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Inequalities for two sine polynomials

Horst AlzerStamatis Koumandos — 2006

Colloquium Mathematicae

We prove: (I) For all integers n ≥ 2 and real numbers x ∈ (0,π) we have α j = 1 n - 1 1 / ( n ² - j ² ) s i n ( j x ) β , with the best possible constant bounds α = (15-√2073)/10240 √(1998-10√2073) = -0.1171..., β = 1/3. (II) The inequality 0 < j = 1 n - 1 ( n ² - j ² ) s i n ( j x ) holds for all even integers n ≥ 2 and x ∈ (0,π), and also for all odd integers n ≥ 3 and x ∈ (0,π - π/n].

A sharp bound for a sine polynomial

Horst AlzerStamatis Koumandos — 2003

Colloquium Mathematicae

We prove that | k = 1 n s i n ( ( 2 k - 1 ) x ) / k | < S i ( π ) = 1 . 8519 . . . for all integers n ≥ 1 and real numbers x. The upper bound Si(π) is best possible. This result refines inequalities due to Fejér (1910) and Lenz (1951).

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