# difference of lattice elements

Let $a,b\in\mathfrak{A}$. A of $a$ and $b$ is an element $c\in\mathfrak{A}$ that $b\cap c=0$ and $a\cup b=b\cup c$. When there is only one difference  of $a$ and $b$, it is denoted $a\setminus b$.

One immediate property is: $0$ is the unique difference of any element $a$ and itself ($a\setminus a=0$). For if $c$ is such a difference, then $a\cap c=0$ and $a=a\cup c$. So $c\leq a$ by the second equation, and hence that $c=a\cap c=0$ by the first equation.

For arbitrary lattices of two given elements do not necessarily exist. For some lattices there may be more than one difference of two given elements.

For a distributive lattice  with bottom element $0$, the difference of two elements, if it exists, must be unique. To see this, let $c$ and $d$ be two differences of $a$ and $b$. Then

• $b\cap c=b\cap d=0$, and

• $a\cup b=b\cup c=b\cup d$.

So $c=c\cap(b\cup c)=c\cap(b\cup d)=(c\cap b)\cup(c\cap d)=0\cup(c\cap d)=c\cap d$. Similarly, $d=d\cap c$. As a result, $c=c\cap d=d\cap c=d$.

Title difference of lattice elements DifferenceOfLatticeElements 2013-03-22 17:57:44 2013-03-22 17:57:44 porton (9363) porton (9363) 10 porton (9363) Definition msc 06B99 ComplementedLattice Pseudodifference SectionallyComplementedLattice