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Norbert Hungerbühler

Publications and source records attributed to Norbert Hungerbühler.

At least 19 recordsLinked to original sources
arXiv2025

Integer triangles with a rational ratio of circumcircle radius to excircle radius

We consider the problem of finding integer triangles with $R/r$ a positive rational, where $R$ and $r$ are the radii of the circumcircle and an excircle, respectively. We show that for general triangles $R/r>1/4$ applies. The equation $R/r=N$ turns out to be related to the elliptic curve $\mathcal{E}_N$ given by $v^2=u^3+2(2N^2+2N-1)u^2-(4N-1)u$. If $N>1/4$ is rational, then the torsion group of $\mathcal{E}_N$ is $\mathbb Z/2\mathbb Z\times\mathbb Z/6\mathbb Z$ if $N(N+2)$ is a square and $\mathbb Z/6\mathbb Z$ otherwise. We show that a rational triangle with rational ratio $R/r=N$ exists if and only if $N>1/4$ and there exists a rational non-torsion point on the curve $\mathcal{E}_N$ which satisfies a certain condition. Furthermore, we show that the rank of $\mathcal{E}_N$ is positive when $N = m^2 \pm 1>1/4$ for a rational $m$. We also show that on every curve $\mathcal{E}_N$ whose rank is positive, there are infinitely many rational points which lead to infinitely many non-similar integer triangles with $R/r=N$.

math.NT↗
arXiv2025

The excenters of bicentric polygons are concyclic

We show that the centers of the excircles of a bicentric polygon $B$ are concyclic on a circle $E$. The center of the circumscribed circle $K$ of $B$ is the midpoint of the center of $E$ and the center of the inscribed circle $C$ of $B$. The radius of $E$ is given by a simple formula in terms of the radii of $C$ and $K$ and the distance between their centers.

math.MG↗
arXiv2025

Closing Theorems for Circle Chains

We consider closed chains of circles $C_1,C_2,\ldots,C_n,C_{n+1}=C_1$ such that two neighbouring circles $C_i,C_{i+1}$ intersect or touch each other with $A_i$ being a common point. We formulate conditions such that a polygon with vertices $X_i$ on $C_i$, and $A_i$ on the (extended) side $X_iX_{i+1}$, is closed for every position of the starting point $X_1$ on $C_1$. Similar results apply to open chains of circles. It turns out that the intersection of the sides $X_iX_{i+1}$ and $X_jX_{j+1}$ of the polygon lies on a circle $C_{ij}$ through $A_i$ and $A_j$ with the property that $C_{ij}, C_{jk}$ and $C_{ki}$ pass through a common point. The six circles theorem of Miquel and Steiner's quadrilateral Theorem appear as special cases of the general results.

math.GM↗
arXiv2025

Poncelet Curves

We examine pairs of closed plane curves that have the same closing property as two conic sections in Poncelet's porism. We show how the vertex curve can be computed for a given envelope and vice versa. Our formulas are universal in the sense that they produce all possible sufficiently regular pairs of such Poncelet curves. We arrive at similar results for sets of curves, analogous to the pencil of conic sections in the full Poncelet theorem. We also study the case of Poncelet curves that carry Poncelet polygons which are equiangular or even congruent.

math.DG↗
arXiv2024

Pairing Powers of Pythagorean Pairs

A pair $(a, b)$ of positive integers is a pythagorean pair if $a^2 + b^2$ is a square. A pythagorean pair $(a, b)$ is called a pythapotent pair of degree $h$ if there is another pythagorean pair $(k,l)$, which is not a multiple of $(a,b)$, such that $(a^hk, b^hl)$ is a pythagorean pair. To each pythagorean pair $(a, b)$ we assign an elliptic curve $Γ_{a^h ,b^h}$ for $h\ge 3$ with torsion group isomorphic to $\mathbb Z/2\mathbb Z \times \mathbb Z/4\mathbb Z$ such that $Γ_{a^h,b^h}$ has positive rank over $\mathbb Q$ if and only if $(a,b)$ is a pythapotent pair of degree $h$. As a side result, we get that if $(a, b)$ is a pythapotent pair of degree $h$, then there exist infinitely many pythagorean pairs $(k,l)$, not multiples of each other, such that $(a^hk,b^hl)$ is a pythagorean pair. In particular, we show that any pythagorean pair is always a pythapotent pair of degree 3. In a previous work, pythapotent pairs of degrees 1 and 2 have been studied.

