I'm trying understand why limε→0(∫|x|≥εφ(x)x2dx−2φ(0)ε) exists, where φ∈C∞c(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 limε→0φ(0)ε not necessarily exist for every φ∈C∞c(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 limε→0(∫|x|≥εφ(x)x2dx−2φ(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 φ2∼cx2 as x→0. So we can reduce to considering φ1(x).
For 0<ϵ<1,
∫|x|≥ϵφ1(x)x2dx=∫|x≥1φ1(x)x2dx+∫1ϵa+bxx2dx+∫−ϵ−1a+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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