Difference between revisions of "2016 USAMO Problems/Problem 2"

Revision as of 02:03, 28 June 2025

Problem

Prove that for any positive integer $k,$ $\left(k^2\right)!\cdot\prod_{j=0}^{k-1}\frac{j!}{\left(j+k\right)!}$ is an integer.

Solution 1

Define $v_p(N)$ for all rational numbers $N$ and primes $p$ , where if $N=\frac{x}{y}$ , then $v_p(N)=v_p(x)-v_p(y)$ , and $v_p(x)$ is the greatest power of $p$ that divides $x$ for integer $x$ . Note that the expression(that we're trying to prove is an integer) is clearly rational, call it $N$ .

$v_p(N)=\sum_{i=1}^\infty \left\lfloor \frac{k^{2}}{p^{i}} \right\rfloor+\sum_{j=0}^{k-1} \sum_{i=1}^\infty \left\lfloor \frac{j}{p^{i}}\right\rfloor-\sum_{j=k}^{2k-1} \sum_{i=1}^\infty \left\lfloor \frac{j}{p^{i}} \right\rfloor$ , by Legendre. Clearly, $\left\lfloor{\frac{x}{p}}\right\rfloor={\frac{x-r(x,p)}{p}}$ , and $\sum_{i=0}^{k-1} r(i,m)\leq \sum_{i=k}^{2k-1} r(i,m)$ , where $r(i,m)$ is the remainder function(we take out groups of $m$ which are just permutations of numbers $1$ to $m$ until there are less than $m$ left, then we have $m$ distinct values, which the minimum sum is attained at $0$ to $k-1$ ). Thus, $v_p(N)=\sum_{m=p^{i}, i\in \mathbb{N}_{+}}-\frac{k^{2}}{m}+\left\lfloor{\frac{k^{2}}{m}}\right\rfloor-\frac{\sum_{i=0}^{k-1} r(i,m)-\sum_{i=k}^{2k-1} r(i,m)}{m} \geq \sum_{m=p^{i}, i\in \mathbb{N}} \left\lceil -\frac{k^{2}}{m}+\lfloor{\frac{k^{2}}{m}}\rfloor\right\rceil \geq 0$ , as the term in each summand is a sum of floors also and is clearly an integer.

Solution 2 (Controversial)

Consider an $k\times k$ grid, which is to be filled with the integers $1$ through $k^2$ such that the numbers in each row are in increasing order from left to right, and such that the numbers in each column are in increasing order from bottom to top. In other words, we are creating an $k\times k$ standard Young tableaux.

The Hook Length Formula is the source of the controversy, as it is very powerful and trivializes this problem. The Hook Length Formula states that the number of ways to create this standard Young tableaux (call this $N$ for convenience) is: $N = \frac{\left(k^2\right)!}{\prod_{1\le i, j\le k}(i+j-1)}.$ Now, we do some simple rearrangement: $N = \left(k^2\right)!\cdot\prod_{j=1}^{k}\prod_{i=1}^{k}\frac{1}{i+j-1} = \left(k^2\right)!\cdot\prod_{j=1}^{k}\frac{\left(j-1\right)!}{\left(j+k-1\right)!}$ $= \left(k^2\right)!\cdot\prod_{j=0}^{k-1}\frac{j!}{\left(j+k\right)!}.$ This is exactly the expression given in the problem! Since the expression given in the problem equals the number of distinct $k\times k$ standard Young tableaux, it must be an integer, so we are done.

Solution 3 (Induction)

Define $A(k) = \left(k^2\right)!\cdot\prod_{j=0}^{k-1}\frac{j!}{\left(j+k\right)!}.$ Clearly, $A(1) = 1$ and $A(2) = 2.$

Then $\frac{A(k+1)}{A(k)} = \frac{\left(k^2+2k+1\right)!\cdot\prod_{j=0}^{k}\frac{j!}{\left(j+k+1\right)!}}{\left(k^2\right)!\cdot\prod_{j=0}^{k-1}\frac{j!}{\left(j+k\right)!}}.$ Lots of terms cancel, and we are left with $\frac{A(k+1)}{A(k)} = \frac{(k^2+1)(k^2+2)\cdots(k^2+2k+1)}{2(2k+1)}.$ The numerator has $2k+1$ consecutive positive integers, so one of them must be divisible by $(2k+1).$ Also, there are $2k$ terms left, $k$ of which are even. We can choose one of these to cancel out the $2$ in the denominator. Therefore, the ratio between $A(k+1)$ and $A(k)$ is an integer. By our inductive hypothesis, $A(k)$ is an integer. Therefore, $A(k+1)$ is as well, and we are done.

