Question:
Let H∈R,Prove that the transcendental equation zcotz+H=0
has a countable number of zeros zn and that
limn→∞(n+12)(zn−(n+12)π)=Hπ
My try: we must only prove this
zn=(n+12)π+H(n+12)π+o(1n2)
if this problem don't tell this limit reslut,then we how find this limit? Thank you
can you someone help me,Thank you very much!
Answer
Your equation can be rearranged to:
cot(z)=−Hz
Put z=y+x where y=(n+12)π and use cot(y+x)=−tan(x):
Hy+x=tan(x)
We need to show limy→infxy=H. Taylor expand both sides:
Hy(1−xy)=x+O(x2)
(Hy2−1)x=−Hy+O(x2)
x=H/y1−H/y2+O(x2)
From this we can see that x∈O(1/y), so O(x2)=O(1/y2), and xy=H+O(1/y) as required.
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