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Difference between revisions of "1986 USAMO Problems/Problem 5"

(Created page with "== Problem == By a partition <math>\pi</math> of an integer <math>n\ge 1</math>, we mean here a representation of <math>n</math> as a sum of one or more positive integers wher...")
 
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== Solution ==
 
== Solution ==
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{{solution}}
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Let <math>S(n) = \sum\limits_{\pi} A(\pi)</math> and let <math>T(n) = \sum\limits_{\pi} B(\pi)</math>. We will use generating functions to approach this problem -- specifically, we will show that the generating functions of <math>S(n)</math> and <math>T(n)</math> are equal.
 +
 
 +
Let us start by finding the generating function of <math>S(n)</math>. This function counts the total number of 1's in all the partitions of <math>n</math>. Another way to count this is by counting the number of partitions of <math>n</math> that contain <math>x</math> 1's and multiplying this by <math>x</math>, then summing for <math>1\leq x \leq n</math>. However, the number of partitions of <math>n</math> that contain <math>x</math> 1's is the same as the number of partitions of <math>n-x</math> that contain no 1's, so
 +
 
 +
<cmath>\begin{align*}
 +
S(n) &= 1*(\text{\# of partitions of n-1 with no 1's})\ &+ 2*(\text{\# of partitions of n-2 with no 1's})\ &... \ &+ n*(\text{\# of partitions of 0 with no 1's})
 +
\end{align*}</cmath>
 +
 
 +
The number of partitions of <math>m</math> with no 1's is the coefficient of <math>x^m</math> in <math>F(x)=(1+x^2+x^4+\ldots)(1+x^3+x^6+\ldots)(1+x^4+x^8+\ldots)\ldots=\prod\limits_{i=2}^{\inf}\frac{1}{1-x^i}</math>. Let <math>c_m</math> be the coefficient of <math>x^m</math> in the expansion, so we can rewrite it as <math>F(x)=c_0+c_1x+c_2x^2+c_3x^3+\ldots</math>. We wish to compute <math>S(n)=1*c_{n-1}+2*c_{n-2}+\ldots+n*c_0</math>.
 +
 
 +
Consider the power series <math>G(x)=(x+2x^2+3x^3+4x^4+\ldots)\cdot F(x)</math>. The coefficient of <math>x^n</math> for any <math>n</math> is <math>1*c_{n-1}+2*c_{n-2}+\ldots+n*c_0</math>, which is exactly <math>S(n)</math>!
 +
 
 +
We can simplify the expression for <math>G(x)</math>:
 +
<cmath>\begin{align*}
 +
G(x) &= G(x)=(x+2x^2+3x^3+4x^4+\ldots)\cdot F(x)\ &= x(1+2x+3x^2+4x^3+\ldots)\cdot \prod\limits_{i=2}^{\inf}\frac{1}{1-x^i}\ &= x\cdot\frac{1}{(1-x)^2}\cdot \prod\limits_{i=2}^{\inf}\frac{1}{1-x^i} &= \frac{x}{1-x} \cdot \prod\limits_{i=1}^{\inf}\frac{1}{1-x^i}
 +
\end{align*}</cmath>
 +
 
 +
Therefore the generating function of <math>S(n)</math> is <math>G(x)=\frac{x}{1-x} \cdot \prod\limits_{i=1}^{\inf}\frac{1}{1-x^i}</math>.
 +
 
 +
Now let's find the generating function of <math>T(n)</math>. Notice that counting number of distinct elements in each partition and summing over all partitions is equivalent to counting how many partitions of <math>n</math> contain i and then summing for all <math>1\leq i \leq n</math>.
 +
 
 +
In other words,
 +
<cmath>\begin{align*}
 +
T(n) &= 1*(\text{\# of partitions of n that contain a 1})\ &+ 2*(\text{\# of partitions of n that contain a 2})\ &... \ &+ n*(\text{\# of partitions of n that contain a n})
 +
\end{align*}</cmath>
 +
 
