Inverse of Vandermonde Matrix/Eisinberg Formula
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Theorem
Let:
\(\ds \prod_{k \mathop = 1}^n \paren {x - x_k}\) | \(=\) | \(\ds a_n x^n + \sum_{m \mathop = 0}^{n - 1} a_m x^m\) | Polynomial expansion in powers of $x$ | |||||||||||
\(\ds \) | \(=\) | \(\ds x^n + \sum_{m \mathop = 0}^{n - 1} \paren {-1}^{n - m} \map {e_{n - m} } {x_1, \ldots, x_n} \, x^m\) | Viète's Formulas and Definition of Elementary Symmetric Function | |||||||||||
\(\ds W_n\) | \(=\) | \(\ds \begin{bmatrix} 1 & x_1 & \cdots & {x_1}^{n - 1} \\ 1 & x_2 & \cdots & {x_2}^{n - 1} \\ \vdots & \vdots & \ddots & \vdots \\ 1 & x_n & \cdots & {x_n}^{n - 1} \\ \end{bmatrix}\) | Definition of Vandermonde Matrix of Order $n$ |
Let $W_n$ have a matrix inverse $W_n^{-1} = \begin {bmatrix} d_{ij} \end {bmatrix}$.
Let $a_n = \map {e_0} {x_1, \ldots, x_n} = 1$.
Then:
\(\text {(1)}: \quad\) | \(\ds d_{ij}\) | \(=\) | \(\ds \dfrac {\ds \sum_{k \mathop = 0}^{n - i} a_{i + k} \, {x_j}^k} {\ds \prod_{m \mathop = 1, m \mathop \ne j }^n \paren {x_j - x_m} }\) | for $i, j = 1, \ldots, n$ | ||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {\ds \sum_{k \mathop = 0}^{n - i} \paren {-1}^{n - i - k} \map {e_{n - i - k} } {x_1, \ldots, x_n} \, {x_j}^k} {\ds \prod_{m \mathop = 1, m \mathop \ne j }^n \paren {x_j - x_m} }\) | for $i, j = 1, \ldots, n$ |
Proof
Lemma 1
Given values $z_1, \ldots, z_{p + 1}$ and $1 \le m \le p$, then:
\(\text {(2)}: \quad\) | \(\ds \ds \map {e_m} {z_1, \ldots, z_p, z_{p + 1} }\) | \(=\) | \(\ds z_{p + 1} \map {e_{m - 1} } {z_1, \ldots, z_p} + \map {e_m} {z_1, \ldots, z_p}\) | Recursion Property of Elementary Symmetric Function |
$\Box$
Lemma 2
Let $X = \set {x_1, \ldots, x_n}$ and $\mathbf u = x_j$ for some $j = 1, \ldots, n$.
Then:
\(\text {(3)}: \quad\) | \(\ds \ds \sum_{k \mathop = 0}^{n - i} \paren {-1}^k \map {e_{n - i - k} } X \mathbf u^k\) | \(=\) | \(\ds \map {e_{n - i} } {X \setminus \set {\mathbf u} }\) | Eisinberg (1981) |
Proof of Lemma 2:
Let $S$ denote the left hand side of $(3)$.
Let $U = X \setminus \set {\mathbf u}$.
Then:
\(\ds S\) | \(=\) | \(\ds \paren {-1}^{n - i} \mathbf u^{n - i} + \sum_{k \mathop = 0}^{n - i - 1} \paren {-1}^k \map {e_{n - i - k} } X {\mathbf u}^k\) | splitting off the term for $k = n - i$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {-1}^{n - i} \, {\mathbf u}^{n - i} + \sum_{k \mathop = 0}^{n - i - 1} \paren {-1}^k \, \paren {\mathbf u \, \map {e_{n - i - k - 1} } U + \map {e_{n - i - k} } U} \, \mathbf u^k\) | by $(2)$ in Lemma 1 with $p = n - 1$ and $m = n - i - k$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 0}^{n - i - 1} \paren {-1}^k \, \map {e_{n - i - k - 1} } U \mathbf u^{k + 1} + \sum_{k \mathop = 0}^{n - i} \paren {-1}^k \map {e_{n - i - k} } U \, \mathbf u^k\) | reassembling summations | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^{n - i} \paren {-1}^{k - 1} \, \map {e_{n - i - k} } U \mathbf u^k + \sum_{k \mathop = 0}^{n - i} \paren {-1}^k \map {e_{n - i - k} } U \, \mathbf u^k\) | reindexing the first sum | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^{n - i} \paren {-1}^k \paren {-\map {e_{n - i - k} } U + \map {e_{n - i - k} } U} \mathbf u^k + \paren {-1}^0 \map {e_{n - i} } U \mathbf u^0\) | splitting off the term for $k = 0$ and collecting under one summation | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {e_{n-i} } U\) |
$\Box$
Proof of the Theorem
\(\ds d_{ij}\) | \(=\) | \(\ds \dfrac {\ds \paren {-1}^{n - i} \map { e_{n - i} } {X \setminus \set {x_j} } } {\ds \prod_{m \mathop = 1, m \mathop \ne j }^n \paren {x_j - x_m} }\) | transposing $W_n$, then applying corollary to Inverse of Vandermonde Matrix | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {\ds \sum_{k \mathop = 0}^{n - i} \paren {-1}^k \, \map {e_{n - i - k} } X \, x_j^k} {\ds \prod_{m \mathop = 1, m \mathop \ne j }^n \paren {x_j - x_m} }\) | $(3)$ in Lemma 2 | |||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac {\ds \sum_{k \mathop = 0}^{n - i} a_{i + k} \, x_j^k} {\ds \prod_{m \mathop = 1, m \mathop \ne j}^n \paren {x_j - x_m} }\) | Viète's Formulas |
$\blacksquare$
Sources
- 1981: A. Eisinberg and C. Picardi: On the inversion of Vandermonde matrix (Proceedings of the 8th Triennal IFAC World Congress, Kyoto, Japan pp. 507 – 511)
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.3$: Sums and Products: Exercise $40$
- 1998: A. Eisinberg, G. Franzé and P. Pugliese: Vandermonde matrices on Chebyshev points (Linear Algebra and its Applications Vol. 283: pp. 205 – 219)
- April 2005: A. Eisinberg and G. Fedele: A property of the elementary symmetric functions (Calcolo Vol. 42, no. 1: pp. 31 – 36)