An interactive column using Java applets
Cut The Knot!
by Alex Bogomolny
Georg Alexander Pick, born in 1859 in Vienna, perished around 1943 in the Theresienstadt concentration camp. 
First published in 1899, the theorem was brought to broad attention in 1969 through the popular Mathematical Snapshots by H. Steinhaus. The theorem gives an elegant formula for the area of simple lattice polygons, where "simple", as usual, only means the absence of self-intersection. Polygons covered by the theorem have their vertices located at nodes of a square grid or lattice whose nodes are spaced at distance 1 from their immediate neighbors. The formula does not require math proficiency beyond middle grade school and can be easily verified with the help of a geoboard.
Let P be a lattice polygon. Assume there are I(P) lattice points in the interior of P, and B(P) lattice points on its boundary. Let A(P) denote the area of P. Then
A(P) = I(P) + B(P)/2 - 1
(The applet uses an adaptation of a scan conversion algorithm from . The book is replete with ideas. It just appears that this one was not worked out completely. The applet also appears on a separate page from where it could be lifted for use by teachers on their own pages.)
With Pick's theorem one may determine area of a (polygonal) portion of a map. On a transparent paper draw a grid to scale and superimpose the grid over the map. Count the number of nodes inside and on the boundary of the map region. Apply Pick's formula with the selected scale.
The Farey series FN of order n is the ascending sequence of irreducible fractions m/n between 0 and 1 whose denominators do not exceed N. A fraction m/n belongs to FN iff
0 ≤ m ≤ N, gcd(m,n) = 1,
where gcd(m,n) is the greatest common divisor of m and n. For example, F5 is
0/1, 1/5, 1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 1/1
Farey series is characterized by two wonderful equivalent properties.
G.H. Hardy who prided himself in not having done anything "useful", found it worthwhile to include three different proofs of the basic property of the Farey series in his and E.M. Wright's book. (This is a classical work with the Index located not at the end of the book but, in a contemporary manner, elsewhere on the Web. You'll have to look hard to find it there as the url has been changing.)
The sequence of denominators of terms in the Farey series is palindromic. The proof may not be immediately obvious. But, as is often the case, having a bigger picture proves useful. The Farey series are embedded into the Stern-Brocot tree for which this property comes almost for free.
The area measurement application of Pick's theorem I mentioned above comes from the real world experience. Grünbaum and Shepard quote D.W. DeTemple who attended a presentation on application of mathematics in the forest industry:
Although the speaker was not aware that he was essentially using Pick's formula, I was delighted to see that one of my favorite mathematical results was not only beautiful, but even useful.
I am rather curious whether the forester shared in the delight. There is no surprise in that mathematics is useful. Even G.H. Hardy will be remembered in part because of the Hardy-Weinberg law which became centrally important in the study of many genetic problems . I am charmed by the title of an undergraduate text, Applied Abstract Algebra (R. Lidl and G. Pilz, Springer-Verlag, 1997, 2nd edition.) What would Hardy say?
The goal of course is to pass the delight on.
- A.H. Beiler, Recreations in The Theory of Numbers, Dover, 1966
- M. Bruckheimer and A. Arcavi, Farey Series and Pick's Area Theorem, The Mathematical Intelligencer v 17 (1995), no 4, pp 64-67.
- J. Cofman, Numbers and Shapes Revisited, Clarendon Press, 1995
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- Encyclopædia Britannica
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- R. Graham, D. Knuth, O. Patashnik, Concrete Mathematics, 2nd edition, Addison-Wesley, 1994.
- B.Grünbaum and G.C.Shepard, Pick's Theorem, The Am Math Monthly v 100(1993), pp 150-161
- G.H. Hardy, A Mathematician's Apology, Cambridge University Press, 1994.
- G.H. Hardy and E.M. Wright, An Introduction to the Theory of Numbers, Oxford University Press, Fifth Edition, 1sshu996
- R. Honsberger, Ingenuity in Mathematics, MAA, 1970
- T. Pavlidis, Algortihms for Graphics and Image Processing, Computer Science Press, 1982
- H. Steinhaus, Mathematical Snapshots, Dover, 1999
- D.E. Varberg, Pick's Theorem Revisited, The Am Math Monthly v 92(1985), pp 584-587
- Euler's Formula, Proof 10: Pick's Theorem
- Farey Sequence
- Farey Series
- Geoboards in Classroom
- Pick's Theorem
- Stern-Brocot Tree
- Pick's Theorem