
A cyclic sum S N ( x ) = ∑ x i / ( x i + 1 + x i + 2 ) {S_N}({\mathbf {x}}) = \sum {{x_i}/({x_{i + 1}} + {x_{i + 2}})} is formed with N components of a vector x, where in the sum x N + 1 = x 1 {x_{N + 1}} = {x_1} , x N + 2 = x 2 {x_{N + 2}} = {x_2} , and where all denominators are positive and all numerators are nonnegative. It is known that there exist vectors x for which S N ( x ) > N / 2 {S_N}({\mathbf {x}}) > N/2 if N ≥ 14 N \geq 14 and even, and if N ≥ 24 N \geq 24 . It has been proved that the inequality S N ( x ) ≥ N / 2 {S_N}({\mathbf {x}}) \geq N/2 holds for N ≤ 13 N \leq 13 . Although it has been conjectured repeatedly that the inequality also holds for odd N between 15 and 23, this has apparently never been proved. Here we will confirm that the inequality indeed holds for all odd N ≤ 23 N \leq 23 . This settles the question for all N.
Inequalities for sums, series and integrals, minimization, Elementary number theory, Shapiro's cyclic inequality, cyclic sum
Inequalities for sums, series and integrals, minimization, Elementary number theory, Shapiro's cyclic inequality, cyclic sum
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