Third Invariant expressed with Cayley-Hamilton Theorem

In summary, the Cayley-Hamilton Theorem, as explained in Chaves - Notes on Continuum Mechanics, can be used to show that the right-hand sides of two equations are equal. The proof involves replacing the values of IT and IIT with those in 1.269 and using some algebraic manipulations. This results in showing that twice the difference of the two equations is zero, thus proving their equality.
  • #1
FluidStu
26
3
The Cayley-Hamilton Theorem can be used to express the third invariant of the characteristic polynomial obtained from the non-trivial solution of the Eigenvector/Eigenvalue problem. I follow the proof (in Chaves – Notes on Continuum Mechanics) down to the following equation, then get stuck at "Replacing the values of IT and IIT with those in 1.269. Could someone please explain? Thanks

upload_2016-4-11_9-14-6.png
upload_2016-4-11_9-17-24.png
upload_2016-4-11_9-17-35.png

with 1.269 being:
upload_2016-4-11_9-14-51.png
 
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  • #2
We need to show that the RHS of the equations in the first two boxes are equal. To minimise the latex coding I'll write ##A## for ##Tr(T)## and ##B## for ##Tr(T^2)##. Then subtract the RHS of the second from the RHS of the first and multiply the result by 2 to get:
$$2II_TA-2I_TB-A^3+3AB=2II_TA-2AB-A^3+3AB=A\left(2II_T-2B-A^2+3B\right)=A\left(2II_T+B-A^2\right)=2A\left(II_T-0.5(A^2-B)\right)
=2A\left(II_T-II_T\right)=0$$
So twice the difference is zero.
So the two RHSs are equal.
 
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Likes FluidStu
  • #3
Great! Thanks Andrew.
 

1. What is the Third Invariant expressed with Cayley-Hamilton Theorem?

The Third Invariant expressed with Cayley-Hamilton Theorem is a mathematical concept that relates to the characteristic polynomial of a square matrix. It states that every square matrix satisfies its own characteristic polynomial, and this polynomial can be used to express the third invariant of the matrix.

2. How is the Third Invariant related to the Cayley-Hamilton Theorem?

The Third Invariant is directly related to the Cayley-Hamilton Theorem, as it is a result of the theorem. The Cayley-Hamilton Theorem states that every square matrix satisfies its own characteristic polynomial, and the third invariant is a specific expression of this polynomial.

3. What is the significance of the Third Invariant?

The Third Invariant has several important applications in mathematics and physics. It can be used to simplify calculations involving determinants, eigenvalues, and eigenvectors of a matrix. It also has implications in the study of dynamical systems and stability analysis.

4. How is the Third Invariant calculated?

The Third Invariant can be calculated using the characteristic polynomial of a square matrix. The characteristic polynomial is found by taking the determinant of the matrix and setting it equal to 0. The third invariant is then expressed as a function of the coefficients of this polynomial.

5. Can the Third Invariant be used for any type of matrix?

Yes, the Third Invariant can be used for any square matrix. It is a general mathematical concept that applies to all square matrices, regardless of their size, elements, or properties. However, the calculation and interpretation of the Third Invariant may vary depending on the specific characteristics of the matrix.

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