Matrix Exponent Problem: Find smallest n such that Sn = I

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Discussion Overview

The discussion revolves around the problem of determining whether there exists a positive integer n such that the matrix S raised to the power of n equals the 2x2 identity matrix I. The focus is on exploring the properties of the matrix S, which represents a rotation transformation, and finding the smallest such integer n.

Discussion Character

  • Homework-related
  • Exploratory
  • Technical explanation

Main Points Raised

  • Post 1 introduces the matrix S and asks if there exists a positive integer n such that Sn = I, seeking guidance on how to approach the problem.
  • Post 2 suggests examining the powers of the matrix S (S2, S3, etc.) to understand its behavior.
  • Post 3 speculates that n might be 10 but expresses uncertainty about how to compute S2 and S3 to find the answer.
  • Post 4 emphasizes the importance of understanding the transformation that the matrix S represents rather than just its matrix form.
  • Post 6 reiterates the calculation of S2 and encourages the use of standard trigonometric identities to aid in understanding.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the value of n or the method to find it. There are differing approaches suggested, with some focusing on matrix multiplication and others on the geometric interpretation of the transformation.

Contextual Notes

Participants have not provided a complete derivation of the matrix powers or a thorough exploration of the implications of the transformation represented by S. There are unresolved mathematical steps regarding the calculations of S2 and S3.

DmytriE
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Homework Statement


Consider the matrix

cos(3*pi/17) -sin(3*pi/17)
S = sin(3*pi/17) cos(3*pi/17)

Does there exist a positive integer n such that Sn = I where I is the 2x2 identity? If so, what is the smallest such integer? Explain.

Excuse the poor matrix formatting. I do not know how to use the latex formatting to put it into pretty print.

Homework Equations


Not sure...


The Attempt at a Solution



Where should I start? I really have no idea.
 
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Hi DmytriE! :smile:

Hint: suppose S is

Code:
cosθ -sinθ
sinθ cosθ

What is S2 ? S3 ? etc? :wink:
 
Let's suppose that n = 10. I don't think I have to multiply S by S 10 times to get the answer. Unfortunately the answer is not 10. How would S2, S3 help me figure it out?

S2:
UL:cos2(θ) - sin2(θ)
UR: -2sin(θ)cos(θ)
LL: 2sin(θ)cos(θ)
LR: -sin2(θ)+cos2(θ)

Each abbreviation represent the place in the matrix that they would appear. UL - Upper left, etc.

S3:
a lot of sines and cosines.
 
That's not what tiny-tim is suggesting. Your matrix represents a certain kind of transformation.

Instead of thinking about what S, S2, S3, etc. are (in terms of their matrix representations), think about what they do to a vector they multiply.
 
Thanks for the help! This forum really is the best!
 
Hi DmytriE! :smile:

(just got up :zzz:)
DmytriE said:
S2:
UL:cos2(θ) - sin2(θ)
UR: -2sin(θ)cos(θ)
LL: 2sin(θ)cos(θ)
LR: -sin2(θ)+cos2(θ)

have you got it now?

if not, use standard trigonometric identities :wink:
 

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