# Math Challenge - September 2021

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Mentor
Summary: Gamma function. Combinatorics. Stochastic. Semisimple Modules. Topological Groups. Metric spaces. Logarithmic inequality. Stochastic. Primes. Approximation theory.

1. (solved by @julian and @benorin ) Let ##f## be a function defined on ##(0,\infty)## such that ##f(x)>0## for all ##x>0.## Suppose that ##f## has the following properties:
1. ##\log f(x)## is a convex function.
2. ##f(x+1)=x\cdot f(x)## for all ##x>0.##
3. ##f(1)=1.##
Then ##f(x)=\displaystyle{\lim_{n \to \infty}\dfrac{n!n^x}{x(x+1)\cdots (x+n)}}=:\Gamma(x)## for all ##x>0.##

2. (solved by @TeethWhitener ) Let ##T = (x_1, x_2, \ldots , x_m)## be a sequence of not necessarily distinct reals. For any positive ##b##, define
$$T_b := \{(x_i, x_j ) \,|\, 1 \leq i, j \leq m, |x_i-x_j | \leq b\}.$$
Show that for any sequence ##T## and for every integer ##r>1,##
$$|T_r| < (2r-1)|T_1|.$$

3. Let ##X,Y## be two independent identically distributed real random variables. For a positive ##b,## define ##p_b:=\operatorname{prob}\left(|X-Y|\leq b\right).## Then for every integer ##r,## ##p_r\leq (2r-1)p_1.## Thus
$$\operatorname{prob}\left(|X-Y|\leq 2\right) \leq 3\cdot \operatorname{prob}\left(|X-Y|\leq 1\right)$$

4. Let ##\mathbb{F}## be a field and ##G## a finite group, such that ##\operatorname{char}\mathbb{F}\,\nmid\,|G|.## Prove that ##\mathbb{F}G## is semisimple, and show that this is not true if ##\operatorname{char}\mathbb{F}\,\mid\,|G|.##

5. A group ##G## together with a topology, such that the mapping on ##G\times G## (equipped with the product topology) to ##G## given by ##(x,y)\longmapsto xy^{-1}## is continuous, is called a topological group (e.g. a Lie group). Prove
1. (solved by @jbergman ) ##G## is a topological group if and only if inversion and multiplication are continuous.
2. (solved by @jbergman ) The mappings ##x\longmapsto xg## and ##x\longmapsto gx## are homeomorphisms for each ##g\in G.##
3. (solved by @jbergman ) Each open subgroup ##U\leq G## is closed, and each closed subgroup ##U\leq G## of finite index is open. If ##G## is compact, then each open subgroup is of finite index.
4. (solved by @jbergman ) Let ##H \leq G## be a subgroup equipped with the subspace topology, ##K\trianglelefteq G## a normal subgroup, and ##G/K## equipped with the quotient space topology. Then ##H## and ##G/K## are again topological groups and the projection ##\pi\, : \,G \twoheadrightarrow G/K## is open.
5. ##G## is Hausdorff if and only if ##\{1\}## is a closed set in ##G.## ##G/K## is Hausdorff for a normal subgroup ##K\trianglelefteq G,## if and only if ##K## is closed in ##G.## If ##G## is totally disconnected, then ##G## is Hausdorff.
6. Let ##G## be a compact topological group and ##\{X_j \;(j\in I)\}\subseteq G## a family of closed subsets such that for all ## i ,j\in I## there is a ##k\in I## with ##X_k\subseteq X_i\cap X_j.## Then we have for any closed subset ##Y\subseteq G##
$$Y\cdot \left(\bigcap_{i\in I}X_i\right) = \bigcap_{i\in I} YX_i$$

