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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 33. A function f:ZZf: \mathbb{Z} \rightarrow \mathbb{Z} satisfies f(1)=0f(-1)=0, f(0)=1f(0)=1, f(1)=1f(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.

Prove that f(3p)0f(3p) \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 22 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)=2t\left(2^{2k}-3\right)=2, t(22k2)=1t\left(2^{2k}-2\right)=1, t(22k1)=3t\left(2^{2k}-1\right)=3, t(22k+13)=1t\left(2^{2k+1}-3\right)=1, t(22k+12)=3t\left(2^{2k+1}-2\right)=3, t(22k+11)=2t\left(2^{2k+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^{2k+1}-3\right)=0, & f\left(2^{2k+1}-2\right)=3^{k}, & f\left(2^{2k+1}-1\right)=-3^{k}, \\ f\left(2^{2k+2}-3\right)=-3^{k}, & f\left(2^{2k+2}-2\right)=-3^{k}, & f\left(2^{2k+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^{2k+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^{2k+1}-3\right)=f\left(2^{2k}+(2^{2k}-3)\right)=f\left(2^{2k}-2\right)-f\left(2^{2k}-3\right)=-3^{k-1}+3^{k-1}=0, \\ & f\left(2^{2k+1}-2\right)=f\left(2^{2k}+(2^{2k}-2)\right)=f\left(2^{2k}-1\right)-f\left(2^{2k}-2\right)=2 \cdot 3^{k-1}+3^{k-1}=3^{k}, \\ & f\left(2^{2k+1}-1\right)=f\left(2^{2k}+(2^{2k}-1)\right)=f\left(2^{2k}-3\right)-f\left(2^{2k}-1\right)=-3^{k-1}-2 \cdot 3^{k-1}=-3^{k} . \end{aligned}
For f(22k+2t(m))f\left(2^{2k+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^{2k+2}-3\right)=f\left(2^{2k+1}+(2^{2k+1}-3)\right)=f\left(2^{2k+1}-1\right)-f\left(2^{2k+1}-3\right)=-3^{k}-0=-3^{k}, \\ & f\left(2^{2k+2}-2\right)=f\left(2^{2k+1}+(2^{2k+1}-2)\right)=f\left(2^{2k+1}-3\right)-f\left(2^{2k}-2\right)=0-3^{k}=-3^{k}, \\ & f\left(2^{2k+2}-1\right)=f\left(2^{2k+1}+(2^{2k+1}-1)\right)=f\left(2^{2k+1}-2\right)-f\left(2^{2k+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 33, 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 33 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 33 ;
(ii) f(2nt(m))0f\left(2^{n}-t(m)\right) \leq 0 if 2n+m2^{n}+m is not divisible by 33 .

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 22. 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 m<2nm<2^{n}, we are done by the inductive hypothesis. If m2nm \geq 2^{n} then m=2n+km=2^{n}+k where 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<3(n+1)/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}<3^{(n+1)/2}
The induction is complete.

We proceed to prove that f(3p)0f(3p) \geq 0 for all integers p0p \geq 0. Since 3p3p is not a power of 22, its binary expansion contains at least two summands. Hence one can write 3p=2a+2b+c3p=2^{a}+2^{b}+c where a>ba>b 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(3p)=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 33, 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 33, 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(3p) \geq 3^{(a-1)/2}-3^{b/2} which is nonnegative because a>ba>b.

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