# Existence of a non-Eulerian atomistic lattice with this property on the Möbius function

Let $L$ be a finite lattice with least element $\hat{0}$, greatest element $\hat{1}$, and Möbius function $\mu$.

Question 1: What class of lattices the following property characterizes? $$\mu(\hat{0},a)=\mu(\hat{0},\hat{1})\mu(a,\hat{1}), \ \forall a \in L$$ It follows that $\mu(\hat{0},\hat{1}) = \pm1$.

Remark: It is satisfied by any boolean lattice, more generally by the face lattice of any convex polytope (as suggested by John Shareshian), and more generally by any Eulerian lattice.
Proof: An Eulerian lattice is a graded lattice $L$ such that for any $a,b \in L$ with $a \le b$, we have $\mu(a,b)= (-1)^{|b|-|a|}$, with $a \mapsto |a|$ the rank function. The result is immediate. $\square$

Question 2: Is there a non-Eulerian lattice with the above property on the Möbius function?
Yes, see the answer of John Machacek.

As suggested by Sam Hopkins:
Question 3: Is there a non-Eulerian atomistic lattice with the above property on the Möbius function?
No for $|L| \le 13$, as checked by the following Sage program (using these lists of Martin Malandro):

from itertools import product
def relationtest(L,n):
for l in L:
P=Poset((range(n),l))
b = P.bottom()
t = P.top()
if all(P.moebius_function(b,x) == P.moebius_function(b,t)*P.moebius_function(x,t) for x in P):
L=LatticePoset(P)
if L.is_atomic():
print(P.cover_relations())
for x, y in product(P, P):
if P.compare_elements(x,y)==-1:
if not P.moebius_function(x,y)==(-1)^(P.rank(y)-P.rank(x)):
print(P.cover_relations())
break


Are the small atomistic lattices listed somewhere?

• Hi Sebastien, how about the face lattice of an arbitrary polytope? – John Shareshian Sep 11 '17 at 23:14
• Hi John, you are right, it works for some non-simplex polytopes I tried. I don't know if it works for any arbitrary polytope, or if we need convexity. In fact, I'm interested to know all the lattices satisfying this property, so I hope you will not mind if I improve the post. – Sebastien Palcoux Sep 12 '17 at 0:09
• You might want to read about Eulerian posets in Chapter 3 of Richard Stanley's "Enumerative Combinatorics". – John Shareshian Sep 12 '17 at 0:24

I found the following example with SageMath. Below is the Hasse diagram and verification in SageMath that the poset is a lattice which is not Eulerian (in fact not graded) that satisfies the desired Möbius function condition. I have nothing more enlightening to say currently, but I'll post this example since it answers the second question about a non-Eulerian example and will perhaps helps toward an answer to the first question about characterizing such lattices.

sage: P = Poset(([0,1,2,3,4,5,6,7,8],[[0, 1], [0, 2], [0, 5], [1, 4], [1, 6], [2, 3], [2, 7], [3, 4], [4, 8], [5, 6], [5, 7], [6, 8], [7, 8]]))
sage: b = P.bottom()
sage: t = P.top()
sage: P.is_lattice()
True

• Yes! From your example I deduce the following process generating new lattices with the expected property: take a lattice $L$ with this property (for example an Eulerian lattice), and take two elements $a, b \in L$ with $$\hat{0} < a \lessdot b < \hat{1}.$$ Create a new lattice $\tilde{L}$ by adding an extra element $c$ between $a$ and $b$. By construction $$\mu_{\tilde{L}}(\hat{0},c) = \mu_{\tilde{L}}(c,\hat{1}) = 0,$$ and the rest is unchanged, so $\tilde{L}$ works also. Your example is this process applied to $B_3$. – Sebastien Palcoux Sep 12 '17 at 19:24
• Question: Is there an example which does not come the application finitely many times of this process to an Eulerian lattice? For example, a lattice $L$ having an element $a$ such that $\mu(\hat{0},a) \not \in \{ -1,0,1 \}$. – Sebastien Palcoux Sep 12 '17 at 19:29