6. Let ##(X_n,d_n)_{n\in \mathbb{N}_0}## be a sequence of complete metric spaces, and let ##(f_n)_{n\in \mathbb{N}_0}## be a sequence of continuous functions ##f_n:X_{n+1}\longrightarrow X_n## such that the image ##f_n(X_{n+1}) \subseteq (X_n,d_n)## is dense for all ##n\in \mathbb{N}_0##. Then
$$M_0:= \left\{v_0\in X_0\,|\,\exists\,(v_n)_{n\in \mathbb{N}} \,\forall\,n\in \mathbb{N}\, : \,v_n\in X_n\,\wedge \,f_{n-1}(v_n)=v_{n-1} \right\}$$
and
$$M_0\subseteq M:= \bigcap_{n=0}^\infty(f_0\circ f_1\circ\ldots\circ f_n)(X_{n+1})$$
are dense in ##(X_0,d_0).## In particular ##M\neq \emptyset## in case ##X_0\neq \emptyset.##

7. (solved by @julian ) Prove for all ##x>-1##
$$x-(1+x)\log (1+x)\leq -\dfrac{3x^2}{2(x+3)}$$

8. Let ##(\Omega, \mathcal{A},\mathcal{P})## be a probability space, ##B,T,\sigma ## positive real numbers, and ##n\in \mathbb{N}.## For independently distributed random variables ##X_1,\ldots,X_n\, : \,\Omega \longrightarrow \mathbb{R}## with expectation values ##E(X_k)=0## and ##E(X_k^2)\leq \sigma^2##, and boundary ##\|X_k\|_\infty \leq B## for all ##k=1,\ldots,n ## prove
$$P\left(\dfrac{1}{n}\sum_{k=1}^nX_k \geq \sqrt{\dfrac{2\sigma^2 T}{n}}+\dfrac{2BT}{3n}\right) \leq e^{-T}.$$

9. Let ##\mathbb{P}## be the set of all primes, ##p\in \mathbb{P},## and ##n\in \mathbb{N}## a positive integer. ##\operatorname{ord}_p(N)## denotes the number of primes ##p## which occur as divisor in ##\{1,2,\ldots,N\}## counted by multiplicity. E.g. ##N=24=4!## and ##p=3## yields in ##\{3=3^1,6=3^1\cdot 2,9=3^2,12=3^1\cdot 4,15=3^1\cdot 5,18=3^2\cdot 2,21=3^1\cdot 7,24=3^1\cdot 8\}##
$$\operatorname{ord}_3(24) = 1+1+2+1+1+2+1+1=10$$
Prove
1. (solved by @TeethWhitener ) ##\displaystyle{\operatorname{ord}_p(n)=\sum_{k\geq 1}\left\lfloor \dfrac{n}{p^k}\right\rfloor}##
2. (solved by @TeethWhitener )##2\left.\,\right|\binom{2n}{n}## and ##p\left.\,\right|\binom{2n}{n}## for all ##n<p\leq 2n##
3. (solved by @TeethWhitener ) ##p\geq 3\,\wedge \,2n/3 <p\leq n \,\Longrightarrow \,p\,\nmid\,\binom{2n}{n}##
4. (solved by @julian ) ##p^r\left.\,\right|\binom{2n}{n}\,\Longrightarrow \,p^r\leq 2n##
5. (solved by @julian ) ##\dfrac{2^{2n-1}}{n}\leq \binom{2n}{n}\leq 2^{2n-1}##
6. (solved by @julian ) ##\displaystyle{\prod_{p\leq n}p<4^n}##

10. Let ##K## be compact and ##C(K):=\{f:K\to \mathbb{R} \text{ or }\mathbb{C}\,|\,f \text{ is continuous}.## A ##n-##dimensional subspace ##M\subseteq C(K)## is called Haar space, if all ##f\in M-\{0\}## have at most ##n-1## zeros. Linear independent functions ##S:=\{\varphi_1,\ldots,\varphi_n \}\subseteq C(K)## are called a Chebyshev- or Haar-system, if ##\operatorname{span}(S)## is a Haar space. We denote the (compact) unit circle ##\mathbb{T} := \{e^{2\pi it} \,|\, t\in [0,1)\}.## Let ##K\subseteq \mathbb{R}## be compact or ##K=\mathbb{T}\, , \,f\in C(K).##

