19:35
• I'm looking for mathematical argument; writing a program with exhaustive check of 16 candidate functions will unlikely give me any insight. For example, generalizing the question to 3 equalities reveals the flaw in the second suggested equivalent equality. – Tegiri Nenashi Jun 6 '14 at 19:39

This is easy to prove with a counting argument. Given a candidate binary operation $?$, count the number $n$ of boolean values $x,y$ such that $x?y$ is true. Out of the 16 possible values for $a,b,c,d$, exactly $m=n^2+(4-n)^2$ of them will satisfy $a?c = b?d$. The possible values for $m$ are $m \in \{16,10,8\}$. However, there are only 4 possible values of $a,b,c,d$ such that $a=b$ and $c=d$, which is not in the set of possible values for $m$.