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Algebra Difficulty 7.0 National olympiad Prove it China

It is known that each term of sequence {an}\{a_n\} is a non-zero real number, and for any positive integer nn holds the equation
(a1+a2++an)2=a13+a23++an3. (a_1 + a_2 + \cdots + a_n)^2 = a_1^3 + a_2^3 + \cdots + a_n^3.

(1) When n=3n = 3, find out all the sequences consisting of three terms a1,a2,a3a_1, a_2, a_3.

(2) Does there exist an infinite sequence {an}\{a_n\} such that a2013=2012a_{2013} = -2012? If it exists, write out the formula of general term; if not, give your reason.

Solution

(1) When n=1n = 1, we have a12=a13a_1^2 = a_1^3. Since a10a_1 \neq 0, we get a1=1a_1 = 1.
When n=2n = 2, we have (1+a2)2=1+a23(1+a_2)^2 = 1+a_2^3. Since a20a_2 \neq 0, we get a2=2a_2 = 2 or a2=1a_2 = -1.
When n=3n = 3, we have (1+a2+a3)2=1+a23+a33(1+a_2+a_3)^2 = 1+a_2^3+a_3^3. For a2=2a_2 = 2, we get a3=3a_3 = 3 or a3=2a_3 = -2; for a2=1a_2 = -1, we get a3=1a_3 = 1.
In summary, we get three sequences consisting of three terms that satisfy the given condition:
{1,2,3},{1,2,2}, and {1,1,1}. \{1, 2, 3\}, \{1, 2, -2\}, \text{ and } \{1, -1, 1\}.

(2) Let Sn=a1+a2++anS_n = a_1 + a_2 + \cdots + a_n. Then we have
Sn2=a13+a23++an3 (nN),(Sn+an+1)2=a13+a23++an3+an+13. S_n^2 = a_1^3 + a_2^3 + \cdots + a_n^3 \ (n \in \mathbb{N}), \\ (S_n + a_{n+1})^2 = a_1^3 + a_2^3 + \cdots + a_n^3 + a_{n+1}^3.
Finding out the difference of the two expressions above and by an+10a_{n+1} \neq 0, we have 2Sn=an+12an+12S_n = a_{n+1}^2 - a_{n+1}.
When n=1n = 1, we know from (1) that a1=1a_1 = 1.
When n2n \ge 2, we have
2an=2(SnSn1)=(an+12an+1)(an2an). 2a_n = 2(S_n - S_{n-1}) = (a_{n+1}^2 - a_{n+1}) - (a_n^2 - a_n).
And that is
(an+1+an)(an+1an1)=0. (a_{n+1} + a_n)(a_{n+1} - a_n - 1) = 0.
Then we get an+1=ana_{n+1} = -a_n or an+1=an+1a_{n+1} = a_n + 1.
Finally, from a1=1a_1 = 1 and a2013=2012a_{2013} = -2012, we find the formula of general term for a required sequence as
an={n,1n2012,2012(1)n,n2013. a_n = \begin{cases} n, & 1 \le n \le 2012, \\ 2012(-1)^n, & n \ge 2013. \end{cases}

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