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Number theory Difficulty 7.9 National olympiad, round 2 Prove it Asia Pacific Mathematics Olympiad (APMO)

Prove that for every positive integer tt there is a unique permutation a0,a1,,at1a_{0}, a_{1}, \ldots, a_{t-1} of 0,1,,t10,1, \ldots, t-1 such that, for every 0it10 \leq i \leq t-1, the binomial coefficient (t+i2ai)\binom{t+i}{2 a_{i}} is odd and 2ait+i2 a_{i} \neq t+i.

Solution

We constantly make use of Kummer's theorem which, in particular, implies that (nk)\binom{n}{k} is odd if and only if kk and nkn-k have ones in different positions in binary. In other words, if S(x)S(x) is the set of positions of the digits 1 of xx in binary (in which the digit multiplied by 2i2^{i} is in position i)i), (nk)\binom{n}{k} is odd if and only if S(k)S(n)S(k) \subseteq S(n). Moreover, if we set k<n,S(k)k<n, S(k) is a proper subset of S(n)S(n), that is, S(k)<S(n)|S(k)|<|S(n)|.
We start with a lemma that guides us how the permutation should be set.

Lemma 1.
i=0t1S(t+i)=t+i=0t1S(2i). \sum_{i=0}^{t-1}|S(t+i)|=t+\sum_{i=0}^{t-1}|S(2 i)| .
The proof is just realizing that S(2i)={1+x,xS(i)}S(2 i)=\{1+x, x \in S(i)\} and S(2i+1)={0}{1+x,xS(i)}S(2 i+1)=\{0\} \cup\{1+x, x \in S(i)\}, because 2i2 i in binary is ii followed by a zero and 2i+12 i+1 in binary is ii followed by a one. Therefore
i=0t1S(t+i)=i=02t1S(i)i=0t1S(i)=i=0t1S(2i)+i=0t1S(2i+1)i=0t1S(i)=i=0t1S(i)+i=0t1(1+S(i))i=0t1S(i)=t+i=0t1S(i)=t+i=0t1S(2i). \begin{aligned} \sum_{i=0}^{t-1}|S(t+i)| & =\sum_{i=0}^{2 t-1}|S(i)|-\sum_{i=0}^{t-1}|S(i)|=\sum_{i=0}^{t-1}|S(2 i)|+\sum_{i=0}^{t-1}|S(2 i+1)|-\sum_{i=0}^{t-1}|S(i)| \\ & =\sum_{i=0}^{t-1}|S(i)|+\sum_{i=0}^{t-1}(1+|S(i)|)-\sum_{i=0}^{t-1}|S(i)|=t+\sum_{i=0}^{t-1}|S(i)|=t+\sum_{i=0}^{t-1}|S(2 i)| . \end{aligned}
The lemma has an immediate corollary: since t+i>2ait+i>2 a_{i} and (t+i2ai)\binom{t+i}{2 a_{i}} is odd for all i,0it1i, 0 \leq i \leq t-1, S(2ai)S(t+i)S\left(2 a_{i}\right) \subset S(t+i) with S(2ai)S(t+i)1\left|S\left(2 a_{i}\right)\right| \leq|S(t+i)|-1. Since the sum of S(2ai)\left|S\left(2 a_{i}\right)\right| is tt less than the sum of S(t+i)|S(t+i)|, and there are tt values of ii, equality must occur, that is, S(2ai)=S(t+i)1\left|S\left(2 a_{i}\right)\right|=|S(t+i)|-1, which in conjunction with S(2ai)S(t+i)S\left(2 a_{i}\right) \subset S(t+i) means that t+i2ai=2kit+i-2 a_{i}=2^{k_{i}} for every i,0it1i, 0 \leq i \leq t-1, kiS(t+i)k_{i} \in S(t+i) (more precisely, {ki}=S(t+i)\S(2ai)\left\{k_{i}\right\}=S(t+i) \backslash S\left(2 a_{i}\right).)
In particular, for t+it+i odd, this means that t+i2ai=1t+i-2 a_{i}=1, because the only odd power of 2 is 1. Then ai=t+i12a_{i}=\frac{t+i-1}{2} for t+it+i odd, which takes up all the numbers greater than or equal to t12\frac{t-1}{2}. Now we need to distribute the numbers that are smaller than t12\frac{t-1}{2} (call these numbers small). If t+it+i is even then by Lucas' Theorem (t+i2ai)(t+iai)(mod2)\binom{t+i}{2 a_{i}} \equiv\left(\frac{t+i}{a_{i}}\right)(\bmod 2), so we pair numbers from t/2\lceil t / 2\rceil to t1t-1 (call these numbers big) with the small numbers.
Say that a set AA is paired with another set BB whenever A=B|A|=|B| and there exists a bijection π:AB\pi: A \rightarrow B such that S(a)S(π(a))S(a) \subset S(\pi(a)) and S(a)=S(π(a))1|S(a)|=|S(\pi(a))|-1; we also say that aa and π(a)\pi(a) are paired. We prove by induction in tt that At={0,1,2,,t/21}A_{t}=\{0,1,2, \ldots,\lfloor t / 2\rfloor-1\} (the set of small numbers) and Bt={t/2,,t2,t1}B_{t}=\{\lceil t / 2\rceil, \ldots, t-2, t-1\} (the set of big numbers) can be uniquely paired.
