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    $f命题1:$$(f{Bendixon判别法})$设$sumlimits_{n = 1}^infty {{u_n}left( x ight)} $为$left[ {a,b} ight]$上的可微函数项级数,且$sumlimits_{n = 1}^infty {{u_n}^prime left( x ight)} $的部分和函数列在$left[ {a,b} ight]$上一致有界

    证明:如果$sumlimits_{n = 1}^infty {{u_n}left( x ight)} $在$left[ {a,b} ight]$上收敛,则必在$left[ {a,b} ight]$上一致收敛

    证明:由一致有界的定义知,存在$C>0$,使得对每个正整数$n$和每个$x in left[ {a,b} ight]$,有
    [left| {sumlimits_{k = 1}^n {{u_k}^prime left( x ight)} } ight| le C]
    对任给$varepsilon > 0$,取区间$left[ {a,b} ight]$的等距分划$left{ {{x_0},{x_1}, cdots ,{x_m}} ight}$,使得当$m$充分大时,分划的细度[Delta {x_i} = frac{{b - a}}{m} < frac{varepsilon }{{4C}}]
    由于$sumlimits_{n = 1}^infty {{u_n}left( x ight)} $在$left[ {a,b} ight]$上处处收敛,则由$f{Cauchy收敛准则}$知,对任给$varepsilon >0$,存在$N>0$,使得当$n>N$时,对任意正整数$p$和分划的每个分点${x_i}$,同时成立

    [left| {sumlimits_{k = n + 1}^{n + p} {{u_k}left( {{x_i}} ight)} } ight| < frac{varepsilon }{2},i = 0,1, cdots ,m]
    于是对任意$x in left[ {a,b} ight]$,不妨设$x in left[ {{x_{i - 1}},{x_i}} ight]$,由微分中值定理知,存在${xi _i} in left( {x,{x_i}} ight)$,使得
    egin{align*}
    left| {sumlimits_{k = n + 1}^{n + p} {{u_k}left( x ight)} } ight| &= left| {sumlimits_{k = n + 1}^{n + p} {{u_k}left( {{x_i}} ight)} + sumlimits_{k = n + 1}^{n + p} {left( {{u_k}left( x ight) - {u_k}left( {{x_i}} ight)} ight)} } ight|\&
    le left| {sumlimits_{k = n + 1}^{n + p} {{u_k}left( {{x_i}} ight)} } ight| + left| {sumlimits_{k = 1}^n {{u_k}^prime left( {{xi _i}} ight)} } ight|left| {x - {x_i}} ight|\&
    < frac{varepsilon }{2} + 2Cleft| {x - {x_i}} ight| le frac{varepsilon }{2} + frac{varepsilon }{2} = varepsilon 
    end{align*}
    从而由函数项级数一致收敛的$f{Cauchy准则}$即证

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  • 原文地址:https://www.cnblogs.com/ly758241/p/3706442.html
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