 David Wilson Henderson showed in 1963 that the existence of an "n"-Venn diagram with "n"-fold rotational symmetry implied that "n" was a prime number.He also showed that such symmetric Venn diagrams exist when "n" is five or seven.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.