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Algebra Difficulty 7.5 National olympiad, round 2 Find the answer

Determine all sequences a0,a1,a2,a_{0}, a_{1}, a_{2}, \ldots of positive integers with a02015a_{0} \geq 2015 such that for all integers n1n \geq 1 : (i) an+2a_{n+2} is divisible by ana_{n}; (ii) sn+1(n+1)an=1\left|s_{n+1}-(n+1) a_{n}\right|=1, where sn+1=an+1an+an1+(1)n+1a0s_{n+1}=a_{n+1}-a_{n}+a_{n-1}-\cdots+(-1)^{n+1} a_{0}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

There are two families of answers: (a) an=c(n+2)na_{n}=c(n+2) n ! for all n1n \geq 1 and a0=c+1a_{0}=c+1 for some integer c2014c \geq 2014, and (b) an=c(n+2)na_{n}=c(n+2) n ! for all n1n \geq 1 and a0=c1a_{0}=c-1 for some integer c2016c \geq 2016. Let {an}n=0\left\{a_{n}\right\}_{n=0}^{\infty} be a sequence of positive integers satisfying the given conditions. We can rewrite (ii) as sn+1=(n+1)an+hns_{n+1}=(n+1) a_{n}+h_{n}, where hn{1,1}h_{n} \in\{-1,1\}. Substituting nn with n1n-1 yields sn=nan1+hn1s_{n}=n a_{n-1}+h_{n-1}, where hn1{1,1}h_{n-1} \in\{-1,1\}. Note that an+1=sn+1+sna_{n+1}=s_{n+1}+s_{n}, therefore there exists δn{2,0,2}\delta_{n} \in\{-2,0,2\} such that an+1=(n+1)an+nan1+δn a_{n+1}=(n+1) a_{n}+n a_{n-1}+\delta_{n} We also have s22a1=1\left|s_{2}-2 a_{1}\right|=1, which yields a0=3a1a2±13a1a_{0}=3 a_{1}-a_{2} \pm 1 \leq 3 a_{1}, and therefore a1a03671a_{1} \geq \frac{a_{0}}{3} \geq 671. Substituting n=2n=2 in (1), we find that a3=3a2+2a1+δ2a_{3}=3 a_{2}+2 a_{1}+\delta_{2}. Since a1a3a_{1} \mid a_{3}, we have a13a2+δ2a_{1} \mid 3 a_{2}+\delta_{2}, and therefore a2223a_{2} \geq 223. Using (1), we obtain that an223a_{n} \geq 223 for all n0n \geq 0. Lemma 1: For n4n \geq 4, we have an+2=(n+1)(n+4)ana_{n+2}=(n+1)(n+4) a_{n}. Proof. For n3n \geq 3 we have an=nan1+(n1)an2+δn1>nan1+3 a_{n}=n a_{n-1}+(n-1) a_{n-2}+\delta_{n-1}>n a_{n-1}+3 By applying (2) with nn substituted by n1n-1 we have for n4n \geq 4, an=nan1+(n1)an2+δn1<nan1+(an13)+δn1<(n+1)an1 a_{n}=n a_{n-1}+(n-1) a_{n-2}+\delta_{n-1}<n a_{n-1}+(a_{n-1}-3)+\delta_{n-1}<(n+1) a_{n-1} Using (1) to write an+2a_{n+2} in terms of ana_{n} and an1a_{n-1} along with (2), we obtain that for n3n \geq 3, an+2=(n+3)(n+1)an+(n+2)nan1+(n+2)δn+δn+1<(n+3)(n+1)an+(n+2)nan1+3(n+2)<(n2+5n+5)an. \begin{aligned} a_{n+2} & =(n+3)(n+1) a_{n}+(n+2) n a_{n-1}+(n+2) \delta_{n}+\delta_{n+1} \\ & <(n+3)(n+1) a_{n}+(n+2) n a_{n-1}+3(n+2) \\ & <\left(n^{2}+5 n+5\right) a_{n} . \end{aligned} Also for n4n \geq 