Maths Olympiad Prep

Library / /500 of 520

Number theory Difficulty 7.6 National olympiad, round 2 Prove it

For an integer mm, denote by t(m)t(m) the unique number in {1,2,3}\{1,2,3\} such that m+t(m)m+t(m) is a multiple of 3. A function f:ZZf: \mathbb{Z} \rightarrow \mathbb{Z} satisfies f(1)=0,f(0)=1,f(1)=1f(-1)=0, f(0)=1, f(1)=-1 and
f(2n+m)=f(2nt(m))f(m) for all integers m,n0 with 2n>m f\left(2^{n}+m\right)=f\left(2^{n}-t(m)\right)-f(m) \quad \text { for all integers } m, n \geq 0 \text { with } 2^{n}>m \text {. }
Prove that f(3p)0f(3 p) \geq 0 holds for all integers p0p \geq 0.

Solution

The given conditions determine ff uniquely on the positive integers. The signs of f(1),f(2),f(1), f(2), \ldots seem to change quite erratically. However values of the form f(2nt(m))f\left(2^{n}-t(m)\right) are sufficient to compute directly any functional value. Indeed, let n>0n>0 have base 2 representation n=2a0+2a1++2ak,a0>a1>>ak0n=2^{a_{0}}+2^{a_{1}}+\cdots+2^{a_{k}}, a_{0}>a_{1}>\cdots>a_{k} \geq 0, and let nj=2aj+2aj1++2ak,j=0,,kn_{j}=2^{a_{j}}+2^{a_{j-1}}+\cdots+2^{a_{k}}, j=0, \ldots, k. Repeated applications of the recurrence show that f(n)f(n) is an alternating sum of the quantities f(2ajt(nj+1))f\left(2^{a_{j}}-t\left(n_{j+1}\right)\right) plus (1)k+1(-1)^{k+1}. (The exact formula is not needed for our proof.) So we focus attention on the values f(2n1),f(2n2)f\left(2^{n}-1\right), f\left(2^{n}-2\right) and f(2n3)f\left(2^{n}-3\right). Six cases arise; more specifically, t(22k3)=2,t(22k2)=1,t(22k1)=3,t(22k+13)=1,t(22k+12)=3,t(22k+11)=2t\left(2^{2 k}-3\right)=2, t\left(2^{2 k}-2\right)=1, t\left(2^{2 k}-1\right)=3, t\left(2^{2 k+1}-3\right)=1, t\left(2^{2 k+1}-2\right)=3, t\left(2^{2 k+1}-1\right)=2.

Claim. For all integers k0k \geq 0 the following equalities hold:
f(22k+13)=0,f(22k+12)=3k,f(22k+11)=3k,f(22k+23)=3k,f(22k+22)=3k,f(22k+21)=23k. \begin{array}{lll} f\left(2^{2 k+1}-3\right)=0, & f\left(2^{2 k+1}-2\right)=3^{k}, & f\left(2^{2 k+1}-1\right)=-3^{k}, \\ f\left(2^{2 k+2}-3\right)=-3^{k}, & f\left(2^{2 k+2}-2\right)=-3^{k}, & f\left(2^{2 k+2}-1\right)=2 \cdot 3^{k} . \end{array}

Proof. By induction on kk. The base k=0k=0 comes down to checking that f(2)=1f(2)=-1 and f(3)=2f(3)=2; the given values f(1)=0,f(0)=1,f(1)=1f(-1)=0, f(0)=1, f(1)=-1 are also needed. Suppose the claim holds for k1k-1. For f(22k+1t(m))f\left(2^{2 k+1}-t(m)\right), the recurrence formula and the induction hypothesis yield
f(22k+13)=f(22k+(22k3))=f(22k2)f(22k3)=3k1+3k1=0,f(22k+12)=f(22k+(22k2))=f(22k1)f(22k2)=23k1+3k1=3k,f(22k+11)=f(22k+(22k1))=f(22k3)f(22k1)=3k123k1=3k. \begin{aligned} & f\left(2^{2 k+1}-3\right)=f\left(2^{2 k}+\left(2^{2 k}-3\right)\right)=f\left(2^{2 k}-2\right)-f\left(2^{2 k}-3\right)=-3^{k-1}+3^{k-1}=0, \\ & f\left(2^{2 k+1}-2\right)=f\left(2^{2 k}+\left(2^{2 k}-2\right)\right)=f\left(2^{2 k}-1\right)-f\left(2^{2 k}-2\right)=2 \cdot 3^{k-1}+3^{k-1}=3^{k}, \\ & f\left(2^{2 k+1}-1\right)=f\left(2^{2 k}+\left(2^{2 k}-1\right)\right)=f\left(2^{2 k}-3\right)-f\left(2^{2 k}-1\right)=-3^{k-1}-2 \cdot 3^{k-1}=-3^{k} . \end{aligned}

