# Solution: 2021-16 Optimal constant

For a given positive integer $$n$$ and a real number $$a$$, find the maximum constant $$b$$ such that
$x_1^n + x_2^n + \dots + x_n^n + a x_1 x_2 \dots x_n \geq b (x_1 + x_2 + \dots + x_n)^n$
for any non-negative $$x_1, x_2, \dots, x_n$$.

The best solution was submitted by 전해구 (기계공학과 졸업생, +4). Congratulations!

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# Solution: 2021-15 Triangles with integer side lengths

For a natural number $$n$$, let $$a_n$$ be the number of congruence classes of triangles whose all three sides have integer length and its perimeter is $$n$$. Obtain a formula for $$a_n$$.

The best solution was submitted by 이도현 (수리과학과 2018학번, +4). Congratulations!

Other solutions were submitted by 강한필 (전산학부 2016학번, +3), 고성훈 (수리과학과 2018학번, +3), 전해구 (기계공학과 졸업생, +3).

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# Solution: 2021-13 Not convex

Prove or disprove the following:

There exist an infinite sequence of functions $$f_n: [0, 1] \to \mathbb{R} , n=1, 2, \dots$$ ) such that

(1) $$f_n(0) = f_n(1) = 0$$ for any $$n$$,

(2) $$f_n(\frac{a+b}{2}) \leq f_n(a) + f_n(b)$$ for any $$a, b \in [0, 1]$$,

(3) $$f_n – c f_m$$ is not identically zero for any $$c \in \mathbb{R}$$ and $$n \neq m$$.

The best solution was submitted by 김기택 (수리과학과 대학원생, +4). Congratulations!

Other solutions were submitted by 강한필 (전산학부 2016학번, +3), 고성훈 (수리과학과 2018학번, +3), 김민서 (수리과학과 2019학번, +3), 박정우 (수리과학과 2019학번, +3), 신주홍 (수리과학과 2020학번, +3), 이도현 (수리과학과 2018학번, +3), 이본우 (수리과학과 2017학번, +3), 이호빈 (수리과학과 대학원생, +3).

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# Solution: 2021-12 A graduation ceremony

In a graduation ceremony, $$n$$ graduating students form a circle and their diplomas are distributed uniformly at random. Students who have their own diploma leave, and each of the remaining students passes the diploma she has to the student on her right, and this is one round. Again, each student with her own diploma leave and each of the remaining students passes the diploma to the student on her right and repeat this until everyone leaves. What is the probability that this process takes exactly $$k$$ rounds until everyone leaves.

The best solution was submitted by 고성훈 (수리과학과 2018학번, +4). Congratulations!

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# Solution: 2021-11 Interesting perfect cubes

Determine if there exist infinitely many perfect cubes such that the sum of the decimal digits coincides with the cube root. If there are only finitely many, how many are there?

The best solution was submitted by 박항 (전산학부 2013학번, +4). Congratulations!

Other solutions were submitted by 강한필 (전산학부 2016학번, +3), 고성훈 (수리과학과 2018학번, +3), 김기수 (수리과학과 2018학번, +3), 최백규 (생명과학과 대학원, +3), 김기택 (2021학번, +3).

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# Solution: 2021-10 Integral inequality

Let $$f: [0, 1] \to \mathbb{R}$$ be a continuous function satisfying
$\int_x^1 f(t) dt \geq \int_x^1 t\, dt$
for all $$x \in [0, 1]$$. Prove that
$\int_0^1 [f(t)]^2 dt \geq \int_0^1 t f(t) dt.$

The best solution was submitted by 김기택 (2021학번, +4). Congratulations!

Other solutions were submitted by 강한필 (전산학부 2016학번, +3), 고성훈 (수리과학과 2018학번, +3), 최백규 (생명과학과 대학원, +3).

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# Solution: 2021-09 Monochromatic solution of an equation

For given $$k\in \mathbb{N}$$, determine the minimum natural number $$n$$ satisfying the following: no matter how one colors each number in $$\{1,2,\dots, n\}$$ red or blue, there always exists (not necessarily distinct) numbers $$x_0, x_1,\dots, x_k \in [n]$$ with the same color satisfying $$x_1+\dots + x_k = x_0$$.

The best solution was submitted by an anonymous participant. Congratulations!

Here is his/her solution of problem 2021-09.

Other solutions were submitted by 고성훈 (수리과학과 2018학번, +3), 김기수 (수리과학과 2018학번, +3).

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# Solution: 2021-08 Self-antipodal sets on the sphere

Prove or disprove that if C is any nonempty connected, closed, self-antipodal (ie., invariant under the antipodal map) set on $$S^2$$, then it equals the zero locus of an odd, smooth function $$f:S^2 -> \mathbb{R}$$.

The best solution was submitted by 신준형 (수리과학과 2015학번, +4). Congratulations!

Here is his solution of problem 2021-08.

Another solution was submitted by 고성훈 (수리과학과 2018학번, +2).

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# Solution: 2021-07 Odd determinant

Let $$A_N$$ be an $$N \times N$$ matrix whose entries are i.i.d. Bernoulli random variables with probability $$1/2$$, i.e.,

$\mathbb{P}( (A_N)_{ij} =0) = \mathbb{P}( (A_N)_{ij} =1) = \frac{1}{2}.$

Let $$p_N$$ be the probability that $$\det A_N$$ is odd. Find $$\lim_{N \to \infty} p_N$$.

The best solution was submitted by 강한필 (전산학부 2016학번, +4). Congratulations!

Here is his solution of problem 2021-07.

Another solution was submitted by 고성훈 (수리과학과 2018학번, +3).

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# Solution: 2021-06 A nondecreasing subsequence

Let $$\mathcal{A}_n$$ be the collection of all sequences $$\mathbf{a}= (a_1,\dots, a_n)$$ with $$a_i \in [i]$$ for all $$i\in [n]=\{1,2,\dots, n\}$$. A nondecreasing $$k$$-subsequence of $$\mathbf{a}$$ is a subsequence $$(a_{i_1}, a_{i_2},\dots, a_{i_k})$$ such that $$i_1< i_2< \dots < i_k$$ and $$a_{i_1}\leq a_{i_2}\leq \dots \leq a_{i_k}$$. For given $$k$$, determine the smallest $$n$$ such that any sequence $$\mathbf{a}\in \mathcal{A}_n$$ has a nondecreasing $$k$$-subsequence.

The best solution was submitted by 고성훈 (수리과학과 2018학번, +4). Congratulations!

Here is his solution of problem 2021-06.

Another solution was submitted by 강한필 (전산학부 2016학번, +3).

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