math.NT↗
arXiv2023

Properties of Hesse derivatives of cubic curves

The Hesse curve or Hesse derivative Hess$(Γ_f)$ of a cubic curve $Γ_{f}$ given by a homogeneous polynomial $f$ is the set of points $P$ such that $\det \left(H_f (P)\right)=0$, where $H_f (P)$ is the Hesse matrix of $f$ evaluated at $P$. Also Hess$(Γ_f)$ is again a cubic curve. We show that for a point $P\in$Hess$(Γ_{f})$, all the contact points of tangents from $P$ to the curves $Γ_{f}$ and Hess$(Γ_{f})$ are intersection points of two straight lines $\ell_1^P$ and $\ell_2^P$ (meeting on Hess$(Γ_{f})$) with $Γ_{f}$ and Hess$(Γ_{f})$, where the product of $\ell_1^P$ and $\ell_2^P$ is the polar conic of $Γ_{f}$ at $P$. The operator Hess defines an iterative discrete dynamical system on the set of the cubic curves. We identify the two fixed points of this system, investigate orbits that end in the fixed points, and discuss the closed orbits of the dynamical system.

math.AG↗
arXiv2022

The Ring of Polyfunctions over $\mathbb Z/n\mathbb Z$

We study the ring of polyfunctions over $\mathbb Z/n\mathbb Z$. The ring of polyfunctions over a commutative ring $R$ with unit element is the ring of functions $f:R\to R$ which admit a polynomial representative $p\in R[x]$ in the sense that $f(x)= p(x)$ for all $x\in R$. This allows to define a ring invariant $s$ which associates to a commutative ring $R$ with unit element a value in $\mathbb N\cup\{\infty\}$. The function $s$ generalizes the number theoretic Smarandache function. For the ring $R=\mathbb Z/n\mathbb Z$ we provide a unique representation of polynomials which vanish as a function. This yields a new formula for the number $Ψ(n)$ of polyfunctions over $\mathbb Z/n\mathbb Z$. We also investigate algebraic properties of the ring of polyfunctions over $\mathbb Z/n\mathbb Z$. In particular, we identify the additive subgroup of the ring and the ring structure itself. Moreover we derive formulas for the size of the ring of polyfunctions in several variables over $\mathbb Z/n\mathbb Z$, and we compute the number of polyfunctions which are units of the ring.

math.CO↗
arXiv2022

Polyfunctions over Commutative Rings

A function $f:R\to R$, where $R$ is a commutative ring with unit element, is called polyfunction if it admits a polynomial representative $p\in R[x]$. Based on this notion we introduce ring invariants which associate to $R$ the numbers $s(R)$ and $s(R';R)$, where $R'$ is the subring generated by $1$. For the ring $R=\mathbb Z/n\mathbb Z$ the invariant $s(R)$ coincides with the number theoretic \emph{Smarandache function} $s(n)$. If every function in a ring $R$ is a polyfunction, then $R$ is a finite field according to the Rédei-Szele theorem, and it holds that $s(R)=|R|$. However, the condition $s(R)=|R|$ does not imply that every function $f:R\to R$ is a polyfunction. We classify all finite commutative rings $R$ with unit element which satisfy $s(R)=|R|$. For infinite rings $R$, we obtain a bound on the cardinality of the subring $R'$ and for $s(R';R)$ in terms of $s(R)$. In particular we show that $|R'|\leqslant s(R)!$. We also give two new proofs for the Rédei-Szele theorem which are based on our results.

math.RA↗
arXiv2021

Reversion Porisms in Conics

We give a projective proof of the butterfly porism for cyclic quadrilaterals and present a general reversion porism for polygons with an arbitrary number of vertices on a conic. We also investigate projective properties of the porisms.

math.AG↗
arXiv2021

Constructing Cubic Curves with Involutions

In 1888, Heinrich Schroeter provided a ruler construction for points on cubic curves based on line involutions. Using Chasles' Theorem and the terminology of elliptic curves, we give a simple proof of Schroeter's construction. In addition, we show how to construct tangents and additional points on the curve using another ruler construction which is also based on line involutions.