We call a point ##\xi \in K## with ##f(\xi)=0## a simple zero of ##f## if ##\xi## is either on the boundary of ##K## or ##f## changes sign in ##\xi.## If ##f(\xi -t)f(\xi +t)>0## in a neighborhood of ##\xi,## then we speak of a double zero.
1. A subspace ##M\subseteq C(K)## with ##\dim M=n## is a Haar space if and only if each ##f\in M-\{0\}## that has ##j\in \mathbb{N}_0## simple zeros and ##k\in \mathbb{N}_0## double zeros holdsSo each element $f\in M-\{0\}$ has at most $n-1$ different zeros.
$$j+2k<n.$$ So each element ##f\in M-\{0\}## has at most ##n-1## different zeros.
2. The space of all real-valued trigonometric polynomials on ##[0,1)## of degree at most ##n## is a Haar space of dimension ##2n+1.##
3. Let ##n\in \mathbb{N}_0,\;p\in T_n,## and ##x\in \mathbb{T}.## Then
$$\left|p'(x)\right|\leq 2\pi n\sqrt{\|p\|^2_\infty -|p(x)|^2}.$$
Remark: Consider the linear differential operator ##D(p)=p'## on ##T_n.## From ##|D(\sin(2\pi nx))|=|2\pi n\cos(2\pi nx)|## we conclude that
$$2\pi n \leq \|D\|=\sup_{\|p\|_\infty \leq 1}\|p'\|_\infty=\sup_{\|p\|_\infty = 1}\sup_{x\in \mathbb{T}}|p'(x)| <\infty$$
because ##T_n## is finite-dimensional.

High Schoolers only (until 26th)

11.
(solved by @Not anonymous ) Let ##S## be a set of real numbers that is closed under multiplication (that is, if ##a## and ##b## are in ##S##, then so is ##ab##). Let ##T## and ##U## be disjoint subsets of ##S## whose union is ##S##. Given that the product of any three (not necessarily distinct) elements of ##T## is in ##T## and that the product of any three elements of ##U## is in ##U##, show that at least one of the two subsets ##T,U## is closed under multiplication.

12. (solved by @Not anonymous ) Suppose we have a necklace of ##n## beads. Each bead is labeled with an integer and the sum of all these labels is ##n-1.## Prove that we can cut the necklace to form a string whose consecutive labels ##x_1, x_2,\ldots, x_n## satisfy
$$\sum_{i=1}^k x_i \leq k-1 \quad (k=1,\ldots,n).$$

13. (solved by @Not anonymous ) Let ##d:=d_1d_2\ldots d_9## be a number with not necessarily distinct nine decimal digits. A number ##e:=e_1e_2\ldots e_9## is such that each of the nine digit numbers formed by replacing just one of the digits ##d_j## by the corresponding digit ##e_j## is divisible by ##7## for all ##1\leq j\leq 9.## A number ##f:=f_1f_2\ldots f_9## is formed the same way by starting with ##e,## i.e. each of the nine numbers formed by replacing a ##e_k## by ##f_k## is divisible by ##7.## Example: If ##d=20210901## then ##e_6\in \{0,7\}## since ##7\,|\,20210001## and ##7\,|\,20210701 \,.##
Show that, for each ##j,## ##d_j-f_j## is divisible by ##7.##

14. An ellipse, whose semi-axes have lengths ##a## and ##b##, rolls without slipping on the curve ##y=c\sin(x/a).## How are ##a,b,c## related, given that the ellipse completes one revolution when it traverses one period of the curve?

15. (solved by @Not anonymous ) For a partition ##\pi## of ##N:=\{1,2,\ldots\, , \,9\}##, let ##\pi(x)## be the number of elements in the part containing ##x.## Prove that for any two partitions ##\pi_1## and ##\pi_2,## there are two distinct numbers ##x## and ##y## in ##N## such that ##\pi_j(x)=\pi_j(y)## for ##j=1,2.##

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Not anonymous, jbergman and berkeman

PeroK
Homework Helper
Gold Member
2020 Award
I was confused by number 15, but I think I understand it now.

martinbn
Small nitpicking: I don't agree that a topological group and a Lie group are the same thing.

