Median Genocchi Numbers

• Median Genocchi numbers are a sequence of integers defined through combinatorial models like the Seidel–Entringer triangle and are characterized by deep algebraic and geometric properties.
• They are intricately linked to constructs such as Dumont derangements, Dellac configurations, and hyperplane arrangements, offering diverse applications in enumerative combinatorics.
• Their analysis employs recursive definitions, generating functions, and bijections that expose rich structural insights and divisibility phenomena such as the 2^n factor.

The median Genocchi numbers are a sequence of integers with deep connections in enumerative combinatorics, algebraic geometry, and the theory of orthogonal polynomials. They arise as the "Genocchi numbers of the second kind," feature prominently as region counts for certain hyperplane arrangements, and admit a rich array of combinatorial incarnations, including Dumont derangements, Dellac configurations, parity-restricted permutations, terrain-like graphs, and alternation acyclic tournaments. Their construction and interpretation involve a mixture of recursive, generating-function, and bijective frameworks.

1. Definitions and Classical Recurrences

The median Genocchi numbers, usually denoted $H_n$ (sometimes $H_{2n+1}$), or, after normalization, $h_n = H_{2n+1}/2^n$, can be defined in several equivalent ways.

Seidel–Entringer Triangle

Let $G_{k,n}$ be entries of the Seidel triangle, defined recursively as

$$\begin{cases} G_{1,1} = 1, \ G_{k,2n} = \sum_{i=k}^{2n-1} G_{i,2n-1}, \ G_{k,2n+1} = \sum_{i=1}^{k-1} G_{i,2n}. \end{cases}$$

The numbers $H_{2n+1} = G_{1,2n+2}$ comprise the median Genocchi sequence. Classical divisibility $2^n \mid H_{2n+1}$ motivates the normalization $h_n = H_{2n+1}/2^n$ (Feigin, 2011, Vanderburg, 2014, Bigeni, 2017, Hetyei, 2017).

Dumont Derangements

A permutation $\sigma \in S_{2n+2}$ is a Dumont permutation of the second kind if $\sigma(2i-1) > 2i-1$ and $\sigma(2i) < 2i$ for $i=1,\dots,n$. The set of such permutations with no fixed points (i.e., Dumont derangements) is counted by $h_n$ (Lazar et al., 2018, Froese et al., 2022, Lazar et al., 2019, Bigeni, 2017).

Dellac Configurations

A Dellac configuration of size $n$ is a placement of dots in an $n \times 2n$ grid such that each column contains two dots, each row contains exactly one dot, and every dot in column $j$ lies in rows $j-n \le i \le j$. The set $DC_n$ of all such configurations has cardinality $h_n$ (Feigin, 2011, Fang et al., 2015, Vanderburg, 2014, Bigeni, 2017).

Binomial Formula

A closed formula via quiver Grassmannian techniques is

$$h_n = \sum_{f_1,\dots,f_{n-1} \ge 0} \prod_{k=1}^{n-1} \binom{1+f_{k-1}}{f_k} \binom{1+f_{k+1}}{f_k}, \quad f_0 = f_n = 0.$$

(Feigin, 2011).

Divisibility

It is a classical result that $h_n \equiv 0 \pmod{2^n}$ (Yuan et al., 15 Oct 2025), and the combinatorial models make this divisibility transparent (e.g., via orbit sizes in Hetyei’s tuples).

First Values

The first few normalized median Genocchi numbers $h_n$ are $1, 2, 8, 56, 608, 9440, \ldots$ (Fang et al., 2015, Hetyei, 2017, Lazar et al., 2018, Bigeni, 2017, Yuan et al., 15 Oct 2025).

2. Combinatorial and Geometric Models

Terrain-Like Graphs

A terrain-like graph on $[n]$ satisfies the X-property: for edges $\{a,c\}$, $\{b,d\}$ with $a < b < c < d$, the edge $\{a,d\}$ must also exist. The set of terrain-like graphs $\mathcal{T}_n$ on $n$ vertices is in bijection with Dumont derangements of the second kind, and $|\mathcal{T}_n| = h_n$ (Froese et al., 2022).

