# Algorithms for Egyptian Fractions

## References

L. Beeckmans. The splitting algorithm for Egyptian fractions.
J. Number Th. 43, 1993, pp. 173­185.
This paper contains a proof that the splitting method terminates; Wagon [Wag91] credits the same result to Graham and Jewett.

M. N. Bleicher. A new algorithm for the expansion of continued fractions.
J. Number Th. 4, 1972, pp. 342­382.
Defines two methods for Egyptian fraction representation: what he calls the Farey sequence method, which is equivalent to the continued fraction method described here, and what he calls the continued fraction method, which is a variant of what we call the grouped continued fraction method.
R. Breusch. A special case of Egyptian fractions, solution to advanced problem 4512.
Amer. Math. Monthly 61, 1954, pp. 200­201.
Shows that every x/y with y odd has an Egyptian fraction representation with all denominators odd, by using a method similar to the binary remainder method but using the base 5(3^k) instead of 2^k.
T. H. Byers and M. S. Waterman. Determining all optimal and near-optimal solutions when solving shortest path problems by dynamic programming.
Oper. Res. 32, 1984, pp. 1381­1384.
Byers and Waterman describe a simple algorithm for finding short paths in a graph, used in our implementation of the grouped continued fraction method.
D. Eppstein. Finding the k shortest paths.
35th IEEE Symp. Foundations of Computer Science, 1994, pp. 154­165.
I describe what is theoretically the fastest known algorithm for the shortest path problem used in the grouped continued fraction method, however my technique is rather more complex than that of [BW84] and has not been implemented in Mathematica.
R.K. Guy. Unsolved Problems in Number Theory.
Springer-Verlag, 1981, pp. 87­93.
Guy lists a number of questions about Egyptian fractions, including the following:
• Does 4/y have a 3-term representation for all y? Does 5/y? Does x/y for x sufficiently large relative to y?
• Does the odd greedy method terminate?
• What different denominators are possible in a t-term representation of one? Do all positive-density sets of integers have a subset forming the denominators of a representation of one?
• If we assign to each of the integers one of a finite set of colors, can we pick a single color that can be used as the denominators for representations of all rationals?
Guy also provides a long bibliography.
V. Klee and S. Wagon. Old and New Unsolved Problems in Plane Geometry and Number Theory. Math. Assoc. of America, 1991, pp. 175­177 and 206­208.
Concentrates primarily on the question of whether the odd greedy method halts; notes that a similar method for finding representations with even denominators does halt; contains a number of further references.
L. J. Mordell. Diophantine Equations. Academic Press, 1969, pp. 287­290.
Discusses the question of whether 4/y has a three-term representation; describes methods of finding such representations depending on the value of n modulo various small primes.
I. Niven and H.S. Zuckerman. An Introduction to the Theory of Numbers. 4th ed., Wiley, 1980, p. 200.
They describe in an exercise here the secondary sequences used in the continued fraction methods of Egyptian fraction representation.
B. M. Stewart. Sums of distinct divisors.
Amer. J. Math. 76, 1954, pp. 779­785.
Uses the binary remainder method to find representations with all denominators even. Also, like [Bre54], uses a method similar to the binary remainder method (using base 35(3^k)) to find odd representations of fractions with odd denominators.
I. Stewart. The riddle of the vanishing camel.
Scientific American, June 1992, pp. 122­124.
Stewart shows that any rational has only a finite number of t-term Egyptian fraction representations, for any fixed number t, and describes a brute force method for finding all such representations.
T. Takenouchi. On an indeterminate equation.
Proc. Physico-Mathematical Soc. of Japan (3rd ser.) 3, 1921, pp. 78-92.
In this paper, discovered for me by Stefan Bartels, Takenouchi proves that any number with a k-term unit fraction representation has a representation in which all terms are distinct. Bartels writes "Takenouchi does not state the theorem itself but gives a proof to it. He uses the fact that the sum of the denominators of the representation increases and the fact that the number of representations of a given length is finite. As far as I know this article has not been recognized in the meantime. I found some proofs (done in the 60s) which for example prove that n/m can be represented as a sum of n Egyptian fractions. This shows that the article is (was) quite unknown in mathematical literature."
G. Tenenbaum and H. Yokota. Length and denominators of Egyptian fractions.
J. Number Th. 35, 1990, pp. 150­156.
This paper provides good simultanous bounds on the number of terms and the denominators of an Egyptian fraction representation, as discussed in the section on the binary remainder method.
M. Vose. Egyptian fractions.
Bull. Lond. Math. Soc. 17, 1985, p. 21.
Shows that every x/y has a t-term representation where t=O(Sqrt Log[y]). The technique is similar to the binary remainder method.

[Wag91]

S. Wagon. Mathematica in Action.
W.H. Freeman, 1991, pp. 271­277.
Wagon implements the greedy and odd greedy methods, and describes the splitting method. He also mentions the open problem of whether the odd greedy method always terminates for the special case of fractions with numerator 2.

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