Mentor
Small nitpicking: I don't agree that a topological group and a Lie group are the same thing.
Me neither. I said for example, and I hope you agree that a Lie group is indeed a topological group.

martinbn
Me neither. I said for example, and I hope you agree that a Lie group is indeed a topological group.
I see.

Chestermiller
Mentor
In question 7, did you mean log or ln?

benorin
Homework Helper
#1) "Um, I'll take what is the Bohr-Mollerup Theorem for 200 points, Alex."

I want you honest opinions here, what do you think of my new signature? Simple yet amazing truth but is it too long?

Mentor
In question 7, did you mean log or ln?
I meant ##\ln##. I just think that ##\log ## has become common if no basis is noted.

Mentor
#1) "Um, I'll take what is the Bohr-Mollerup Theorem for 200 points, Alex."

I want you honest opinions here, what do you think of my new signature? Simple yet amazing truth but is it too long?
I take Gauß for 500. (He defined it that way.)

benorin
Homework Helper
I meant ##\ln##. I just think that ##\log ## has become common if no basis is noted.
In Computer Science ##\log := \log _2##, in algebra 2 ##\log := \log _{10}##, in Analysis ##\log := \ln ##.

LCSphysicist
2020 Award
I am really puzzled about the exercise 7. I think the use of calculus can simplify a lot the demonstration of the inequalitty, but i ask myself (and of course it is more elegante if it exist) if there is a way to show it without calculus (not derivatives basically)...

Mentor
I am really puzzled about the exercise 7. I think the use of calculus can simplify a lot the demonstration of the inequalitty, but i ask myself (and of course it is more elegante if it exist) if there is a way to show it without calculus (not derivatives basically)...
I don't know. My solution uses derivatives. At least it is rather short.

TeethWhitener
Gold Member
Here's my completely non-rigorous first stab at problem 2:
Thought process: try to make the right side as small as possible and the left side as big as possible and show the inequality still holds.

(NB--I'm pretty sure we can assume WLOG that ##x_1\leq x_2\leq\dots\leq x_m##, so that's what I'll be doing)

For any given sequence ##T = (x_1,x_2,\dots,x_m)##, we have ##\min(|T_1|)=m##. Such a situation occurs, e.g., for ##T=(2,4,6,8,10)##. Here the only members of ##T_1## are the pairs where ##i=j##; that is: ##T_1=\{(x_1,x_1),(x_2,x_2),\dots,(x_m,x_m)\}##. For this case to hold, it must be true that ##\forall i \neq j, |x_i-x_j|>1##. Using the assumption above that the sequence ##T## is ordered low to high, we'll let ##|x_i-x_{i+1}|=1+\epsilon_{i}, \epsilon_{i}>0##. This means that, in this case, ##\max(|x_i-x_j|)=(m-1)(1+\max(\epsilon_i))##, because there are ##m-1## gaps between the ##m## members of the sequence, and each gap is less than or equal to ##1+\max(\epsilon_i)##.

We also have ##\max(|T_r|)=m^2##. This arises in the case where all possible pairs are present in ##T_r##. However, for the maximum case to hold, it must be the case that ##r\geq\max(|x_i-x_j|)##. Keeping with the theme of my thought process from earlier, we want to make ##r## and ##|T_1|## as small as possible. So we have ##r\geq (m-1)(1+\max(\epsilon_i))## and we choose to make ##\epsilon_i\ll 1## to get ##r## as small as possible. Now, we know that ##r## is an integer, and that ##r>m-1##, so that means ##\min(r)=m##.

If we then plug into the inequality ##\max(|T_r|)=m^2##, ##\min(r)=m##, and ##\min(|T_1|)=m##, we get
$$m^2<(2m-1)m$$
or
$$m>1$$
To finish up, when ##m=1##, we have ##|T_r|=|T_1|=1## for all ##r##, and the inequality holds trivially.

I'm not sure this constitutes a full proof, because I'm not sure if it properly excludes some intermediate values of ##|T_1|##, ##r##, and ##|T_r|## that could mess things up.

Mentor
Here's my completely non-rigorous first stab at problem 2:
Thought process: try to make the right side as small as possible and the left side as big as possible and show the inequality still holds.

