George Washington University Differential Geometry Worksheet

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Please solve the Question and show all the steps. you may need the note(pdf) but you cannot copy from it

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Consider the plane curves ve : [0, 1] + R2, defined by formulae ve(t) =
(cos(2nlt), sin(2nlt)), if l is a non zero integer;
(cos(2nt), sin(4nt)) if l = 0.
(i) What are rotation indices of these curves? Briefly explain your answer.
(ii) Using (a)(i), deduce that any regular closed curve 7 : [0, 1] + R2 is regularly
homotopic to a curve ve for some integer l e Z.
(iii) Draw a plane curve that is regularly homotopic to 72 and has five self-intersection
points. Explain why your drawing satisfies these conditions.
Consider the family of closed curves
(cos(2nt), sin(4nt))+p(cos(2nlt), sin(2mlt)),
where l is a non-zero integer, p > 0 is a real number, and t€ [0, 1].
(i) Prove that there exists a constant C > 0 such that if pl > C, then y is a
regular closed curve.
(ii) Give one possible value for the constant C in (b)(i). Justify your answer.
(ii) What are possible values of the rotation index r() when p satisfies the con-
dition in (b)(i)? Justify your answer.
Let a : [0, 1] + R” and B : [0, 1] + R” be two unit speed curves such that
for all t € [0, 1] the angle between the vectors a'(t) and B'(t) is not greater
than 7/3. Prove that the curves a and B are regularly homotopic.

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Tags:
plane curves

rotation indices

homotopic curve

non zero integer

regular closed curve

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