Match of the Day

Some interesting shapes are mentioned in Derrick Niederman’s Number Freak (2010). Using identical matchsticks, what’s the smallest fully connected shape you can make in which two matches meet at every vertex? That is, what is the smallest 2-regular matchstick graph?

It’s an equilateral triangle:

2match

Now, what is the smallest fully connected shape you can make in which three matches meet at every vertex? That is, what is the smallest 3-regular matchstick graph? It uses twelve identical matches and looks like this:

3match

And here is the smallest known 4-regular matchstick graph, discovered by the German mathematician Heiko Harborth and using 104 identical matches:

4match

But Niederman says that “it’s impossible to create any arrangement in which five or more matchsticks meet at every vertex” (entry for “104”, pg. 230 of the 2012 paperback).

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He Say, He Sigh, He Sow #34

• Mathematik ist die Wissenschaft von dem, was an sich klar. — Carl Jacobi (1804-51).
  • “Mathematics is the science of that which is clear by itself.” — Carl Jacobi.