The claim is immediate for t=1t=1 and t=2t=2. For t>2t>2, there is exactly one power of two in BtB_{t}, since t/22a<ta=log2(t/2)t / 2 \leq 2^{a}<t \Longleftrightarrow a=\left\lceil\log _{2}(t / 2)\right\rceil. Let 2a2^{a} be this power of two. Then, since 2at/22^{a} \geq t / 2, no number in AtA_{t} has a one in position aa in binary. Since for every number x,2ax<t,aS(x)x, 2^{a} \leq x<t, a \in S(x) and aS(y)a \notin S(y) for all yAt,xy \in A_{t}, x can only be paired with x2ax-2^{a}, since S(x)S(x) needs to be stripped of exactly one position. This takes cares of xBt,2ax<tx \in B_{t}, 2^{a} \leq x<t, and yAt,0y<t2ay \in A_{t}, 0 \leq y<t-2^{a}.
Now we need to pair the numbers from A={t2a,t2a+1,,t/21}AA^{\prime}=\left\{t-2^{a}, t-2^{a}+1, \ldots,\lfloor t / 2\rfloor-1\right\} \subset A with the numbers from B={t/2,t/2+1,,2a1}BB^{\prime}=\left\{\lceil t / 2\rceil,\lceil t / 2\rceil+1, \ldots, 2^{a}-1\right\} \subset B. In order to pair these t2(t2a)=2a+1t<tt-2\left(t-2^{a}\right)= 2^{a+1}-t<t numbers, we use the induction hypothesis and a bijection between ABA^{\prime} \cup B^{\prime} and B2a+1tA2a+1tB_{2^{a+1}-t} \cup A_{2^{a+1}-t}. Let S=S(2a1)={0,1,2,,a1}S=S\left(2^{a}-1\right)=\{0,1,2, \ldots, a-1\}. Then take a pair x,y,xA2a+1tx, y, x \in A_{2^{a+1}-t} and yB2a+1ty \in B_{2^{a+1}-t} and biject it with 2a1xB2^{a}-1-x \in B^{\prime} and 2a1yA2^{a}-1-y \in A^{\prime}. In fact,
0x2a+1t21=2at21t22a1x2a1 0 \leq x \leq\left\lfloor\frac{2^{a+1}-t}{2}\right\rfloor-1=2^{a}-\left\lceil\frac{t}{2}\right\rceil-1 \Longleftrightarrow\left\lceil\frac{t}{2}\right\rceil \leq 2^{a}-1-x \leq 2^{a}-1
and
2a+1t2=2at2y2a+1t1t2a2a1yt21. \left\lceil\frac{2^{a+1}-t}{2}\right\rceil=2^{a}-\left\lfloor\frac{t}{2}\right\rfloor \leq y \leq 2^{a+1}-t-1 \Longleftrightarrow t-2^{a} \leq 2^{a}-1-y \leq\left\lfloor\frac{t}{2}\right\rfloor-1 .
Moreover, S(2a1x)=S\S(x)S\left(2^{a}-1-x\right)=S \backslash S(x) and S(2a1y)=S\S(y)S\left(2^{a}-1-y\right)=S \backslash S(y) are complements with respect to SS, and S(x)S(y)S(x) \subset S(y) and S(x)=S(y)1|S(x)|=|S(y)|-1 implies S(2a1y)S(2a1x)S\left(2^{a}-1-y\right) \subset S\left(2^{a}-1-x\right) and S(2a1y)=S(2a1x)1\left|S\left(2^{a}-1-y\right)\right|=\left|S\left(2^{a}-1-x\right)\right|-1. Therefore a pairing between AA^{\prime} and BB^{\prime} corresponds to a pairing between A2a+1tA_{2^{a+1}-t} and B2a+1tB_{2^{a+1}-t}. Since the latter pairing is unique, the former pairing is also unique, and the result follows.
We illustrate the bijection by showing the case t=23t=23 :
A23={0,1,2,,10},B23={12,13,14,,22}. A_{23}=\{0,1,2, \ldots, 10\}, \quad B_{23}=\{12,13,14, \ldots, 22\} .
The pairing is
(1213141516171819202122891070123456), \left(\begin{array}{ccccccccccc} 12 & 13 & 14 & 15 & 16 & 17 & 18 & 19 & 20 & 21 & 22 \\ 8 & 9 & 10 & 7 & 0 & 1 & 2 & 3 & 4 & 5 & 6 \end{array}\right),
in which the bijection is between
(1213141589107) and (32107658)(56781230). \left(\begin{array}{cccc} 12 & 13 & 14 & 15 \\ 8 & 9 & 10 & 7 \end{array}\right) \text{ and }\left(\begin{array}{llll} 3 & 2 & 1 & 0 \\ 7 & 6 & 5 & 8 \end{array}\right) \rightarrow\left(\begin{array}{llll} 5 & 6 & 7 & 8 \\ 1 & 2 & 3 & 0 \end{array}\right) .

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.