 +
However, the number of partitions of n that contain a <math>i</math> is the same as the total number of partitions of <math>n-i</math>, so
 +
<cmath>\begin{align*}
 +
T(n) &= 1*(\text{\# of partitions of n-1})\ &+ 2*(\text{\# of partitions of n-2)\ &... \ &+ n*(\text{\# of partitions of 0})
 +
\end{align*}</cmath>
 +
 
 +
The generating function for the number of partitions is <math>P(x)=(1+x+x^2+\ldots)(1+x^2+x^4+\ldots)(1+x^3+x^6+\ldots)\ldots=\prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}</math>. Let's write the expansion of P(x) as <math>P(x)=d_0+d_1x+d_2x^2+d_3x^3+\ldots</math>, so we wish to find <math>T(n)=d_0+d_1+d_2+\ldots+d_{n-1}</math>.
 +
 
 +
Consider the power series <math>H(x)=(x+x^2+x^3+x^4+\ldots)P(x)=(x+x^2+x^3+x^4+\ldots)(d_0+d_1x+d_2x^2+d_3x^3+\ldots)</math>. The coefficient of <math>x^n</math> in <math>H(x)</math> is <math>d_{n-1}+d_{n-2}+\ldots+d_0</math>, which is precisely <math>T(n)</math>, so <math>H(x)</math> is the generating function of <math>T(n)</math>.
 +
 
 +
<math>H(x)</math> can be simplified:
 +
<cmath>\begin{align*}
 +
H(x) &= (x+x^2+x^3+x^4+\ldots)P(x)\ &= x(1+x+x^2+x^3+\ldots)\cdot  \prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}\ &= x\cdot \frac{1}{1-x} \cdot \prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}\ &= \frac{x}{1-x} \cdot \prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}
 +
\end{align*}</cmath>
 +
 
 +
Thus, the generating functions of <math>S(n)</math> and <math>T(n)</math> are the same, which means <math>S(n)=T(n)</math> for all <math>n</math>, and we are done.
 +
 
 +
~Peggy
  
 
== See Also ==
 
== See Also ==

Revision as of 10:45, 30 August 2018

Problem

By a partition $\pi$ of an integer $n\ge 1$, we mean here a representation of $n$ as a sum of one or more positive integers where the summands must be put in nondecreasing order. (E.g., if $n=4$, then the partitions $\pi$ are $1+1+1+1$, $1+1+2$, $1+3, 2+2$, and $4$).

For any partition $\pi$, define $A(\pi)$ to be the number of $1$'s which appear in $\pi$, and define $B(\pi)$ to be the number of distinct integers which appear in $\pi$. (E.g., if $n=13$ and $\pi$ is the partition $1+1+2+2+2+5$, then $A(\pi)=2$ and $B(\pi) = 3$).

Prove that, for any fixed $n$, the sum of $A(\pi)$ over all partitions of $\pi$ of $n$ is equal to the sum of $B(\pi)$ over all partitions of $\pi$ of $n$.

Solution

Let $S(n) = \sum\limits_{\pi} A(\pi)$ and let $T(n) = \sum\limits_{\pi} B(\pi)$. We will use generating functions to approach this problem -- specifically, we will show that the generating functions of $S(n)$ and $T(n)$ are equal.

Let us start by finding the generating function of $S(n)$. This function counts the total number of 1's in all the partitions of $n$. Another way to count this is by counting the number of partitions of $n$ that contain $x$ 1's and multiplying this by $x$, then summing for $1\leq x \leq n$. However, the number of partitions of $n$ that contain $x$ 1's is the same as the number of partitions of $n-x$ that contain no 1's, so

\begin{align*} S(n) &= 1*(\text{\# of partitions of n-1 with no 1's})\\ &+ 2*(\text{\# of partitions of n-2 with no 1's})\\ &... \\ &+ n*(\text{\# of partitions of 0 with no 1's}) \end{align*}

The number of partitions of $m$ with no 1's is the coefficient of $x^m$ in $F(x)=(1+x^2+x^4+\ldots)(1+x^3+x^6+\ldots)(1+x^4+x^8+\ldots)\ldots=\prod\limits_{i=2}^{\inf}\frac{1}{1-x^i}$. Let $c_m$ be the coefficient of $x^m$ in the expansion, so we can rewrite it as $F(x)=c_0+c_1x+c_2x^2+c_3x^3+\ldots$. We wish to compute $S(n)=1*c_{n-1}+2*c_{n-2}+\ldots+n*c_0$.