4, an+2=(n+3)(n+1)an+(n+2)nan1+(n+2)δn+δn+1>(n+3)(n+1)an+nan=(n2+5n+3)an. \begin{aligned} a_{n+2} & =(n+3)(n+1) a_{n}+(n+2) n a_{n-1}+(n+2) \delta_{n}+\delta_{n+1} \\ & >(n+3)(n+1) a_{n}+n a_{n} \\ & =\left(n^{2}+5 n+3\right) a_{n} . \end{aligned} Since anan+2a_{n} \mid a_{n+2}, we obtain that an+2=(n2+5n+4)an=(n+1)(n+4)ana_{n+2}=\left(n^{2}+5 n+4\right) a_{n}=(n+1)(n+4) a_{n}, as desired. Lemma 2: For n4n \geq 4, we have an+1=(n+1)(n+3)n+2ana_{n+1}=\frac{(n+1)(n+3)}{n+2} a_{n}. Proof. Using the recurrence an+3=(n+3)an+2+(n+2)an+1+δn+2a_{n+3}=(n+3) a_{n+2}+(n+2) a_{n+1}+\delta_{n+2} and writing an+3a_{n+3}, an+2a_{n+2} in terms of an+1,ana_{n+1}, a_{n} according to Lemma 1 we obtain (n+2)(n+4)an+1=(n+3)(n+1)(n+4)an+δn+2 (n+2)(n+4) a_{n+1}=(n+3)(n+1)(n+4) a_{n}+\delta_{n+2} Hence n+4δn+2n+4 \mid \delta_{n+2}, which yields δn+2=0\delta_{n+2}=0 and an+1=(n+1)(n+3)n+2ana_{n+1}=\frac{(n+1)(n+3)}{n+2} a_{n}, as desired. Suppose there exists n1n \geq 1 such that an+1(n+1)(n+3)n+2ana_{n+1} \neq \frac{(n+1)(n+3)}{n+2} a_{n}. By Lemma 2, there exist a greatest integer 1m31 \leq m \leq 3 with this property. Then am+2=(m+2)(m+4)m+3am+1a_{m+2}=\frac{(m+2)(m+4)}{m+3} a_{m+1}. If δm+1=0\delta_{m+1}=0, we have am+1=(m+1)(m+3)m+2ama_{m+1}=\frac{(m+1)(m+3)}{m+2} a_{m}, which contradicts our choice of mm. Thus δm+10\delta_{m+1} \neq 0. Clearly m+3am+1m+3 \mid a_{m+1}. Write am+1=(m+3)ka_{m+1}=(m+3) k and am+2=(m+2)(m+4)ka_{m+2}=(m+2)(m+4) k. Then (m+(m+ 1) am+δm+1=am+2(m+2)am+1=(m+2)ka_{m}+\delta_{m+1}=a_{m+2}-(m+2) a_{m+1}=(m+2) k. So, am(m+2)kδm+1a_{m} \mid(m+2) k-\delta_{m+1}. But ama_{m} also divides am+2=(m+2)(m+4)ka_{m+2}=(m+2)(m+4) k. Combining the two divisibility conditions, we obtain am(m+4)δm+1a_{m} \mid(m+4) \delta_{m+1}. Since δm+10\delta_{m+1} \neq 0, we have am2m+814a_{m} \mid 2 m+8 \leq 14, which contradicts the previous result that an223a_{n} \geq 223 for all nonnegative integers nn. So, an+1=(n+1)(n+3)n+2ana_{n+1}=\frac{(n+1)(n+3)}{n+2} a_{n} for n1n \geq 1. Substituting n=1n=1 yields 3a13 \mid a_{1}. Letting a1=3ca_{1}=3 c, we have by induction that an=n!(n+2)ca_{n}=n!(n+2) c for n1n \geq 1. Since s22a1=1\left|s_{2}-2 a_{1}\right|=1, we then get a0=c±1a_{0}=c \pm 1, yielding the two families of solutions. By noting that (n+2)n!=n!+(n+1)!(n+2) n!=n!+(n+1)!, we have sn+1=c(n+2)!+(1)n(ca0)s_{n+1}=c(n+2)!+(-1)^{n}\left(c-a_{0}\right). Hence both families of solutions satisfy the given conditions.

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