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Generating Simple Convex Venn Diagrams

Khalegh Mamakania, Wendy Myrvolda, Frank Ruskeya

aDepartment of Computer Science, University of Victoria, BC, Canada

Abstract

In this paper we are concerned with producing exhaustive lists of simplemonotone Venn diagrams that have some symmetry (non-trivial isometry)when drawn on the sphere. A diagram is simple if at most two curves intersectat any point, and it is monotone if it has some embedding on the planein which all curves are convex. We show that there are 23 such 7-Venndiagrams with a 7-fold rotational symmetry about the polar axis, and that 6of these have an additional 2-fold rotational symmetry about an equatorialaxis. In the case of simple monotone 6-Venn diagrams, we show that thereare 39020 non-isomorphic planar diagrams in total, and that 375 of themhave a 2-fold symmetry by rotation about an equatorial axis, and amongstthese we determine all those that have a richer isometry group on the sphere.Additionally, 270 of the 6-Venn diagrams also have the 2-fold symmetryinduced by reflection about the center of the sphere.

Since such exhaustive searches are prone to error, we have implementedthe search in a couple of ways, and with independent programs. These dis-tinct algorithms are described. We also prove that the Grunbaum encodingcan be used to efficiently identify any monotone Venn diagram.

Keywords: Spherical Venn diagram, symmetry, exhaustive enumeration,Grunbaum encoding.

1. Introduction

Named after John Venn (1834 1923), who used diagrams of overlap-ping circles to represent propositions, Venn diagrams are commonly used inset theory to visualize the relationships between different sets. The familiar

Email addresses: aahmadi@cs.uvic.ca (Khalegh Mamakani), wendy@csc.uvic.ca(Wendy Myrvold), ruskey@csc.uvic.ca (Frank Ruskey)

Preprint submitted to Elsevier April 3, 2012

three circle Venn diagram is usually drawn with a three-fold rotational sym-metry (Figure1(a)) and the question naturally arises as to whether there areother Venn diagrams with rotational and other symmetries. Some researchhas been done recently on generating and drawing Venn diagrams of morethan three sets, particularly in regard to symmetric Venn diagrams, whichare those where rotating the diagram by 360/n degrees results in the samediagram up to a relabelling of the curves. Grunbaum [10] discovered a ro-tationally symmetric 5-Venn diagram (Figure 1(b)). Henderson [12] provedthat if an n-curve Venn diagram has an n-fold rotational symmetry then nmust be prime. Recently, Wagon and Webb [17] clarified some details ofHendersons argument. The necessary condition that n be prime was shownto be sufficient by Griggs, Killian and Savage [9] and an overview of theseresults was given by Ruskey, Savage, and Wagon [14].

A Venn diagram is simple if at most two curves intersect at any point.In this paper we only consider simple Venn diagrams. There is one simplesymmetric 3-Venn diagram and one simple symmetric 5-Venn diagram. Ed-wards wrote a program to exhaustively search for simple polar symmetric7-Venn diagrams and he discovered 5 of them, but somehow overlooked a6-th [7]. His search was in fact restricted to monotone Venn diagrams, whichare equivalent to those that can be drawn with convex curves [2]. Figure1(c) is a 7-set Venn diagram with 7-fold rotational symmetry, called Ade-laide by Edwards, and which was discovered independently by Grunbaum[11] and Edwards [7]. It should be noted that the diagrams constructed in [9]are inherently non-simple, and the existence question for simple symmetric11-Venn diagrams remains an open problem.

It is known that Venn diagrams exist for any number of curves and severalconstructions of them are known [15], but the total number of simple Venndiagrams is known only up to n = 5. In this paper, we determine that thenumber of simple monotone 6-Venn diagrams is 39020; undoubtedly thereare many other non-monotone diagrams.

Symmetric spherical Venn diagrams were first systematically investigatedby Weston [18] and the recent paper [16] shows that Venn diagrams exhibit-ing each of the possible order 2 isometries exist for all n. The underlyingconstructions of [16] are inherently non-simple and the diagrams presentedin the current paper are the first known simple Venn diagrams with certainorder 2 isometries for 6-Venn diagrams.

