Division Theorem for Polynomial Forms over Field/Proof 3

Proof
Proof by induction:

For all $n \in \N_{> 0}$, let $P \left({n}\right)$ be the proposition:
 * $\forall f \in F \left[{X}\right]: \exists q, r \in F \left[{X}\right]: f = q \circ d + r$ provided that $\deg \left({f}\right) < n$

$P \left({0}\right)$ is the statement that $q$ and $r$ exist when $f = 0$.

This is shown trivially to be true by taking $q = r = 0$.

Basis for the Induction
$P \left({0}\right)$ is the statement that $q$ and $r$ exist when $f = 0$.

This is shown trivially to be true by taking $q = r = 0$.

This is our basis for the induction.

Induction Hypothesis
Now we need to show that, if $P \left({k}\right)$ is true, where $k \ge 0$, then it logically follows that $P \left({k+1}\right)$ is true.

So this is our induction hypothesis:
 * $\forall f \in F \left[{X}\right]: \exists q, r \in F \left[{X}\right]: f = q \circ d + r$ provided that $\deg \left({f}\right) < k$

Then we need to show:
 * $\forall f \in F \left[{X}\right]: \exists q, r \in F \left[{X}\right]: f = q \circ d + r$ provided that $\deg \left({f}\right) < k + 1$

Induction Step
This is our induction step:

Let $f$ be such that $\deg \left({f}\right) = n$.

Let:
 * $\displaystyle f = a_0 + a_1 \circ x + a_2 \circ x^2 + \cdots + a_n \circ x^n$ where $a_n \ne 0$

Let:
 * $\displaystyle d = b_0 + b_1 \circ x + b_2 \circ x^2 + \cdots + b_j \circ x^j$ where $b_j \ne 0$

If $n < l$ then take $q = 0, r = f$.

If $n \ge l$, consider:
 * $c := f - a_n b_j^{-1} x^{n-j} d$

This has been carefully arranged so that the coefficient of $x^n$ in $c$ is zero.

Thus $\deg \left({c}\right) < n$.

Therefore, by the induction hypothesis:
 * $c = d q_0 + r$

where $\deg \left({r}\right) < \deg \left({d}\right)$.

Therefore:
 * $f = d \left({q_0 + a_n b_j^{-1} x^{n-j}}\right) + r$

Thus the existence of $q$ and $r$ have been established.

As for uniqueness, assume:
 * $d q + r = d q' + r'$

with $\deg \left({r}\right) < \deg \left({d}\right), \deg \left({r'}\right) < \deg \left({d}\right)$

Then:
 * $d \left({q - q'}\right) = r' - r$

By Degree of Sum of Polynomials:
 * $\deg \left({r' - r}\right) \le \max \left\{{\deg \left({r'}\right), \deg \left({r}\right)}\right\} < \deg \left({d}\right)$

and by Degree of Product of Polynomials over Integral Domain:
 * $\deg \left({d \left({q - q'}\right)}\right) = \deg \left({d}\right) \deg \left({d}\right) + \deg \left({q - q'}\right)$

That is:
 * $\deg \left({d}\right) < \deg \left({d}\right) + \deg \left({q - q'}\right)$

and the only way for that to happen is for:
 * $\deg \left({q - q'}\right) = - \infty$

that is, for $q - q'$ to be the null polynomial.

That is, $q - q' = 0_F$ and by a similar argument $r' - r = 0_F$, demonstrating the uniqueness of $q$ and $r$.

So $P \left({k}\right) \implies P \left({k+1}\right)$ and the result follows by the Principle of Mathematical Induction.

Therefore:
 * $\forall f \in F \left[{X}\right]: \exists q, r \in F \left[{X}\right]: f = q \circ d + r$ such that either:
 * $(1): \quad r = 0_F$
 * or:
 * $(2): \quad r \ne 0_F$ and $r$ has degree that is less than $n$.