math.HO↗
arXiv2021

Pairing Pythagorean Pairs

A pair $(a, b)$ of positive integers is a pythagorean pair if $a^2 + b^2 = \Box$ (i.e., $a^2 + b^2$ is a square). A pythagorean pair $(a, b)$ is called a double-pythapotent pair if there is another pythagorean pair $(k,l)$ such that $(ak,bl)$ is a pythagorean pair, and it is called a quadratic pythapotent pair if there is another pythagorean pair $(k,l)$ which is not a multiple of $(a,b)$, such that $(a^2k,b^2l)$ is a pythagorean pair. To each pythagorean pair $(a, b)$ we assign an elliptic curve $Γ_{a,b}$ with torsion group $\mathbb Z/2\mathbb Z\times\mathbb Z/4\mathbb Z$, such that $Γ_{a,b}$ has positive rank if and only if $(a, b)$ is a double-pythapotent pair. Similarly, to each pythagorean pair $(a, b)$ we assign an elliptic curve $Γ_{a^2 ,b^2}$ with torsion group $\mathbb Z/2\mathbb Z\times\mathbb Z/8\mathbb Z$, such that $Γ_{a^2,b^2}$ has positive rank if and only if $(a,b)$ is a quadratic pythapotent pair. Moreover, in the later case we obtain that every elliptic curve $Γ$ with torsion group $\mathbb Z/2\mathbb Z\times\mathbb Z/8\mathbb Z$ is isomorphic to a curve of the form $Γ_{a^2 ,b^2}$ , where $(a,b)$ is a pythagorean pair. As a side-result we get that if $(a,b)$ is a double-pythapotent pair, then there are infinitely many pythagorean pairs $(k, l)$, not multiples of each other, such that $(ak, bl)$ is a pythagorean pair; the analogous result holds for quadratic pythapotent pairs.

math.NT↗
arXiv2019

Equilibria of plane convex bodies

We obtain a formula for the number of horizontal equilibria of a planar convex body $K$ with respect to a center of mass $O$ in terms of the winding number of the evolute of $\partial K$ with respect to $O$. The formula extends to the case where $O$ lies on the evolute of $\partial K$ and a suitably modified version holds true for non-horizontal equilibria.

math.DG↗
arXiv2019

An alternative quadratic formula

The classical quadratic formula and some of its lesser known variants for solving the quadratic equation are reviewed. Then, a new formula for the roots of a quadratic polynomial is presented.

math.HO↗
arXiv2019

Lights Out on graphs

We model the Lights Out game on general simple graphs in the framework of linear algebra over the field $\mathbb F_2$. Based upon a version of the Fredholm alternative, we introduce a separating invariant of the game, i.e., an initial state can be transformed into a final state if and only if the invariant of both states agrees. We also investigate certain states with particularly interesting properties. Apart from the classical version of the game, we propose several variants, in particular a version with more than only two states (light on, light off), where the analysis resides on systems of linear equations over the ring $\mathbb Z_n$. Although it is easy to find a concrete solution of the Lights Out problem, we show that it is NP-hard to find a minimal solution. We also propose electric circuit diagrams to actually realize the Lights Out game.

math.CO↗
arXiv2019

Non-integer valued winding numbers and a generalized Residue Theorem

We define a generalization of the winding number of a piecewise $C^1$ cycle in the complex plane which has a geometric meaning also for points which lie on the cycle. The computation of this winding number relies on the Cauchy principal value, but is also possible in a real version via an integral with bounded integrand. The new winding number allows to establish a generalized residue theorem which covers also the situation where singularities lie on the cycle. This residue theorem can be used to calculate the value of improper integrals for which the standard technique with the classical residue theorem does not apply.

math.CA↗
arXiv2019

An integral that counts the zeros of a function

Given a real function $f$ on an interval $[a,b]$ satisfying mild regularity conditions, we determine the number of zeros of $f$ by evaluating a certain integral. The integrand depends on $f, f'$ and $f''$. In particular, by approximating the integral with the trapezoidal rule on a fine enough grid, we can compute the number of zeros of $f$ by evaluating finitely many values of $f,f'$ and $f''$. A variant of the integral even allows to determine the number of the zeros broken down by their multiplicity.

math.CA↗
arXiv2018

Exotic Steiner Chains in Miquelian Möbius Planes

In the Euclidean plane, two intersecting circles or two circles which are tangent to each other clearly do not carry a finite Steiner chain. However, in this paper we will show that such exotic Steiner chains exist in finite Miquelian Möbius planes of odd order. We state and prove explicit conditions in terms of the order of the plane and the capacitance of the two carrier circles $C_1$ and $C_2$ for the existence, length, and number of Steiner chains carried by $C_1$ and $C_2$.

math.CO↗