# Newton's Identities/Proof 1

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## Theorem

Let $X$ be a set of $n$ numbers $\set {x_1, x_2, \ldots, x_n}$.

Define:

\(\ds \mathbf S_m\) | \(=\) | \(\ds \set {\paren {j_1, \ldots, j_m} : 1 \le j_1 < \cdots < j_m \le n}\) | $1 \le m \le n$ | |||||||||||

\(\ds \map {e_m} X\) | \(=\) | \(\ds \begin {cases} 1 & : m = 0 \\ \ds \sum_{\mathbf S_m} x_{j_1} \cdots x_{j_m} & : 1 \le m \le n \\ 0 & : m > n \\ \end {cases}\) | Definition of Elementary Symmetric Function | |||||||||||

\(\ds \map {p_k} X\) | \(=\) | \(\ds \begin{cases} n & : k = 0 \\ \ds \sum_{i \mathop = 1}^n x_i^k & : k \ge 1 \end {cases}\) | Definition of Power Sum |

Then **Newton's identities** are:

\(\text {(1)}: \quad\) | \(\ds k \map {e_k} X\) | \(=\) | \(\ds \sum_{i \mathop = 1}^k \paren {-1}^{i - 1} \map {e_{k - i} } X \map {p_i} X\) | for $1 \le k \le n$ | ||||||||||

\(\text {(2)}: \quad\) | \(\ds 0\) | \(=\) | \(\ds \sum_{i \mathop = k - n}^k \paren {-1}^{i - 1} \map {e_{k - i} } X \map {p_i} X\) | for $1 \le n < k$ |

## Proof

### Outline

The proof is divided into three cases: $k < n$, $k = n$ and $k > n$.

The tools are Viète's Formulas, Recursion Property of Elementary Symmetric Function, telescoping sums and homogeneous functions of degree $k$.

### Details

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Discovery of formulas (1)-(2) has humble beginnings:

\(\text {(3)}: \quad\) | \(\ds \prod_{r \mathop = 1}^n \paren { x - x_r }\) | \(=\) | \(\ds \sum_{i \mathop = 0}^n \paren {-1}^{n-i} {\map {e_{n - i} } X} x^i\) | Viète's Formulas |

Create $n$ equations all with left hand side zero by substitution $x = x_j$ in (3), $1 \le j \le n$.

Then multiply the $j$th equation by $x_j^r$ and add the $n$ equations, for $1 \le j \le n$:

\(\text {(4)}: \quad\) | \(\ds 0\) | \(=\) | \(\ds \sum_{j \mathop = 1}^n \sum_{i \mathop = 0}^n \paren {-1}^{n - i} \map {e_{n - i} } X x_j^{i + r}\) | |||||||||||

\(\ds \) | \(=\) | \(\ds \sum_{i \mathop = 0}^n \paren {-1}^{n - i} \map {e_{n - i} } X \map {p_{i + r} } X\) | Term $\map {e_m} X$ is zero for $m \gt n$, affecting case $r \gt 0$. |

Case $r = 0$ in (4) gives (1) for $k = n$, by isolating term $n \map {e_n} X$.

Case $r > 0$ in (4) gives (2) by re-indexing: $m = k - n + i$ with $m = k - n$ to $k$.

Case $k < n$ in (1), not yet discussed, does not use multiply and add as in (4).

The key ingredient:

\(\text {(5)}: \quad\) | \(\ds \map {e_m} {z_1, \ldots, z_p, z_{p + 1} }\) | \(=\) | \(\ds z_{p + 1} \map {e_{m - 1} } {z_1, \ldots, z_p} + \map {e_m} {z_1, \ldots, z_p}\) | Recursion Property of Elementary Symmetric Function, valid for $1 \le m \le p$. |

The right hand side of (1) is:

\(\text {(6)}: \quad\) | \(\ds \) | \(\) | \(\ds \sum_{i \mathop = 1}^k \paren {-1}^{i - 1} \sum_{j \mathop = 1}^n \map {e_{k - i} } X x_j^i\) | Replace in (1) each power sum $p_i$ by its summation. | ||||||||||

\(\ds \leadsto \ \ \) | \(\ds \) | \(\) | \(\ds \paren {-1}^{k - 1} \sum_{m \mathop = 0}^{k - 1} \paren {-1}^m \sum_{j \mathop = 1}^n \map {e_m} X x_j^{k - m}\) | Re-index with $m = k - i$ and adjust powers of $-1$. | ||||||||||

\(\ds \leadsto \ \ \) | \(\ds \) | \(\) | \(\ds \paren {-1}^{k - 1} \sum_{j \mathop = 1}^n \paren {\map {e_0} X x_j^k + \sum_{m \mathop = 1}^{k - 1} \paren {-1}^m \paren {x_j \map {e_{m - 1} } {U_j} x_j^{k - m} + \map {e_m} {U_j} x_j^{k - m} } }\) | by (5), where $U_j = X \setminus \set {x_j}$ | ||||||||||

\(\ds \leadsto \ \ \) | \(\ds \) | \(\) | \(\ds \paren {-1}^{k - 1} \sum_{j \mathop = 1}^n \paren {\map {e_0} X x_j^k + \sum_{m \mathop = 1}^{k - 1} \paren {-1}^m \map {e_{m - 1} } {U_j} x_j^{k - m + 1} + \sum_{m \mathop = 1}^{k - 1} \paren {-1}^m \map {e_m} {U_j} {x_j^{k - m} } }\) | adjust summations and collect powers of $x_j$ | ||||||||||

\(\text {(7)}: \quad\) | \(\ds \leadsto \ \ \) | \(\ds \) | \(\) | \(\ds \paren {-1}^{k - 1} \sum_{j \mathop = 1}^n \paren {-1}^{k - 1} x_j \map {e_k} {U_j}\) | telescoping sum reduces to a single term |

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It remains to prove (7) matches (1):

\(\text {(8)}: \quad\) | \(\ds \sum_{j \mathop = 1}^n x_j \map {e_k} {X \setminus \set {x_j} }\) | \(=\) | \(\ds k \map {e_k} X\) | for $1 \le k \le n$ |

Define:

- $\map {y_i} t = t\, x_i, 1 \le i \le n$

- $X_t = \set {\map {y_1} t, \ldots, \map {y_n} t}$

- $\map f t = \map {e_k} {tx_1, \ldots, tx_n}$.

Then there are two equations (9)-(10) for $\map {f'} t$:

\(\text {(9)}: \quad\) | \(\ds \map {f'} t\) | \(=\) | \(\ds \sum_{j \mathop = 1}^n x_j \map {e_k} {X_t \setminus \set {\map {y_j} t} }\) | chain rule for several variables | ||||||||||

\(\text {(10)}: \quad\) | \(\ds \map f t\) | \(=\) | \(\ds t^k \map {e_k} X\) | because $f$ is a homogeneous function of degree $k$ | ||||||||||

\(\ds \leadsto \ \ \) | \(\ds \map {f'} t\) | \(=\) | \(\ds k t^{k - 1} \map {e_k} X\) | Power Rule for Derivatives |

Let $t = 1$ in (9) and (10) to prove (8).

$\blacksquare$