(NB--I'm pretty sure we can assume WLOG that ##x_1\leq x_2\leq\dots\leq x_m##, so that's what I'll be doing)

For any given sequence ##T = (x_1,x_2,\dots,x_m)##, we have ##\min(|T_1|)=m##. Such a situation occurs, e.g., for ##T=(2,4,6,8,10)##. Here the only members of ##T_1## are the pairs where ##i=j##; that is: ##T_1=\{(x_1,x_1),(x_2,x_2),\dots,(x_m,x_m)\}##. For this case to hold, it must be true that ##\forall i \neq j, |x_i-x_j|>1##. Using the assumption above that the sequence ##T## is ordered low to high, we'll let ##|x_i-x_{i+1}|=1+\epsilon_{i}, \epsilon_{i}>0##. This means that, in this case, ##\max(|x_i-x_j|)=(m-1)(1+\max(\epsilon_i))##, because there are ##m-1## gaps between the ##m## members of the sequence, and each gap is less than or equal to ##1+\max(\epsilon_i)##.

We also have ##\max(|T_r|)=m^2##. This arises in the case where all possible pairs are present in ##T_r##. However, for the maximum case to hold, it must be the case that ##r\geq\max(|x_i-x_j|)##. Keeping with the theme of my thought process from earlier, we want to make ##r## and ##|T_1|## as small as possible. So we have ##r\geq (m-1)(1+\max(\epsilon_i))## and we choose to make ##\epsilon_i\ll 1## to get ##r## as small as possible. Now, we know that ##r## is an integer, and that ##r>m-1##, so that means ##\min(r)=m##.

If we then plug into the inequality ##\max(|T_r|)=m^2##, ##\min(r)=m##, and ##\min(|T_1|)=m##, we get
$$m^2<(2m-1)m$$
or
$$m>1$$
To finish up, when ##m=1##, we have ##|T_r|=|T_1|=1## for all ##r##, and the inequality holds trivially.

I'm not sure this constitutes a full proof, because I'm not sure if it properly excludes some intermediate values of ##|T_1|##, ##r##, and ##|T_r|## that could mess things up.
Looks ok to me. My solution uses induction on ##m## and intervals ##[x_i-1,x_i+1].##

TeethWhitener
TeethWhitener
Gold Member
For problem 9 part 1, I think you meant ##\displaystyle{\operatorname{ord}_p(n!)=\sum_{k\geq 1}\left\lfloor \dfrac{n!}{p^k}\right\rfloor}##? If so, I think I have a bead on parts 1 and 2--maybe.
##\displaystyle{\operatorname{ord}}_p(N)## counts the number of times a prime appears in the divisor list of ##\{1,2,\dots,N\}## along with the multiplicity. But let's consider a different operator ##\overline{\displaystyle{\operatorname{ord}}}_m(N)## that is the same as ##\displaystyle{\operatorname{ord}}_p(N)## except it doesn't count multiplicity, and ##m\in\mathbb{N}## doesn't have to be prime. If we look, e.g., at ##\overline{\displaystyle{\operatorname{ord}}}_2(n!)##, we add 1 every time we pass an even number in ##\{1,2,\dots,n!\}##, which means ##\overline{\displaystyle{\operatorname{ord}}}_2(n!)=\left\lfloor\frac{n!}{2}\right\rfloor##. Similarly, ##\overline{\displaystyle{\operatorname{ord}}}_3(n!)=\left\lfloor\frac{n!}{3}\right\rfloor##, ##\overline{\displaystyle{\operatorname{ord}}}_4(n!)=\left\lfloor\frac{n!}{4}\right\rfloor##, etc., and generally
$$\overline{\displaystyle{\operatorname{ord}}}_m(n!)=\left\lfloor\frac{n!}{m}\right\rfloor$$
To go from ##\overline{\displaystyle{\operatorname{ord}}}_m(n!)## to ##\displaystyle{\operatorname{ord}}_p(n!)##, we note that
$$\displaystyle{\operatorname{ord}}_p(n!)=\sum_{k\geq1}{\overline{\displaystyle{\operatorname{ord}}}_{p^k}(n!)}=\sum_{k\geq 1}{\left\lfloor \dfrac{n!}{p^k}\right\rfloor}$$
##p|\binom{2n}{n}## is really easy. Since ##p>n##, there is no ##p## in the denominator of ##\binom{2n}{n}##. But since ##p\leq 2n##, there is a ##p## in the numerator. So ##p|\binom{2n}{n}##.

