Möbius function - understanding of relations I am trying to understand Möbius function from the wikipedia article (and also few others that I have come across so far). This function is defined in posets and so the relations in Special elements section of the wikipedia article confuse me. So what do relations $=$, $\leq$, $<$ mean in this section? Do these have the traditional meaning or does these relations mean something different?

In the original number-theoretic case, $\leq$ would be replaced by $\mid$. The integers are partially ordered by divisibility and the Mobius function is the inverse of the zeta function, using the language of the general poset case.