Consider the power series $G(x)=(x+2x^2+3x^3+4x^4+\ldots)\cdot F(x)$. The coefficient of $x^n$ for any $n$ is $1*c_{n-1}+2*c_{n-2}+\ldots+n*c_0$, which is exactly $S(n)$!

We can simplify the expression for $G(x)$: \begin{align*} G(x) &= G(x)=(x+2x^2+3x^3+4x^4+\ldots)\cdot F(x)\\ &= x(1+2x+3x^2+4x^3+\ldots)\cdot \prod\limits_{i=2}^{\inf}\frac{1}{1-x^i}\\ &= x\cdot\frac{1}{(1-x)^2}\cdot \prod\limits_{i=2}^{\inf}\frac{1}{1-x^i} &= \frac{x}{1-x} \cdot \prod\limits_{i=1}^{\inf}\frac{1}{1-x^i} \end{align*}

Therefore the generating function of $S(n)$ is $G(x)=\frac{x}{1-x} \cdot \prod\limits_{i=1}^{\inf}\frac{1}{1-x^i}$.

Now let's find the generating function of $T(n)$. Notice that counting number of distinct elements in each partition and summing over all partitions is equivalent to counting how many partitions of $n$ contain i and then summing for all $1\leq i \leq n$.

In other words, \begin{align*} T(n) &= 1*(\text{\# of partitions of n that contain a 1})\\ &+ 2*(\text{\# of partitions of n that contain a 2})\\ &... \\ &+ n*(\text{\# of partitions of n that contain a n}) \end{align*}

However, the number of partitions of n that contain a $i$ is the same as the total number of partitions of $n-i$, so 

\begin{align*}
T(n) &= 1*(\text{\# of partitions of n-1})\\ &+ 2*(\text{\# of partitions of n-2)\\ &... \\ &+ n*(\text{\# of partitions of 0})
\end{align*} (Error compiling LaTeX. Unknown error_msg)

The generating function for the number of partitions is $P(x)=(1+x+x^2+\ldots)(1+x^2+x^4+\ldots)(1+x^3+x^6+\ldots)\ldots=\prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}$. Let's write the expansion of P(x) as $P(x)=d_0+d_1x+d_2x^2+d_3x^3+\ldots$, so we wish to find $T(n)=d_0+d_1+d_2+\ldots+d_{n-1}$.

Consider the power series $H(x)=(x+x^2+x^3+x^4+\ldots)P(x)=(x+x^2+x^3+x^4+\ldots)(d_0+d_1x+d_2x^2+d_3x^3+\ldots)$. The coefficient of $x^n$ in $H(x)$ is $d_{n-1}+d_{n-2}+\ldots+d_0$, which is precisely $T(n)$, so $H(x)$ is the generating function of $T(n)$.

$H(x)$ can be simplified: \begin{align*} H(x) &= (x+x^2+x^3+x^4+\ldots)P(x)\\ &= x(1+x+x^2+x^3+\ldots)\cdot \prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}\\ &= x\cdot \frac{1}{1-x} \cdot \prod\limits_{i=1}^{\inf} \frac{1}{1-x^i}\\ &= \frac{x}{1-x} \cdot \prod\limits_{i=1}^{\inf} \frac{1}{1-x^i} \end{align*}

Thus, the generating functions of $S(n)$ and $T(n)$ are the same, which means $S(n)=T(n)$ for all $n$, and we are done.

~Peggy

See Also

1986 USAMO (Problems • Resources)
Preceded by
Problem 4 		Followed by
Last Question
1 • 2 • 3 • 4 • 5
All USAMO Problems and Solutions

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