设f(x)在(0,+∞)三次可导,且当x∈(0,+∞)时|f(x)|≤M0, |f’’’(x)|≤M3,其中M0,M3为非负常数,求证f’’(x)在(0,+∞)上有界.

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问题 设f(x)在(0,+∞)三次可导,且当x∈(0,+∞)时|f(x)|≤M0,  |f’’’(x)|≤M3,其中M0,M3为非负常数,求证f’’(x)在(0,+∞)上有界.

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答案分别讨论x>1与0<x≤1两种情形. 1)当x>1时考察二阶泰勒公式 [*] 两式相加并移项即得 f’’(x)=f(x+1)+f(x-1)-2f(x)+[*][f’’’(η)-f’’’(ξ)], 故当x>1时有|f’’(x)|≤4M0+[*] 2)当0<x≤1时对f’’(x)用拉格朗51中值定理,有 f’’(x)=f’’(x)-f’’(1)+f’’(1)=f’’(ξ)(x-1)+f’’(1),其中ξ∈(x,1). 从而 |f’’(x)|≤|f’’’(ξ)||x-1|+|f’’(1)|≤M3+|f’’(1)|(x∈(0,1]). 综合即知f’’(x)在(0,+∞)上有界.

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