L_POLYCOMPAN

=L_POLYCOMPAN(coeffs)

Description

Finding the roots of a polynomial involves finding the eigenvalues of the corresponding companion matrix C(p).

Keeping with the same definition for a polynomial as the other functions in the library, p is a row vector of coefficients in decreasing order of power such that p(x) = β_n*x_n + β_n-1*x_n-1 + … + β₂*x₂ + β₁*x₁ + β₀.

The first row of the polynomial companion matrix is the subvector m(x) of length n where the elements are equal to m_k=-β_k/β_n+1 for k from n to 1.

m(x) = [ -βn-1/βn ... -β2/βn -β1/βn -β0/βn ]

$$C(p) = \begin{bmatrix}-{\beta}_{n-1}/{\beta}_n & \cdots & -{\beta}_2/{\beta}_1 & -{\beta}_1/{\beta}_n & -{\beta}_0/{\beta}_n \\ 1 & \cdots & 0 & 0 & 0 \\ \vdots & \ddots & \vdots & \vdots & \vdots \\ 0 & \cdots & 1 & 0 & 0 \\ 0 & \cdots & 0 & 1 & 0 \end{bmatrix}$$

The rest of the companion matrix is assembled using the identity matrix and zeros. This form of the companion matrix is designed to be similar to that used in the scipy.linalg.companion function in SciPy and the compan function in Matlab.

Lambda Formula

This code for using L_POLYCOMPAN in Excel is provided under the License as part of the LAMBDA Library, but to use just this function, you may copy the following code directly into your spreadsheet.

/**
* Returns the companion matrix for the polynomial defined by coeffs
*/
L_POLYCOMPAN = LAMBDA(coeffs,
LET(doc,"https://www.vertex42.com/lambda/polycompan.html",
    coeffs,IF(AND(ROWS(coeffs)>1,COLUMNS(coeffs)=1),TRANSPOSE(coeffs),coeffs),
    n,COLUMNS(coeffs)-1,
    m,-CHOOSECOLS(coeffs,SEQUENCE(1,n,2,1))/INDEX(coeffs,1),
    VSTACK(m,HSTACK(MUNIT(n-1),SEQUENCE(n-1,1,0,0)))
));

Name: L_POLYCOMPAN
Description: Returns the companion matrix for a vector of polynomial coefficients
Arguments: coeffs
Function:

=LET(doc,"https://www.vertex42.com/lambda/polycompan.html",
    coeffs,IF(AND(ROWS(coeffs)>1,COLUMNS(coeffs)=1),TRANSPOSE(coeffs),coeffs),
    n,COLUMNS(coeffs)-1,
    m,ARRAYFORMULA(-CHOOSECOLS(coeffs,SEQUENCE(1,n,2,1))/INDEX(coeffs,1)),
    VSTACK(m,HSTACK(MUNIT(n-1),SEQUENCE(n-1,1,0,0)))
)

L_POLYCOMPAN Examples

Test: Copy and Paste this LET function into a cell
=LET(
    coeffs, {2,3,-9,10},
    L_POLYCOMPAN(coeffs)
)

Result:
{-1.5, 4.5,  5;
    1,   0,  0;
    0,   1,  0}