Alternation Acyclic Tournaments

A tournament on $[n]$ is alternation acyclic if it contains no cycle alternating between descents and ascents. The number of such tournaments equals $H_{2n-1}$, and normalization by $2^n$ gives $h_n$ (Hetyei, 2017).

Degenerate Flag Varieties and Torus Fixed Points

The number of torus fixed points in a degenerate flag variety (type $A$) equals $h_n$. Dellac configurations index these torus fixed points via explicit bijections involving rook placements and Schubert varieties. The symplectic case yields a “symplectic Dellac configuration,” conjecturally giving a median Euler number (Fang et al., 2015, Bigeni, 2017).

Multiset Tuples (Hetyei’s Model)

Objects $(a_1,\dots,a_n), (b_1,\dots,b_n)$ with $0 \leq a_i \leq n$, $1 \leq b_i \leq n$, such that their multiset covers $[n]$, and their orbit representatives (unordered pairs covering $[n]$) provide a new simple model for $h_n$, and connect bijectively to Dellac configurations and Dumont permutations (Bigeni, 2017, Hetyei, 2017).

Parity Pattern Permutations

Region labelings of certain hyperplane arrangements can be described via permutations where ascents or descents are subject to parity restrictions (e.g., every ascent is from odd to even). Four distinct but equinumerous pattern-avoidance classes each label regions by permutations with $h_n$ elements (Yuan et al., 15 Oct 2025).

Even-Odd Drop Cycles

Cycles with only even-odd or odd-odd drops (certain parity-restricted descents in permutations) are equinumerous with median Genocchi numbers, and refined enumerators yield bivariate generating functions interpolating between Genocchi numbers of both kinds (Chern, 2021, Pan et al., 2021).

3. Hyperplane Arrangements, Characteristic Polynomials, and Region Counts

The Homogenized Linial Arrangement

The arrangement $$\mathcal{H}_{2n-1} = \{x_i - x_j = y_i : 1 \le i < j \le n+1\}$$ in $\mathbb{R}^{2n+1}$ has the property that the number of its regions equals $h_n$, by Zaslavsky's theorem and explicit determination of its characteristic polynomial (Lazar et al., 2018, Lazar et al., 2019, Yuan et al., 15 Oct 2025).

Intersection Lattices and D-Permutations

The intersection lattice of $\mathcal{H}_{2n-1}$ is isomorphic to the bond lattice of a Ferrers bipartite graph. D-permutations (where $\sigma(i) \geq i$ for odd, $\sigma(i) \leq i$ for even) label regions or NBC forests; $h_n$ counts such permutations (Lazar et al., 2018, Deb et al., 2022, Lazar et al., 2019).

Cycle Statistics and Gamma-Positivity

Combinatorial statistics on parity-restricted permutations (number of descents, cycles) yield descent polynomials and Eulerian-type polynomials with $\gamma$-positivity, refined via continued fractions and moment sequences (Eu et al., 2021, Pan et al., 2021).

4. Generating Functions and Continued Fractions

Exponential Generating Functions

For the (non-normalized) median Genocchi numbers,

$$\sum_{n \ge 1} H_{2n+1} \frac{x^{2n+1}}{(2n+1)!} = \tan x + \sec x,$$

or via ordinary Genocchi numbers

$$\sum_{n \ge 1} G_{2n} \frac{x^{2n}}{(2n)!} = \frac{2x}{e^{x} + 1}.$$

Normalization is achieved by rescaling arguments and dividing by powers of $2$ (Feigin, 2011, Feigin, 2011, Bigeni, 2017).

Ordinary Generating Function—Jacobi and Stieltjes Continued Fractions

The ordinary generating function for $h_n$ can be expressed as

$$H(z) = \sum_{n \ge 0} h_n z^n = \cfrac{1}{1 - 2z - \cfrac{2z^2}{1 - 4z - \cfrac{3z^2}{1 - 6z - \ddots}}}$$

(Jacobi continued fraction), or as a Stieltjes $S$-fraction: $$\sum_{n \ge 0} h_n t^n = \cfrac{1}{1 - \cfrac{1 t}{1 - \cfrac{1 t}{1 - \cfrac{4 t}{1 - \cfrac{4 t}{1 - \ddots}}}}}$$ (Deb et al., 2022, Feigin, 2011, Vanderburg, 2014, Pan et al., 2021, Lazar et al., 2018).

