How to Determine the Dimension of Span(1, cos(2x), cos²(x)) in C[-π, π]?

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SUMMARY

The dimension of the span of the functions 1, cos(2x), and cos²(x) in the interval C[-π, π] is determined by their linear independence. The function cos(2x) can be expressed as a linear combination of the other two functions using the trigonometric identity cos(2x) = 2cos²(x) - 1. This indicates that the three functions are not linearly independent, confirming that the dimension of their span is 2. The Wronskian method is not necessary for this analysis, as a simpler approach using trigonometric identities suffices.

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  • Understanding of linear combinations in vector spaces
  • Familiarity with trigonometric identities, specifically cos(2x) = 2cos²(x) - 1
  • Basic knowledge of function spaces, particularly C[-π, π]
  • Concept of linear independence in mathematical analysis
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  • Learn about the Wronskian and its role in determining linear independence
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Dustinsfl
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C[-pi, pi]
Span(1, cos (2x), cos2 (x))

Doing the Wronskian here is pain so what other method would be more appropriate?
 
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One of the functions is a linear combination of the other two, so the three functions can't be linearly independent.
 
How can I go about identifying if one is?
 
Use a trig identity for cos2x
 
cos2x=2cos^2 x-1 but I don't see how that is a multiple of cos2x sine it doesn't = cos^2 x outright.
 
Can cos2x can be written as a linear combination of cos2x and 1?
 
I see now.
 
Great :smile:
 
Dustinsfl said:
cos2x=2cos^2 x-1 but I don't see how that is a multiple of cos2x sine it doesn't = cos^2 x outright.
I realize that you already figured this out, but if you look at what I said, I didn't say that one of the functions was a multiple of another. I said that one function was a linear combination of the other two.
 

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