Note: This is incorrect.

Solution 4

Throughout this proof we work in the polynomial ring $\mathbb{Z}[q]$ .

For any positive integer $n$ , define the $q$ -integer and Gaussian factorial (also called $q$ -factorial) by $[n]_q := 1 + q + q^2 + \dots + q^{n-1}, \quad \text{so that} \quad [n]_q! := \prod_{i=1}^{n}[i]_q.$ Each $[i]_q$ is a polynomial of degree $i-1$ with integer coefficients, so $[n]_q!$ is a polynomial in $\mathbb{Z}[q]$ . Moreover, evaluating at $q=1$ recovers the ordinary factorial: $\lim_{q\to1}[n]_q! = n!.$

We now consider the expression $P_k(q) := [k^2]_q! \cdot \prod_{j=0}^{k-1} \frac{[j]_q!}{[j+k]_q!},$ and aim to prove that $P_k(q) \in \mathbb{Z}[q]$ and $P_k(1)$ is an integer.

Let $\nu_d(F)$ denote the multiplicity of $(1 - q^d)$ in a polynomial $F$ .

We use the identity $\nu_d([n]_q!) = \sum_{j=1}^n \left\lfloor \frac{j}{d} \right\rfloor - n \cdot \delta_{d=1},$ where $\delta_{d=1}$ is the Kronecker delta.

Then $\nu_d(P_k(q)) = \nu_d([k^2]_q!) + \sum_{j=0}^{k-1} \nu_d([j]_q!) - \sum_{j=0}^{k-1} \nu_d([j+k]_q!).$

Substitute the formula for each term: $\nu_d(P_k(q)) = \left( \sum_{i=1}^{k^2} \left\lfloor \frac{i}{d} \right\rfloor - k^2 \delta_{d=1} \right) + \sum_{j=0}^{k-1} \left( \sum_{i=1}^{j} \left\lfloor \frac{i}{d} \right\rfloor - j \delta_{d=1} \right) - \sum_{j=0}^{k-1} \left( \sum_{i=1}^{j+k} \left\lfloor \frac{i}{d} \right\rfloor - (j+k) \delta_{d=1} \right).$

Group terms: $\nu_d(P_k(q)) = \sum_{i=1}^{k^2} \left\lfloor \frac{i}{d} \right\rfloor + \sum_{j=0}^{k-1} \sum_{i=1}^{j} \left\lfloor \frac{i}{d} \right\rfloor - \sum_{j=0}^{k-1} \sum_{i=1}^{j+k} \left\lfloor \frac{i}{d} \right\rfloor - \left(k^2 + \sum_{j=0}^{k-1} j - \sum_{j=0}^{k-1} (j + k)\right) \delta_{d=1}.$

Compute the final delta term: $\sum_{j=0}^{k-1} j = \frac{k(k-1)}{2}, \quad \sum_{j=0}^{k-1} (j+k) = \frac{k(k-1)}{2} + k^2,$ so $-k^2 - \frac{k(k-1)}{2} + \left( \frac{k(k-1)}{2} + k^2 \right) = 0.$

Hence $\nu_d(P_k(q)) = \sum_{i=1}^{k^2} \left\lfloor \frac{i}{d} \right\rfloor + \sum_{j=0}^{k-1} \sum_{i=1}^{j} \left\lfloor \frac{i}{d} \right\rfloor - \sum_{j=0}^{k-1} \sum_{i=1}^{j+k} \left\lfloor \frac{i}{d} \right\rfloor.$

All floor terms are nonnegative, so $\nu_d(P_k(q)) \ge 0$ for all $d \ge 1$ .

Therefore $P_k(q) \in \mathbb{Z}[q]$ . Evaluating at $q = 1$ gives $P_k(1) = (k^2)! \cdot \prod_{j=0}^{k-1} \frac{j!}{(j+k)!} \in \mathbb{Z},$ which completes the proof. ~Lopkiloinm

Note: This solution is fundamentally different from the first, which works purely in the integers. Here, we work in the integer polynomial ring $\mathbb{Z}[q]$ —a graded algebra—which allows us to test integrality by tracking the multiplicities of reducible elements like $1 - q^d$ for all integers $1 \le d \le k^2$ . In contrast, working in $\mathbb{Z}$ —a non-graded algebra—requires using $p$ -adic valuations via Legendre’s formula, which only considers irreducibles (primes). The graded structure of $\mathbb{Z}[q]$ simplifies the analysis, making it a powerful strategy to lift problems into a graded setting whenever possible.

