Chicken McNugget Theorem
The Chicken McNugget Theorem (or Postage Stamp Problem) states that for any two relatively prime positive integers , the greatest integer that cannot be written in the form
for nonnegative integers
is
.
A consequence of the theorem is that there are exactly positive integers which cannot be expressed in the form
. The proof is based on the fact that in each pair of the form
, exactly one element is expressible.
Contents
[hide]Origins
The story goes that the Chicken McNugget Theorem got its name because in McDonalds, people bought Chicken McNuggets in 9 and 20 piece packages. Somebody wondered what the largest amount you could never buy was, assuming that you did not eat or take away any McNuggets. They found the answer to be 151 McNuggets, thus creating the Chicken McNugget Theorem.
Proof
Definition. An integer will be called purchasable if there exist nonnegative integers
such that
.
We would like to prove that is the largest non-purchasable integer. We are required to show that (1)
is non-purchasable, and (2) every
is purchasable.
Note that all purchasable integers are nonnegative, thus the set of non-purchasable integers is nonempty.
Lemma. Let be the set of solutions
to
. Then
for any
.
Proof: By Bezout, there exist integers such that
. Then
. Hence
is nonempty. It is easy to check that
for all
. We now prove that there are no others. Suppose
and
are solutions to
. Then
implies
. Since
and
are coprime and
divides
,
divides
and
. Similarly
. Let
be integers such that
and
. Then
implies
. We have the desired result.
Lemma. For any integer , there exists unique
such that
.
Proof: By the division algorithm, there exists such that
.
Lemma. is purchasable if and only if
.
Proof: If , then we may simply pick
so
is purchasable. If
, then
if
and
if
, hence at least one coordinate of
is negative for all
. Thus
is not purchasable.
Thus the set of non-purchasable integers is . We would like to find the maximum of this set.
Since both
are positive, the maximum is achieved when
and
so that
.
Problems
Introductory
Marcy buys paint jars in containers of 2 and 7. What's the largest number of paint jars that Marcy can't obtain?
Intermediate
Ninety-four bricks, each measuring are to stacked one on top of another to form a tower 94 bricks tall. Each brick can be oriented so it contributes
or
or
to the total height of the tower. How many different tower heights can be achieved using all ninety-four of the bricks? Source
Olympiad
See Also
This article is a stub. Help us out by expanding it.