Math 2.0

Linear and multilinear algebra; matrix theory Example

Circulant matrix

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Example

Let FF be a field and let c0,c1,…,cn−1∈Fc_0, c_1, \dots, c_{n-1} \in F. The circulant matrix with first column (c0,c1,…,cn−1)T(c_0, c_1, \dots, c_{n-1})^T is the n×nn \times n matrix CC with entries Cij=c(i−j) mod nC_{ij} = c_{(i - j) \bmod n}, for 0≤i,j≤n−10 \le i, j \le n - 1. Each column is the previous column shifted down by one place, with the last entry moved to the top. For n=3n = 3: C=(c0c2c1c1c0c2c2c1c0).C = \begin{pmatrix} c_0 & c_2 & c_1 \\ c_1 & c_0 & c_2 \\ c_2 & c_1 & c_0 \end{pmatrix}.

Remarks

Some authors define circulant matrices by their first row; the two conventions give transposed matrices. Over ℂ\C, the vectors (1,ωk,ω2k,…,ω(n−1)k)(1, \omega^k, \omega^{2k}, \dots, \omega^{(n-1)k}) with ω=e2πi/n\omega = e^{2\pi i/n} are eigenvectors of every n×nn \times n circulant matrix. The product of two circulant matrices of the same order is circulant, and circulant matrices commute with each other.

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Further reading

  1. Web https://en.wikipedia.org/wiki/Circulant_matrix, Definition
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