Commit 8569a1cd authored by Jim Hefferon's avatar Jim Hefferon

finish leontief topics

parent 73d2b6e1
......@@ -3,6 +3,8 @@ TODO list for Linear Algebra
* Look through bug reports
** Do I need lemma 2.4 where it is? Could I move it to after the defn
of linear independence?
** In the index, make page references hyperlinks
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......@@ -1289,15 +1289,19 @@ If we have a non-\( \vec{0} \) row written as a combination
of the others
then we can rewrite the equation as
then we can rewrite that as
where $c_i=-1$.
We will use induction on the row index~$i$
to show that all of the coefficients~$c_i$ in that equation are~$0$.
where not all the coefficients are zero; specifically, $c_i=-1$.
The converse holds also:~given equation~($*$) where some $c_i\neq 0$ then we
could express $\rho_i$ as a combination of the other rows by
moving $c_i\rho_i$ to the left side and dividing by $c_i$.
Therefore, we will have proved the theorem if we
show that in~($*$) all of the coefficients are~$0$.
For that, we use induction on the row index~$i$.
The base case is the first row~$i=1$.
Equation~($*$) defines an equation among the column $\ell_1$ entries of those
......@@ -1308,9 +1312,9 @@ the leading entry in row~$i$).
The matrix is in echelon form so
every row after the first has a zero in that column, and thus
every row after the first has a zero in that column
We conclude that $c_1=0$ because as the leading entry
Thus $c_1=0$ because, as the leading entry
in the row, $r_{1,\ell_1}\neq 0$.
The inductive step is to prove the implication:~if
......@@ -1318,14 +1322,14 @@ for each row index $k\in\set{1,\ldots,i}$ the coefficient $c_k$ is $0$
then $c_{i+1}$ is also $0$.
Consider the entries from column~$\ell_{i+1}$ in equation~($*$).
By the inductive hypothesis the coefficients $c_1$, \ldots $c_i$ are
all $0$ so the equation reduces to
As in the base case we next note that the matrix is in echelon form
so $r_{i+2,\ell_{i+1}}=\cdots=r_{m,\ell_{i+1}}=0$, and
thus $c_{i+1}=0$ because $r_{i+1,\ell_{i+1}}\neq 0$ as it is the row's leading entry.
The matrix is in echelon form
so $r_{i+2,\ell_{i+1}}=\cdots=r_{m,\ell_{i+1}}=0$ and $r_{i+1,\ell_{i+1}}\neq 0$.
Thus $c_{i+1}=0$.
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