Thursday, 24 October 2013

calculus - How prove this limit limntoinftyleft(n+frac12right)left(znleft(n+frac12right)piright)=fracHpi



Question:



Let HR,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 limyinfxy=H. Taylor expand both sides:



Hy(1xy)=x+O(x2)



(Hy21)x=Hy+O(x2)




x=H/y1H/y2+O(x2)



From this we can see that xO(1/y), so O(x2)=O(1/y2), and xy=H+O(1/y) as required.


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