##2|\binom{2n}{n}## is trickier. I think the point is to use the result from part 1, but I'm pretty sure it's easier via induction. Denoting ##n!=2^xa## and ##(2n)!=2^yb##, where ##2\nmid a## and ##2\nmid b##, we have that
$$\binom{2n}{n}=\frac{(2n)!}{n!n!}=\frac{2^yb}{2^{2x}a}$$
(This is where I start to lose rigor and question my approach.)
We can map each even number in the set ##\{1,2,\dots,n\}## to its double in the set ##\{1,2,\dots,2n\}##. We can map each multiple of 4 in the original set to its double in the larger set, and do the same for each multiple of 8, etc., or ##2^k## generally. This mapping lets us write ##y\geq (x+\left\lfloor\frac{n}{2}\right\rfloor)##. But this mapping isn’t surjective: we haven’t accounted for the factors of 2 in ##(2n)!## of the form ##4j+2##. When we do that, we get that ##y\geq (x+n)##.

To show that ##\binom{2n}{n}## is divisible by 2, we have to show that ##y>2x##, or equivalently, ##x<n##. Edit: the following is all incorrect. We do this by induction by proving: ##n!<2^na, \forall a\in\mathbb{N}##.
Base step:
##1!=1<2a, \forall a\in\mathbb{N}##
Inductive step:

Let ##n!<2^na, \forall a\in\mathbb{N}##. Then ##(n+1)!=(n+1)n!<(n+1)2^na##. Since ##(n+1)2^n<2^{n+1}##, we have ##(n+1)!<2^{n+1}a##.
Edit: crap, ignore. Found an error in problem 9 part 2. May work on it later.

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Mentor
For problem 9 part 1, I think you meant ##\displaystyle{\operatorname{ord}_p(n!)=\sum_{k\geq 1}\left\lfloor \dfrac{n!}{p^k}\right\rfloor}##? If so, I think I have a bead on parts 1 and 2--maybe.
Uhm, no. Consider my example: ##10=8+2+0##. ##24!## has many, real many more divisors ##3^k.##
Edit: crap, ignore. Found an error in problem 9 part 2. May work on it later.

TeethWhitener
Gold Member
Uhm, no. Consider my example: ##10=8+2+0##. ##24!## has many, real many more divisors ##3^k.##
##\displaystyle{\operatorname{ord}_3(4!)=\sum_{k\geq 1}\left\lfloor \dfrac{4}{3^k}\right\rfloor}=1##? I think it has to be factorial, or else I'm really missing something.

Mentor
##\displaystyle{\operatorname{ord}_3(4!)=\sum_{k\geq 1}\left\lfloor \dfrac{4}{3^k}\right\rfloor}=1##? I think it has to be factorial, or else I'm really missing something.
Oops, you're right. I confused ##4!=24## with ##24!##, sorry. I will correct the OP.

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TeethWhitener
TeethWhitener
Gold Member
Thanks for the clarification. I still think my problem 9 part 1 is right. I think half of problem 9 part 2 is right too, but I'll have to work on the other half (I've struck through the wrong part in post 15).

Mentor
Thanks for the clarification. I still think my problem 9 part 1 is right. I think half of problem 9 part 2 is right too, but I'll have to work on the other half (I've struck through the wrong part in post 15).
It would be nice if you wrote it again for ##n## instead of the faculty, and if possible with ##n## and ##p## fixed. It's actually quite easy, which is why I gathered so many parts.