@@ Line 45: / Line 45: @@
 Let <math>\nu_d(F)</math> denote the multiplicity of <math>(1 - q^d)</math> in a polynomial <math>F</math>.
-Instead of using the naive approximation <math>\left\lfloor \frac{n}{d} \right\rfloor</math>, we recall the correct formula for the multiplicity of <math>(1 - q^d)</math> in the Gaussian factorial:
+We use the identity
 <cmath>
 \nu_d([n]_q!) = \sum_{j=1}^n \left\lfloor \frac{j}{d} \right\rfloor - n \cdot \delta_{d=1},
@@ Line 51: / Line 51: @@
 where <math>\delta_{d=1}</math> is the Kronecker delta.
-Applying this to <math>P_k(q)</math> gives:
+Then
 <cmath>
 \nu_d(P_k(q)) = \nu_d([k^2]_q!) + \sum_{j=0}^{k-1} \nu_d([j]_q!) - \sum_{j=0}^{k-1} \nu_d([j+k]_q!).
 </cmath>
-Substituting the correct multiplicity formula:
+Substitute the formula for each term:
 <cmath>
 \nu_d(P_k(q)) = \left( \sum_{i=1}^{k^2} \left\lfloor \frac{i}{d} \right\rfloor - k^2 \delta_{d=1} \right) + \sum_{j=0}^{k-1} \left( \sum_{i=1}^{j} \left\lfloor \frac{i}{d} \right\rfloor - j \delta_{d=1} \right) - \sum_{j=0}^{k-1} \left( \sum_{i=1}^{j+k} \left\lfloor \frac{i}{d} \right\rfloor - (j+k) \delta_{d=1} \right).
 </cmath>
-Grouping and simplifying terms:
+Group terms:
 <cmath>
 \nu_d(P_k(q)) = \sum_{i=1}^{k^2} \left\lfloor \frac{i}{d} \right\rfloor + \sum_{j=0}^{k-1} \sum_{i=1}^{j} \left\lfloor \frac{i}{d} \right\rfloor - \sum_{j=0}^{k-1} \sum_{i=1}^{j+k} \left\lfloor \frac{i}{d} \right\rfloor - \left(k^2 + \sum_{j=0}^{k-1} j - \sum_{j=0}^{k-1} (j + k)\right) \delta_{d=1}.
 </cmath>
-Note that
+Compute the final delta term:
 <cmath>
-\sum_{j=0}^{k-1} j = \frac{k(k-1)}{2}, \quad \sum_{j=0}^{k-1} (j+k) = \frac{k(k-1)}{2} + k^2.
+\sum_{j=0}^{k-1} j = \frac{k(k-1)}{2}, \quad \sum_{j=0}^{k-1} (j+k) = \frac{k(k-1)}{2} + k^2,
 </cmath>
-So the <math>\delta_{d=1}</math> term becomes:
+so
 <cmath>
 -k^2 - \frac{k(k-1)}{2} + \left( \frac{k(k-1)}{2} + k^2 \right) = 0.
 </cmath>
-Thus
+Hence
 <cmath>
 \nu_d(P_k(q)) = \sum_{i=1}^{k^2} \left\lfloor \frac{i}{d} \right\rfloor + \sum_{j=0}^{k-1} \sum_{i=1}^{j} \left\lfloor \frac{i}{d} \right\rfloor - \sum_{j=0}^{k-1} \sum_{i=1}^{j+k} \left\lfloor \frac{i}{d} \right\rfloor.
 </cmath>
-Since all floor terms are non-negative integers and all summations are finite, it follows that <math>\nu_d(P_k(q)) \ge 0</math> for all <math>d</math>. Hence every factor <math>1 - q^d</math> appears with non-negative multiplicity in <math>P_k(q)</math>, and so
+All floor terms are nonnegative, so <math>\nu_d(P_k(q)) \ge 0</math> for all <math>d \ge 1</math>.
-<cmath>
-P_k(q) \in \mathbb{Z}[q].
-</cmath>
-Evaluating at <math>q = 1</math> gives:
+Therefore <math>P_k(q) \in \mathbb{Z}[q]</math>. Evaluating at <math>q = 1</math> gives
 <cmath>
-P_k(1) = (k^2)! \cdot \prod_{j=0}^{k-1} \frac{j!}{(j+k)!} \in \mathbb{Z}.
+P_k(1) = (k^2)! \cdot \prod_{j=0}^{k-1} \frac{j!}{(j+k)!} \in \mathbb{Z},
 </cmath>
-Thus, the original expression is an integer.
+which completes the proof.
 ~Lopkiloinm

2016 USAMO (Problems • Resources)
Preceded by Problem 1	Followed by Problem 3
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