$q$-Analogues and Poincaré Polynomials

A $q$-version $h_n(q)$ arises as the Poincaré polynomial of specific degenerate flag varieties, refined by length statistics on Dellac configurations: $h_n(q) = \sum_{D \in DC_n} q^{l(D)}, \text{ where %%%%56%%%% = number of inversions}$ (Feigin, 2011, Vanderburg, 2014). Han–Zeng’s $q$-Genocchi polynomials $C_n(q)$ coincide (after an explicit normalization) with $h_n(q)$.

Three-term Recurrence Relations

From J-fraction expansions, the normalized median Genocchi numbers satisfy

$h_{n+1} = b_n h_n - \lambda_n h_{n-1}$

with explicit expressions for $b_n$, $\lambda_n$ in terms of $n$ (Pan et al., 2021, Lazar et al., 2019, Vanderburg, 2014).

5. Advanced Enumeration and Combinatorial Equivalences

Bijections Among Models

There exist explicit, statistic-preserving bijections between Dellac configurations, normalized Dumont permutations, parity pattern permutations, Hetyei’s tuples, surjective "pistol" functions, Dyck paths with histories, and region labelings in hyperplane arrangements (Bigeni, 2017, Vanderburg, 2014, Pan et al., 2021, Yuan et al., 15 Oct 2025, Froese et al., 2022).

Refined Triangles and Gamma Structures

The Kreweras triangle $(h_{n,k})$ refines the enumeration of median Genocchi objects by a parameter (e.g., first-letter in permutations, position of 1 in subset tuples), with explicit recurrences, symmetries, and underlying combinatorial interpretations (Bigeni, 2017, Yuan et al., 15 Oct 2025).

Cycle and Drop Statistics

Combinatorial models encode cycle-moment and drop-moment statistics, for instance, via (p,q)-Eulerian polynomials, descent polynomials, and Motzkin-path moment sequences, often admitting gamma-positive expansions and factorization properties (Pan et al., 2021, Eu et al., 2021).

6. Connections, Generalizations, and Algebraic Properties

Dowling Arrangements and Other Types

Generalization to Dowling arrangements (type B, cyclic groups) and $m$-labeled D-permutations yield B-type median Genocchi numbers and Gandhi polynomials, with analogous product-form and continued-fraction expansions (Lazar et al., 2018).

Symplectic and Other Degenerate Varieties

Symplectic Dellac configurations related to symplectic Schubert varieties yield analogues conjecturally equivalent to the "median Euler numbers" (Fang et al., 2015).

Total Positivity

The Hankel matrices associated with the continued fractions for $h_n$ are totally positive, and all coefficients are manifestly nonnegative (Deb et al., 2022).

7. Summary Table: Key Enumerative Models for Median Genocchi Numbers

Model	Definition	Cardinality
Seidel–Entringer Triangle	$h_n = G_{1,2n+2}/2^n$	$h_n$
Dumont Derangements (2nd kind)	$\pi(2i-1) > 2i-1$, $\pi(2i) < 2i$, no fixed points	$h_n$
Dellac Configurations	$n \times 2n$ grid, restrictions as above	$h_n$
Hetyei Multiset Tuples	Unordered pairs covering $[n]$	$h_n$
Parity Pattern Permutations	Avoids $(eE, eO, oO)$ ascents, or other parity conditions	$h_n$
Terrain-like Graphs	X-property on edges	$h_n$
Alternation-Acyclic tournaments	No alternating cycles on $[n]$	$H_{2n-1}$

Further combinatorial and geometric descriptions, generating functions, recurrences, continued fractions, and $q$-analogues provide an extensive toolkit for the paper and application of median Genocchi numbers in algebraic combinatorics, geometry, and representation theory. The robust structure and interrelations of their many incarnations exemplify the deep connections among permutation statistics, lattice path enumerations, hyperplane arrangements, and geometric objects (Yuan et al., 15 Oct 2025, Froese et al., 2022, Lazar et al., 2018, Fang et al., 2015, Lazar et al., 2019, Bigeni, 2014, Feigin, 2011, Feigin, 2011, Hetyei, 2017, Bigeni, 2017, Pan et al., 2021, Eu et al., 2021, Deb et al., 2022, Chern, 2021).