12. Suppose we have a necklace of beads. Each bead is labeled with an integer and the sum of all these labels is ##n-1##. Prove that we can cut the necklace to form a string whose consecutive labels ##x_1, x_2, \ldots, x_n## satisfy $$\sum_{i=1}^k x_i \leq k-1 \, \, (k=1,...,n).$$

Does "cut the necklace to form a string" mean any rearrangement of the individual beads is allowed, or does it only mean making at most one cut anywhere in the necklace and swapping the 2 resulting parts to form a new sequence (meaning, for e.g., (1, 2, 3, 4, 5) →(3, 4, 5, 1, 2) is allowed but not (5, 2, 1, 4, 3))? If any reordering of beads is allowed, here is my attempted solution.

Let ##A = (a_1, a_2, ..., a_n)## be the original sequence of integer-valued labels of the necklace's beads. We create a new sequence ##X = (x_1, x_2, ..., x_n)## using only values from ##A## (also, if some value is repeated in ##A##, that value will be repeated the same number of times in the new sequence too, but the order may differ, i.e. cardinality of values is retained) but arranged in ascending (more precisely, non-decreasing) order, i.e. ##x_i \leq x_{i+1} \, \forall i \in \{1,...,n-1\}##.

Define ##S_k = \sum_{i=1}^k x_k, \, k=(1,...,n)##. Since ##X## contains the same values as ##A## and with same cardinality, the sums of all values in ##X## must be the same as that for ##A##. Hence ##S_n = \sum_{i=1}^k a_k = n-1##. ∴ ##S_k## meets the condition ##S_k \leq k-1## when ##k=n##, and this becomes the initial case for induction.

Inductive step: We now show that if ##S_k \leq k-1## for ##k=j## where integer ##j > 1##, then it must be true for ##k=j-1## too.

Proof by contradiction: Suppose ##S_{j-1} > j-1##. Since ##S_j = S_{j-1} + x_j##, ##S_{j-1} > j-1## implies that ##x_j = S_j - S_{j-1} < (j-1) - (j-1) \Rightarrow x_j < 0##. (C1)

But if ##S_{j-1} > (j-1)## as assumed, it follows that ##S_{j-1}## is a positive integer (since ##j > 1##) and therefore there must be at least one positive integer in ##(x_1, ..., x_{j-1})## since ##S_{j-1}## is the sum of these values by definition. (C2)

Conditions (C1) and (C2) cause a contradiction since ##x_j < 0## and ##(x_1, ..., x_{j-1})## containing a positive value imply that the values in ##(x_1, ..., x_{j-1}, x_{j})## do not follow a non-decreasing order as required by the definition of the sequence ##X##. Since this contradiction eventually arises from the assumption that ##S_{j-1} > j-1##, it must be the case that this assumption is incorrect and we must have ##S_{j-1} \leq j-1## instead. Hence the inductive step is proved, and by the method of induction, it follows that $$S_k = \sum_{i=1}^k x_i \leq k-1 \, \, \forall k \in \{1,..., n\}$$. Hence, the beads can be rearranged as required.

Mentor
Does "cut the necklace to form a string" mean any rearrangement of the individual beads is allowed, or does it only mean making at most one cut anywhere in the necklace and swapping the 2 resulting parts to form a new sequence (meaning, for e.g., (1, 2, 3, 4, 5) →(3, 4, 5, 1, 2) is allowed but not (5, 2, 1, 4, 3))? If any reordering of beads is allowed, here is my attempted solution.
Cut means one cut, i.e. we make a string out of the circle and can so determine ##x_n##. Order and given labels remain unchanged.

