Find all positive integer \( k \) satisfying the following statement: For any positive integers \( m \) and \( n \),
\[
\frac{((k+1)m)! ((k+1)n)!}{m! n! ((k-1)m + n)! (m + (k-1)n)!}
\]
is an integer.
2026-02 Is it an integer?
Leave a reply
Category Archives: problem
Notice
The email account pow@mathsci.kaist.ac.kr was not working normally. It is now fixed.
POW2026-01 is revised to clarify the problem. (The revision is only for the clarification and there is essentially no change in the problem.)
2026-01 Covering a right triangle by two tiles
We want to find the maximum area of two disjoint, simply connected, congruent tiles that can be packed inside a right triangle, one of whose angles is \( \pi/6 \) (30 degrees). What would be the maximal coverage of the right triangle by the tiles? (There is no restriction on the shape of the tiles, especially it does not need to be rectangular, as long as they are simply connected.) (4 points will be given to the one with the best answer, and 3 points for the next four best answers.)
2025-19 To make it into an integer
Prove that for every positive integer \( k \) there exists a positive integer \( n \) such that
\[
\frac{(n+1)(n+2) \dots (2n-k)}{n(n-1) \dots (n-k+1)}
\]
is an integer and that \( k = o(n) \) for such \( n \).
2025-18 Infinitely many squares
Find all integers \( k \) such that the sequence \( (3n^2 + 3nk^2 + k^3 )_{n=1, 2, \dots} \) contains infinitely many squares.
Note: The numbering was wrong. It should be 2025-18.
2025-17 Combinatorial identity
Prove the following identity:
\[
\sum_{k=0}^{n-1} \binom{z}{k} \frac{x^{n-k}}{n-k} = \sum_{k=1}^n \binom{z-k}{n-k} \frac{(x+1)^k -1}{k}.
\]
2015-16 Poisson variable
Show that if \(X\) is a Poisson random variable with parameter \(\mu\), there exists a constant \(c>0\) such that for \(t>\mu+1\), \(\mathbb{P}(X-\mu \geq t)\geq ce^{-2t\log (1+(t+1)/\mu)}\).
2025-15 locally Lipschitz functions
Denote \(P = \{(x, y, z) \in \mathbb{R^3}: 10< x,y,z <31\}\). Suppose a function \(f (v): \mathbb{R^3} \to \mathbb{R_{\geq 0}}\) satisfies:
(a) \(f(\lambda v) = \lambda^{25} f(v)\) for all \(v\in P\) and \(0<\lambda \in \mathbb{R}\),
(b) \(f(v+w) \geq f(v)\) for every \(v, w \in P\),
(c) \(f (v)\) is locally bounded.
Show that \(f (v)\) is locally Lipschitz in \(P\).
2025-14 Convex hulls
Show that any set of d + 2 points in R^d can be partitioned into two sets whose convex hulls intersect.
2025-13 Covering the Donut with Unit Punches
Each punch can be centered anywhere in the plane and removes all points within distance 1 from its center. What is the minimum number of punches needed to remove every point in the annulus between the circles of radius 7 and 10 (with the same center)? Describe your construction. The person with the smallest number of punches earns +4, and the next four best answers earn +3.