636

ANDREW

SIMOSON

[August-September

The author is grateful to Professor

S.

Zaks for suggesting the problem.

REFERENCES

1.

S.

Zaks

and

C.

L.

Liu, Decomposition of Graphs into Trees, Proc. 8th

S. E.

conf.,

Combinatorics,

Graph Theo~ and Computing,

643-654.

Fermat's Last Theorem in Combinatorial Form

W.

V.

QUNE

Department of Philosophy, Harvard University, Cambridge,

MA

02138

Fermat's Last Theorem can be vividly stated in terms of sorting objects into a

row of bins, some of which are red, some blue, and the rest unpainted. The theorem

amounts to saying that when there are more than two objects, the following

statement is never true:

STATEMENT. The number of ways of sorting them that shun both colors is equal to

the number of ways that shun neither.

I shall show that this statement is equivalent to Fermat's equation xn

+

yn

=

zn,

when n is the number of objects, z is the number of bins, x is the number of bins

that are not red, and y is the number of bins that are not blue. There are zn ways of

sorting the objects into bins; xn of these ways shun red and yn of them shun blue.

So where A is the number of ways that shun both colors, B is the number of ways

that shun red but not blue, C is the number of ways that shun blue but not red, and

D is the number of ways that shun neither, we have

xn=A+B, yn=A+C, zn=A+B+C+D.

These equations give xn

+

yn

=

zn if and only if A

=

D, which is to say, if and only

if the statement above holds.

A

Difference Equation for Strings of Ones

ANDREW SIMOSON

Mathematics Department, King College, Bristol,

TN

37620

The classic proof

[2,

pp. 122-31 that

almost every number in its binary expansion contains arbitrarily

long strings of the digit

1

(*

>

can be translated into an elegant difference equation argument, as shown below.

In this note, for each number such as which has a dual binary expansion,

interpret its binary expansion as the one terminating in all zeroes. Let

I

be the half

open interval [0, 1). For each positive integer n, let Bn be the set of numbers in

I

containing no string of n 1's in their binary expansions. To prove

(*)

it is sufficient

to show that Bn has measure zero. For all integers k

2

0, 0

I

j

<