For f(22k+2t(m))f\left(2^{2 k+2}-t(m)\right) we use the three equalities just established:
f(22k+23)=f(22k+1+(22k+13))=f(22k+11)f(22k+13)=3k0=3k,f(22k+22)=f(22k+1+(22k+12))=f(22k+13)f(22k2)=03k=3k,f(22k+21)=f(22k+1+(22k+11))=f(22k+12)f(22k+11)=3k+3k=23k. \begin{aligned} & f\left(2^{2 k+2}-3\right)=f\left(2^{2 k+1}+\left(2^{2 k+1}-3\right)\right)=f\left(2^{2 k+1}-1\right)-f\left(2^{2 k+1}-3\right)=-3^{k}-0=-3^{k}, \\ & f\left(2^{2 k+2}-2\right)=f\left(2^{2 k+1}+\left(2^{2 k+1}-2\right)\right)=f\left(2^{2 k+1}-3\right)-f\left(2^{2 k}-2\right)=0-3^{k}=-3^{k}, \\ & f\left(2^{2 k+2}-1\right)=f\left(2^{2 k+1}+\left(2^{2 k+1}-1\right)\right)=f\left(2^{2 k+1}-2\right)-f\left(2^{2 k+1}-1\right)=3^{k}+3^{k}=2 \cdot 3^{k} . \end{aligned}

The claim follows. A closer look at the six cases shows that f(2nt(m))3(n1)/2f\left(2^{n}-t(m)\right) \geq 3^{(n-1) / 2} if 2nt(m)2^{n}-t(m) is divisible by 3, and f(2nt(m))0f\left(2^{n}-t(m)\right) \leq 0 otherwise. On the other hand, note that 2nt(m)2^{n}-t(m) is divisible by 3 if and only if 2n+m2^{n}+m is. Therefore, for all nonnegative integers mm and nn,
(i) f(2nt(m))3(n1)/2f\left(2^{n}-t(m)\right) \geq 3^{(n-1) / 2} if 2n+m2^{n}+m is divisible by 3;
(ii) f(2nt(m))0f\left(2^{n}-t(m)\right) \leq 0 if 2n+m2^{n}+m is not divisible by 3.

One more (direct) consequence of the claim is that f(2nt(m))233n/2\left|f\left(2^{n}-t(m)\right)\right| \leq \frac{2}{3} \cdot 3^{n / 2} for all m,n0m, n \geq 0. The last inequality enables us to find an upper bound for f(m)|f(m)| for mm less than a given power of 2. We prove by induction on nn that f(m)3n/2|f(m)| \leq 3^{n / 2} holds true for all integers m,n0m, n \geq 0 with 2n>m2^{n}>m.

The base n=0n=0 is clear as f(0)=1f(0)=1. For the inductive step from nn to n+1n+1, let mm and nn satisfy 2n+1>m2^{n+1}>m. If m2nm \leq 2^{n}, the inductive assumption gives f(m)3n/2|f(m)| \leq 3^{n / 2}. If m>2nm > 2^{n}, write m=2n+km=2^{n}+k with 2n>k02^{n}>k \geq 0. Now, by f(2nt(k))233n/2\left|f\left(2^{n}-t(k)\right)\right| \leq \frac{2}{3} \cdot 3^{n / 2} and the inductive assumption,
f(m)=f(2nt(k))f(k)f(2nt(k))+f(k)233n/2+3n/2 |f(m)|=\left|f\left(2^{n}-t(k)\right)-f(k)\right| \leq\left|f\left(2^{n}-t(k)\right)\right|+|f(k)| \leq \frac{2}{3} \cdot 3^{n / 2}+3^{n / 2}

Finally, let p=2a+2b+cp=2^{a}+2^{b}+c with a>b0a>b \geq 0 and 2b>c02^{b}>c \geq 0. Applying the recurrence formula twice yields
f(3p)=f(2a+2b+c)=f(2at(2b+c))f(2bt(c))+f(c). f(3 p)=f\left(2^{a}+2^{b}+c\right)=f\left(2^{a}-t\left(2^{b}+c\right)\right)-f\left(2^{b}-t(c)\right)+f(c) .

Since 2a+2b+c2^{a}+2^{b}+c is divisible by 3, we have f(2at(2b+c))3(a1)/2f\left(2^{a}-t\left(2^{b}+c\right)\right) \geq 3^{(a-1) / 2} by (i). Since 2b+c2^{b}+c is not divisible by 3, we have f(2bt(c))0f\left(2^{b}-t(c)\right) \leq 0 by (ii). Finally f(c)3b/2|f(c)| \leq 3^{b / 2} as 2b>c02^{b}>c \geq 0, so that f(c)3b/2f(c) \geq-3^{b / 2}. Therefore f(3p)3(a1)/23b/2f(3 p) \geq 3^{(a-1) / 2}-3^{b / 2} which is nonnegative because a>ba>b.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.