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[SOLVED] Tensor notation-Determinants
I'm trying to learn the basics of Tensor calculus using a free online book (Introduction to Tensor Calculus and Continuum Mechanics), and I got stuck on this question (Part 2 in book, after non-math introduction).
link to part 2, questions (mine #19) at end:
http://www.math.odu.edu/~jhh/part2.PDF"
Let A and B denote 3x3 matrices with elements [tex]A_{ij}[/tex] and [tex]B_{ij}[/tex]
respectively. Show that if C = AB is a matrix product, then det(C) = det(A)*det(B)
where det = determinant.
Hint: use the result from example 1.1-9
det(A) = [tex]e_{ijk}A_{1i}A_{2j}A_{3k}[/tex]
The matrix multiplication of A*B =C, in indical notation, is
[tex]C_{ij}=A_{im}B_{mj}[/tex] (I think) where the first subscript in A,B and C is the row number and the second subscript the column of the matrix.
Then, plugging into 'relevant equation' above, we get
det(C) = [tex]e_{ijk}C_{1i}C_{2j}C_{3k}[/tex]
det(C) = [tex]e_{ijk}(A_{1m}B_{mi})(A_{2n}B_{nj})(A_{3x}B_{xk})[/tex]
Then, I compare this to just multiplying det(A)*det(B)
det(A) * det(B) = [tex](e_{ijk}A_{1i}A_{2j}A_{3k})(e_{rst}B_{1r}B_{2s}B_{3t})[/tex]
However, from here, I can't seem to make the connection between the two
I would try to expand it, but I don't think this is the way to do it, since it would get rid of the advantage of indical notation. Help would be appreciated! Thxs!
PS: on a side note, whenever you preview a post, the template is pasted again to the end of the post, which is annoying. Maybe someone can fix this...
I'm trying to learn the basics of Tensor calculus using a free online book (Introduction to Tensor Calculus and Continuum Mechanics), and I got stuck on this question (Part 2 in book, after non-math introduction).
link to part 2, questions (mine #19) at end:
http://www.math.odu.edu/~jhh/part2.PDF"
Homework Statement
Let A and B denote 3x3 matrices with elements [tex]A_{ij}[/tex] and [tex]B_{ij}[/tex]
respectively. Show that if C = AB is a matrix product, then det(C) = det(A)*det(B)
where det = determinant.
Hint: use the result from example 1.1-9
Homework Equations
det(A) = [tex]e_{ijk}A_{1i}A_{2j}A_{3k}[/tex]
The Attempt at a Solution
The matrix multiplication of A*B =C, in indical notation, is
[tex]C_{ij}=A_{im}B_{mj}[/tex] (I think) where the first subscript in A,B and C is the row number and the second subscript the column of the matrix.
Then, plugging into 'relevant equation' above, we get
det(C) = [tex]e_{ijk}C_{1i}C_{2j}C_{3k}[/tex]
det(C) = [tex]e_{ijk}(A_{1m}B_{mi})(A_{2n}B_{nj})(A_{3x}B_{xk})[/tex]
Then, I compare this to just multiplying det(A)*det(B)
det(A) * det(B) = [tex](e_{ijk}A_{1i}A_{2j}A_{3k})(e_{rst}B_{1r}B_{2s}B_{3t})[/tex]
However, from here, I can't seem to make the connection between the two
I would try to expand it, but I don't think this is the way to do it, since it would get rid of the advantage of indical notation. Help would be appreciated! Thxs!
PS: on a side note, whenever you preview a post, the template is pasted again to the end of the post, which is annoying. Maybe someone can fix this...
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