Shift matrix

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In mathematics, a shift matrix is a binary matrix with ones only on the superdiagonal or subdiagonal, and zeroes elsewhere. A shift matrix U with ones on the superdiagonal is an upper shift matrix. The alternative subdiagonal matrix L is unsurprisingly known as a lower shift matrix. The (i,j):th component of U and L are

where is the Kronecker delta symbol.

For example, the 5×5 shift matrices are

Clearly, the transpose of a lower shift matrix is an upper shift matrix and vice versa.

As a linear transformation, a lower shift matrix shifts the components of a row vector one position to the right, with a zero appearing in the first position. An upper shift matrix shifts the components of a row vector one position to the left, with a zero appearing in the last position.[1]

Premultiplying a matrix A by a lower shift matrix results in the elements of A being shifted downward by one position, with zeroes appearing in the top row. Postmultiplication by a lower shift matrix results in a shift left. Similar operations involving an upper shift matrix result in the opposite shift.

Clearly all shift matrices are nilpotent; an n by n shift matrix S becomes the null matrix when raised to the power of its dimension n.

Properties[edit]

Let L and U be the n by n lower and upper shift matrices, respectively. The following properties hold for both U and L. Let us therefore only list the properties for U:

The following properties show how U and L are related:

  • The spectrum of U and L is . The algebraic multiplicity of 0 is n, and its geometric multiplicity is 1. From the expressions for the null spaces, it follows that (up to a scaling) the only eigenvector for U is , and the only eigenvector for L is .
  • For LU and UL we have
These matrices are both idempotent, symmetric, and have the same rank as U and L
  • Ln-aUn-a + LaUa = Un-aLn-a + UaLa = I (the identity matrix), for any integer a between 0 and n inclusive.

If N is any nilpotent matrix, then N is similar to a block diagonal matrix of the form

where each of the blocks S1S2, ..., Sr is a shift matrix (possibly of different sizes).[2][3]

Examples[edit]

Then

Clearly there are many possible permutations. For example, is equal to the matrix A shifted up and left along the main diagonal.

See also[edit]

Notes[edit]

  1. ^ Beauregard & Fraleigh (1973, p. 312)
  2. ^ Beauregard & Fraleigh (1973, pp. 312,313)
  3. ^ Herstein (1964, p. 250)

References[edit]

External links[edit]