A program was written to search for monotone simple symmetric 7-Venndiagrams and 23 of them were reported in the original version of the Survey

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{1,2,3}

{1,3}{1,2}

{2,3}{3}{2}

{1}

(a) (b)

(c)

Figure 1: (a) A 3-Venn diagram whose curves are circles. (b) A 5-Venn diagram whosecurves are ellipses. (c) A symmetric 7-Venn known as Adelaide.

of Venn Diagrams (Ruskey and Weston [15]) from 1997, but no descriptionof the method was ever published and the isomorphism check was unjustified.Later Cao [3] checked those numbers, and provided a proof of the isomor-phism check, but again no journal paper with the result was ever published.In this paper, we justify that isomorphism check with a new simpler proofand yet again recompute and verify the number of symmetric simple 7-Venndiagrams.

In this paper we are restricting our attention to the special (and moststudied) class of Venn diagrams; diagrams that are both simple and monotone(drawable with convex curves). Our eventual aim is to provide a completeenumeration of such diagrams for small values of n, determining also the dia-

3

grams which have non-trivial isometries when embedded on the sphere. Theunderlying techniques rely on exhaustive backtrack searches with intelligentpruning rules whose use is justified by structural theorems. Such computersearches are prone to error and so we have made considerable effort to ensurethat our computations are correct by using different representations, differentmethods for checking isomorphism, and independent programming efforts.

Our main concern in this paper are Venn diagrams with n curves, wheren = 6 and n = 7. The case of n = 7 is done first, but only on those diagramsthat have an order 7 rotational symmetry, because of the overwhelming num-ber of possibilities otherwise. Two different representations are used, withthree independent programs. We find that there are 23 non-isomorphic ro-tationally symmetric simple monotone 7-Venn diagrams. Of these 23, thereare 6 that have an additional 2-fold polar symmetry (Figure 10), and 17that do not (Figure 11).

In the case of n = 6 we again used 2 different representations and threeindependent programs. There are 39020 non-isomorphic simple monotone6-Venn diagrams. Of these, 375 have polar symmetry, of those 27 have anisometry group order of 4, and 6 have an isometry group order of 8 (Figure12). Additionally, 270 of the 6-Venn diagrams also have the 2-fold symmetryinduced by reflection about the center of the sphere.

We introduce several different representations of these diagrams. Al-though these representations are somewhat similar in nature and there areefficient algorithms for getting from one representation to the other, we usedthem to implement independent generating algorithms for each class of stud-ied Venn diagrams.

The remainder of this paper is organized as follows. In Section 2 weintroduce the terminology and basic definitions. In Section 3 we explainvarious representations of Venn diagrams. The generating algorithms andresults are described in Section 4.

Some of the results of this paper were first published in the conferenceproceedings [13] and [4]. In this paper we have combined, extended, andrefined those results.

2. Definitions

Let C = {C0, C1, . . . , Cn1} be a collection of n finitely intersecting simpleclosed curves in the plane. We call C an independent family if each of 2n sets

X0 X1 Xn1

4

is nonempty, whereXi is either the unbounded open exterior or open boundedinterior of curve Ci. If each set is a nonempty and connected region, then Cis called an n-Venn diagram. A simple Venn diagram is one in which exactlytwo curves cross each other at each intersection point.

A k-region in a Venn diagram is a region that is in the interior of preciselyk curves. In an n-Venn diagram, each k-region corresponds to a k-elementsubset of a set with n elements. Thus, there are

(nk

)distinct k-regions. A

Venn diagram is monotone if every k-region is adjacent to both some (k1)-region (if k > 0) and also to some (k + 1)-region (if k < n). The rank ofa region of a Venn diagram is defined to be

n1i=0 2

ixi, where xi = 1 if it isin the interior of curve i and xi = 0 otherwise. By the definition of a Venndiagram, each region has a unique rank r in the range 0 r < 2n.

An n-Venn diagram is rotationally symmetric, if rotation of the diagramby an angle of 2/n about a fixed point in the plane does not change thediagram, except for a relabeling of the curves. Therefore, a 1/nth circularsector of a rotationally symmetric n-Venn diagram is enough to generate thewhole diagram.

Polar symmetry is another type of symmetry, which was introduced byGrunbaum . Consider a Venn diagram as being projected onto a sphere withthe rank 0 and rank 2n 1 regions mapped to the north and south poles.A polar flip is the rotation of sphere by radians about an equatorial axis;thus the northern and southern hemispheres are exchanged. The polar flipof a plane Venn diagram is then obtained by projecting the flipped sphereback onto the plane this has the effect of interchanging the insides andoutsides of all the curves. A Venn diagram is polar symmetric if it can bedrawn so that it is invariant under some polar flip.

Given a planar Venn diagram V , let VM be its mirror image, let VP