Venn diagram
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Venn diagrams (or set diagrams) are illustrations used in the branch of mathematics known as set theory. Invented in 1881 by John Venn, they show all of the possible mathematical or logical relationships between sets (groups of things). They normally consist of overlapping circles. For instance, in a two-set Venn diagram, one circle may represent the group of all wooden objects, while another circle may represent the set of all tables. The overlapping area (intersection) would then represent the set of all wooden tables. Shapes other than circles can be employed (see below), and this is necessary for more than three sets.
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[edit] Origins
The Hull-born British philosopher and mathematician John Venn (1834-1923) introduced the Venn diagram in 1881.
A stained glass window in Caius College, Cambridge, where Venn studied and spent most of his life, commemorates him and represents a Venn diagram.
[edit] A simple example
The following example involves two sets, A and B, represented here as coloured circles. The orange circle, set A, represents all living creatures that are two-legged. The blue circle, set B, represents the living creatures that can fly. Each separate type of creature can be imagined as a point somewhere in the diagram. Living creatures that both can fly and have two legs — for example, parrots — are then in both sets, so they correspond to points in the area where the blue and orange circles overlap. That area contains all such and only such living creatures.
Humans and penguins are bipedal, and so are then in the orange circle, but since they cannot fly they appear in the left part of the orange circle, where it does not overlap with the blue circle. Mosquitoes have six legs, and fly, so the point for mosquitoes is in the part of the blue circle that does not overlap with the orange one. Creatures that are not two-legged and cannot fly (for example, whales and spiders) would all be represented by points outside both circles.
The combined area of sets A and B is called the union of A and B, denoted by A ∪ B. The union in this case contains all things that either have two legs, or that fly, or both.
The area in both A and B, where the two sets overlap, is called the intersection of A and B, denoted by A ∩ B. For the example, the intersection of the two sets is not empty, because there are points representing creatures that are in both the orange and blue circles.
Sometimes a rectangle called the "Universal set" is drawn around the Venn diagram to show the space of all possible things. As mentioned above, a whale would be represented by a point that is not in the union, but is in the Universe (of living creatures, or of all things, depending on how one chose to define the Universe for a particular diagram).
[edit] Extensions to higher numbers of sets
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Venn diagrams typically have three sets. Venn was keen to find symmetrical figures…elegant in themselves representing higher numbers of sets and he devised a four-set diagram using ellipses. He also gave a construction for Venn diagrams for any number of sets, where each successive curve delimiting a set is interleaved with previous curves, starting with the 3-circle diagram.
[edit] Simple symmetric Venn diagrams
D. W. Henderson showed in 1963 that the existence of an n-Venn diagram with n-fold rotational symmetry implied that n was prime.[1] He also showed that such symmetric Venn diagrams exist when n is 5 or 7. In 2002 Peter Hamburger found symmetric Venn diagrams for n = 11 and in 2003, Griggs, Killian, and Savage showed that symmetric Venn diagrams exist for all other primes. Thus symmetric Venn diagrams exist if and only if n is a prime number.[2]
[edit] Edwards' Venn diagrams
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A. W. F. Edwards gave a construction to higher numbers of sets that features some symmetries. His construction is achieved by projecting the Venn diagram onto a sphere. Three sets can be easily represented by taking three hemispheres at right angles (x≥0, y≥0 and z≥0). A fourth set can be represented by taking a curve similar to the seam on a tennis ball which winds up and down around the equator. The resulting sets can then be projected back to the plane to give cogwheel diagrams with increasing numbers of teeth. These diagrams were devised while designing a stained-glass window in memoriam to Venn.
[edit] Other diagrams
Edwards' Venn diagrams are topologically equivalent to diagrams devised by Branko Grünbaum which were based around intersecting polygons with increasing numbers of sides. They are also 2-dimensional representations of hypercubes.
Smith devised similar n-set diagrams using sine curves with equations y=sin(2ix)/2i, 0≤i≤n-2.
Charles Lutwidge Dodgson (a.k.a. Lewis Carroll) devised a five set diagram.
[edit] Classroom use
Venn diagrams are often used by teachers in the classroom as a mechanism to help students compare and contrast two items. Characteristics are listed in each section of the diagram, with shared characteristics listed in the overlapping section.
[edit] See also
- Boolean algebra (logic)
- Carroll diagram
- Diagram
- Euler diagram
- Graphic organizers
- Mrs. Miniver's problem
- Spider diagram
- Bubble map
- Double bubble map
[edit] Notes
- ^ D. W. Henderson, "Venn diagrams for more than four classes". American Mathematical Monthly, 70 (1963) 424–426.
- ^ Ruskey, Frank; Carla D. Savage, and Stan Wagon (December 2006). "The Search for Simple Symmetric Venn Diagrams" (PDF). Notices of the AMS 53 (11): 1304-1311.
[edit] References
- A Survey of Venn Diagrams by F. Ruskey and M. Weston, is an extensive site with much recent research and many beautiful figures.
- I. Stewart Another Fine Math You've Got Me Into 1992 ch4
- A.W.F. Edwards. Cogwheels of the Mind: the story of Venn diagrams, Johns Hopkins University Press, Baltimore and London, 2004.
- "Venn Diagram Survey: Symmetric Diagrams", The Electronic Journal of Combinatorics, June 2005).
- John Venn (1880). "On the Diagrammatic and Mechanical Representation of Propositions and Reasonings". Dublin Philosophical Magazine and Journal of Science 9 (59): 1--18.
[edit] External links
- What is a Venn diagram?, from the survey page (below).
- LogicTutorial.com - interactive Johnston diagram
- Lewis Carroll's Logic Game — Venn vs. Euler at cut-the-knot
- A Survey of Venn Diagrams
