设f(x)在区间[a,b]上满足a≤f(x)≤b,且有|f’(x)|≤q<1,令un=f(un一1)(n=1,2,…),u0∈[a,b],证明:级数(un+1一un)绝对收敛.

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问题 设f(x)在区间[a,b]上满足a≤f(x)≤b,且有|f’(x)|≤q<1,令un=f(un一1)(n=1,2,…),u0∈[a,b],证明:级数(un+1一un)绝对收敛.

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答案由|un+1一un|=|f(un)一f(un一1)|=|f’(ξ1)||un一un一1| ≤q|un一un一1≤q2|un一1一un一2|≤…≤qn|u1一u0| 且[*]qn|un+1一un|收敛,于是[*](un+1一un)绝对收敛.

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