TeethWhitener
Gold Member
Here it is rewritten without the factorial. I wasn't quite sure what you meant by n and p fixed.
##\displaystyle{\operatorname{ord}}_p(N)## counts the number of times a prime appears in the divisor list of ##\{1,2,\dots,N\}## along with the multiplicity. But let's consider a different operator ##\overline{\displaystyle{\operatorname{ord}}}_m(N)## that is the same as ##\displaystyle{\operatorname{ord}}_p(N)## except it doesn't count multiplicity, and ##m\in\mathbb{N}## doesn't have to be prime. If we look, e.g., at ##\overline{\displaystyle{\operatorname{ord}}}_2(n)##, we add 1 every time we pass an even number in ##\{1,2,\dots,n\}##, which means ##\overline{\displaystyle{\operatorname{ord}}}_2(n)=\left\lfloor\frac{n}{2}\right\rfloor##. Similarly, ##\overline{\displaystyle{\operatorname{ord}}}_3(n)=\left\lfloor\frac{n}{3}\right\rfloor##, ##\overline{\displaystyle{\operatorname{ord}}}_4(n)=\left\lfloor\frac{n}{4}\right\rfloor##, etc., and generally
$$\overline{\displaystyle{\operatorname{ord}}}_m(n)=\left\lfloor\frac{n}{m}\right\rfloor$$
To take into account multiplicity and go from ##\overline{\displaystyle{\operatorname{ord}}}_m(n)## to ##\displaystyle{\operatorname{ord}}_p(n)##, we note that
$$\displaystyle{\operatorname{ord}}_p(n)=\sum_{k\geq1}{\overline{\displaystyle{\operatorname{ord}}}_{p^k}(n)}=\sum_{k\geq 1}{\left\lfloor \dfrac{n}{p^k}\right\rfloor}$$
I also found another error in 9-2, but I haven't figured out how to complete the problem yet. I'll think on it some more.

Mentor
Here it is rewritten without the factorial. I wasn't quite sure what you meant by n and p fixed.
##\displaystyle{\operatorname{ord}}_p(N)## counts the number of times a prime appears in the divisor list of ##\{1,2,\dots,N\}## along with the multiplicity. But let's consider a different operator ##\overline{\displaystyle{\operatorname{ord}}}_m(N)## that is the same as ##\displaystyle{\operatorname{ord}}_p(N)## except it doesn't count multiplicity, and ##m\in\mathbb{N}## doesn't have to be prime. If we look, e.g., at ##\overline{\displaystyle{\operatorname{ord}}}_2(n)##, we add 1 every time we pass an even number in ##\{1,2,\dots,n\}##, which means ##\overline{\displaystyle{\operatorname{ord}}}_2(n)=\left\lfloor\frac{n}{2}\right\rfloor##. Similarly, ##\overline{\displaystyle{\operatorname{ord}}}_3(n)=\left\lfloor\frac{n}{3}\right\rfloor##, ##\overline{\displaystyle{\operatorname{ord}}}_4(n)=\left\lfloor\frac{n}{4}\right\rfloor##, etc., and generally
$$\overline{\displaystyle{\operatorname{ord}}}_m(n)=\left\lfloor\frac{n}{m}\right\rfloor$$
To take into account multiplicity and go from ##\overline{\displaystyle{\operatorname{ord}}}_m(n)## to ##\displaystyle{\operatorname{ord}}_p(n)##, we note that
$$\displaystyle{\operatorname{ord}}_p(n)=\sum_{k\geq1}{\overline{\displaystyle{\operatorname{ord}}}_{p^k}(n)}=\sum_{k\geq 1}{\left\lfloor \dfrac{n}{p^k}\right\rfloor}$$
I also found another error in 9-2, but I haven't figured out how to complete the problem yet. I'll think on it some more.
Yes, but you had not to define ##\overline{ord}##. The same argument at the end works with ##\left\lfloor \dfrac{n}{p}\right\rfloor\, , \,\left\lfloor \dfrac{n}{p^2}\right\rfloor\, , \,\left\lfloor \dfrac{n}{p^3}\right\rfloor,\ldots## from the start.

TeethWhitener
TeethWhitener
To show that ##2\,|\binom{2n}{n}##, we proceed via induction, which we should’ve just done from the beginning. We’re trying to establish that $$\frac{(2n)!}{n!n!}$$ is divisible by 2.
$$\frac{2!}{1!1!}=2$$
Let ##\frac{(2n)!}{n!n!}## be divisible by 2. Then consider $$\frac{(2n+2)!}{(n+1)!(n+1)!}=\frac{(2n+2)(2n+1)}{(n+1)(n+1)}\frac{(2n)!}{n!n!}$$ We have two cases: