# A Finite Calculus Approach to Ehrhart Polynomials

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### Text of A Finite Calculus Approach to Ehrhart Polynomials

Kevin Woods, Oberlin College (joint work with Steven Sam, MIT)
Ehrhart Theory
LP(t) = #tP ∩ Zd
Ehrhart’s Theorem:
where ci (t) are periodic.
When P is integral, period = 1, so Lp(t) is a polynomial.
An Analogy
∑ a∈tP∩Zd
I the origin
vol(td) =
Inductively,
I the origin
vol(td) =
Inductively,
I ∫ t
I ∫ t
0 acts nicely on a basis of R[x ].∫ t
0 sn =
d! .
I ∑t
I ∑t
s=0 acts nicely on a basis of R[x ].
t∑ s=0
where tn = t(t − 1)(t − 2) · · · (t − d + 1).
Is it always this easy?
No.
No.
Yes.
Yes.
Tools
I Quasi-polynomial version of finite calculus: If f (s) is a quasi-polynomial with period r , then
F (t) =
rb
gcd(a, r) .
Key: It’s the smallest t such that at b is integer multiple of r .
Tools
I Induction. Need quasi-polynomial version even to get polynomial version.
Bug? Feature?
Theorem (McMullen)
LP(t) = c0(t) + c1(t)t + · · ·+ cd(t)td ,
and given r and i such that the affine hull of rF contains integer points, for all i -dimensional faces F . Then r is a period of ci (t).
I Let D be smallest positive integer such that DP is integral. Then D is a period of each ci (t).
I If P is integral, D = 1 and LP(t) is a polynomial.
I If P is full-dimensional: Affine hull of 1 · P contains integer points. Period of cd(t) is 1. (cd(t) = vol(P))
Theorem (Ehrhart-Macdonald Reciprocity)
If P is the relative interior of P and LP(t) = #tP ∩ Zd ,
LP(t) = (−1)dim(P)LP(−t).
n∑ s=0
−n∑ s=0
n∑ s=0
−n∑ s=0
f (s) = − −1∑
s=−n+1
n∑ s=0
−n∑ s=0
f (s) = − −1∑
s=−n+1
LQ(s)
LQ(s)
LQ(s)
LQ(s)
Okay, Not Quite Free
You do have to be careful with the triangulation and Inclusion-Exclusion for reciprocity.
Need Euler characteristic, topologically or combinatorially.
True of most proofs (though check out irrational version, Beck–Sottile).
Context
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I McMullen (valuations)
Contemporary (simpler) Proofs
I 1, t + 1, (t + 1)(t + 2)/2, . . .
I (t+d
If
(1− t)d+1 .
I 1, t + 1, (t + 1)(t + 2)/2, . . .
I (t+d
If
(1− t)d+1 .
I 1, t + 1, (t + 1)(t + 2)/2, . . .
I (t+d
If
(1− t)d+1 .
t∑ s=0
I Apply Barvinok’s algorithm.
Questions
t∑ s=0
I Apply Barvinok’s algorithm.
Questions
t∑ s=0
I Apply Barvinok’s algorithm.
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