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KöMaL 2007 B3987

    Let n≥n \ge4 be an integer, and let a1,a2,…,ana_1,a_2,\ldots,a_n denote nonnegative real numbers. Prove that (a1+a2+a3)2(a2+a3+a4)2⋅…⋅(an−1+an+a1)2(an+a1+a2)2{(a_1+a_2+a_3)}^2 {(a_2+a_3+a_4)}^2\cdot \ldots \cdot {(a_{n-1}+a_n+a_1)}^2{(a_n+a_1+a_2)}^2 ≥2n(a1+a2)2(a2+a3)2⋅…⋅(an−1+an)2(an+a1)2.\ge 2^n{(a_1+a_2)}^2{(a_2+a_3)}^2\cdot \ldots \cdot {(a_{n-1}+a_n)}^2{(a_n+a_1)}^2. Discuss also the case of equality.

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