Proof without words

Summary

In mathematics, a proof without words (or visual proof) is a proof of an identity or mathematical statement which can be demonstrated as self-evident by a diagram without any accompanying explanatory text. Such proofs can be considered more elegant than formal or mathematically rigorous proofs due to their self-evident nature.[1] When the diagram demonstrates a particular case of a general statement, to be a proof, it must be generalisable.[2]

Proof without words of the Nicomachus theorem (Gulley (2010))

ExamplesEdit

Sum of odd numbersEdit

 
A proof without words for the sum of odd numbers theorem.

The statement that the sum of all positive odd numbers up to 2n − 1 is a perfect square—more specifically, the perfect square n2—can be demonstrated by a proof without words.[3]

In one corner of a grid, a single block represents 1, the first square. That can be wrapped on two sides by a strip of three blocks (the next odd number) to make a 2 × 2 block: 4, the second square. Adding a further five blocks makes a 3 × 3 block: 9, the third square. This process can be continued indefinitely.

Pythagorean theoremEdit

 
A proof without words for the Pythagorean theorem derived in Zhoubi Suanjing.

The Pythagorean theorem can be proven without words. The two different methods for determining the area of the large square give the relation

 

between the sides. This proof is more subtle than that for the sum of odd numbers, but still can be considered a proof without words.[4]

Jensen's inequalityEdit

 
A graphical proof of Jensen's inequality.

Jensen's inequality can also be proven graphically. A dashed curve along the X axis is the hypothetical distribution of X, while a dashed curve along the Y axis is the corresponding distribution of Y values. The convex mapping Y(X) increasingly "stretches" the distribution for increasing values of X.[5]

UsageEdit

Mathematics Magazine and the College Mathematics Journal run a regular feature titled "Proof without words" containing, as the title suggests, proofs without words.[3] The Art of Problem Solving and USAMTS websites run Java applets illustrating proofs without words.[6][7]

See alsoEdit

NotesEdit

  1. ^ Dunham 1994, p. 120
  2. ^ Weisstein, Eric W. "Proof without Words". MathWorld. Retrieved on 2008-6-20
  3. ^ a b Dunham 1994, p. 121
  4. ^ Nelsen 1997, p. 3
  5. ^ McShane, E. J. (1937), "Jensen's Inequality", Bulletin of the American Mathematical Society, American Mathematical Society, vol. 43, no. 8, p. 527, doi:10.1090/S0002-9904-1937-06588-8
  6. ^ Gallery of Proofs, Art of Problem Solving, retrieved 2015-05-28
  7. ^ Gallery of Proofs, USA Mathematical Talent Search, retrieved 2015-05-28

ReferencesEdit

  • Dunham, William (1994), The Mathematical Universe, John Wiley and Sons, ISBN 0-471-53656-3
  • Nelsen, Roger B. (1997), Proofs without Words: Exercises in Visual Thinking, Mathematical Association of America, p. 160, ISBN 978-0-88385-700-7
  • Nelsen, Roger B. (2000), Proofs without Words II: More Exercises in Visual Thinking, Mathematical Association of America, pp. 142, ISBN 0-88385-721-9
  • Gulley, Ned (March 4, 2010), Shure, Loren (ed.), Nicomachus's Theorem, Matlab Central.