Wednesday, 16 January 2013

real analysis - Prove the following limit exists



I'm trying understand why limε0(|x|εφ(x)x2dx2φ(0)ε) exists, where φCc(R). The motivation for my question is a linear functional which is defined on the space of distributions by this limit, where φ is the input of the linear functional. I would be able to show that the limit of the integral exists, but limε0φ(0)ε not necessarily exist for every φCc(R), then I need compute the whole limit instead of compute the limit by the pieces, but I don't have idea how to do this. I would like to understand why limε0(|x|εφ(x)x2dx2φ(0)ε) exists.


Answer



The Maclaurin series of φ begins a+bx+cx2+. If a=b=0,
of course there is no problem. We'd like to reduce to this case by subtracting off

a+bx, but that does not have compact support. But we can get around that by
taking a compactly supported and smooth ψ which is equal to 1 on the
interval [1,1]. Then
φ=φ1+φ2


where φ1(x)=(a+bx)ψ(x)
and φ2cx2 as x0. So we can reduce to considering φ1(x).



For 0<ϵ<1,
|x|ϵφ1(x)x2dx=|x1φ1(x)x2dx+1ϵa+bxx2dx+ϵ1a+bxx2dx=C+1ϵ2ax2dx=C+2φ1(0)ϵ


where C and C are independent of ϵ. In this case, the limit
is just C.


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