Newton's Identities/Proof 2

Proof
Calculus will be used to prove identities (1) and (2) in a single effort.

Lemma 1
Proof of Lemma 1

Begin with:

Change variables in (11): $x = -1/z$.

Details: Generating Function for Elementary Symmetric Function.

Lemma 2
Denote by $D^k \map f z$ the $k$th calculus derivative of $\map f z$.

Let:

Then:

Proof of Lemma 2

Then identity (12) holds by Maclaurin series expansion applied to polynomial $G$.

Identity (13) will be proved after mathematical induction establishes (14) infra.

Let $\map {\mathbf P} m$ be the statement:

Basis for the induction: $m=0$

By calculus and the definition of $G$:

Then $\map {\mathbf P} 0$ is true.

Induction step $\map {\mathbf P} m$ implies $\map {\mathbf P} {m+1}$:

Simplify to prove $\map {\mathbf P} {m+1}$ is true.

The induction is complete.

To prove equation (13), first let $z=0$ in equation (14).

Divide by $m!$ to isolate $\map {p_{m+1} } X$, which proves (13).

Lemma 3
Proof of Lemma 3

Begin with $D \map G z = {\map F z} {\map G z}$ and differentiate $m$ times on variable $z$:

Proof of the Theorem

To prove (1), start with equation (15) in Lemma 3.

Change indices via equations $m+1=k$, $r+1=i$.

The summation is from $i=0+1$ to $i=m+1$, which gives range $i=1$ to $k$.

Subscript $m-r$ equals $k-1-i+1$, which simplifies to $k-i$.

Then:

To prove (2), assume $k > n \ge 1$ and $X = \set {x_1,\ldots,x_n}$.

Equation (16) implies:

Then (2) holds.