Nilpotent matrix
In linear algebra, a nilpotent matrix is a square matrix N such that
for some positive integer . The smallest such is called the index of ,[1] sometimes the degree of .
More generally, a nilpotent transformation is a linear transformation of a vector space such that for some positive integer (and thus, for all ).[2][3][4] Both of these concepts are special cases of a more general concept of nilpotence that applies to elements of rings.
Examples
Example 1
The matrix
is nilpotent with index 2, since .
Example 2
More generally, any -dimensional triangular matrix with zeros along the main diagonal is nilpotent, with index . For example, the matrix
is nilpotent, with
The index of is therefore 4.
Example 3
Although the examples above have a large number of zero entries, a typical nilpotent matrix does not. For example,
although the matrix has no zero entries.
Example 4
Additionally, any matrices of the form
such as
or
square to zero.
Example 5
Perhaps some of the most striking examples of nilpotent matrices are square matrices of the form:
The first few of which are:
These matrices are nilpotent but there are no zero entries in any powers of them less than the index.[5]
Example 6
Consider the linear space of polynomials of a bounded degree. The derivative operator is a linear map. We know that applying the derivative to a polynomial decreases its degree by one, so when applying it iteratively, we will eventually obtain zero. Therefore, on such a space, the derivative is representable by a nilpotent matrix.
Characterization
For an square matrix with real (or complex) entries, the following are equivalent:
- is nilpotent.
- The characteristic polynomial for is .
- The minimal polynomial for is for some positive integer .
- The only complex eigenvalue for is 0.
The last theorem holds true for matrices over any field of characteristic 0 or sufficiently large characteristic. (cf. Newton's identities)
This theorem has several consequences, including:
- The index of an nilpotent matrix is always less than or equal to . For example, every nilpotent matrix squares to zero.
- The determinant and trace of a nilpotent matrix are always zero. Consequently, a nilpotent matrix cannot be invertible.
- The only nilpotent diagonalizable matrix is the zero matrix.
See also: Jordan–Chevalley decomposition#Nilpotency criterion.
Classification
Consider the (upper) shift matrix:
This matrix has 1s along the superdiagonal and 0s everywhere else. As a linear transformation, the shift matrix "shifts" the components of a vector one position to the left, with a zero appearing in the last position:
This matrix is nilpotent with degree , and is the canonical nilpotent matrix.
Specifically, if is any nilpotent matrix, then is similar to a block diagonal matrix of the form
where each of the blocks is a shift matrix (possibly of different sizes). This form is a special case of the Jordan canonical form for matrices.[7]
For example, any nonzero 2 × 2 nilpotent matrix is similar to the matrix
That is, if is any nonzero 2 × 2 nilpotent matrix, then there exists a basis b1, b2 such that Nb1 = 0 and Nb2 = b1.
This classification theorem holds for matrices over any field. (It is not necessary for the field to be algebraically closed.)
Flag of subspaces
A nilpotent transformation on naturally determines a flag of subspaces
and a signature
The signature characterizes up to an invertible linear transformation. Furthermore, it satisfies the inequalities
Conversely, any sequence of natural numbers satisfying these inequalities is the signature of a nilpotent transformation.
Additional properties
- If is nilpotent of index , then and are invertible, where is the identity matrix. The inverses are given by
- If is nilpotent, then
Conversely, if is a matrix and
- Every singular matrix can be written as a product of nilpotent matrices.[8]
- A nilpotent matrix is a special case of a convergent matrix.
Generalizations
A linear operator is locally nilpotent if for every vector , there exists a such that
For operators on a finite-dimensional vector space, local nilpotence is equivalent to nilpotence.
Notes
- Herstein (1975, p. 294)
- Beauregard & Fraleigh (1973, p. 312)
- Herstein (1975, p. 268)
- Nering (1970, p. 274)
- Mercer, Idris D. (31 October 2005). "Finding "nonobvious" nilpotent matrices" (PDF). idmercer.com. self-published; personal credentials: PhD Mathematics, Simon Fraser University. Retrieved 5 April 2023.
- Beauregard & Fraleigh (1973, p. 312)
- Beauregard & Fraleigh (1973, pp. 312, 313)
- R. Sullivan, Products of nilpotent matrices, Linear and Multilinear Algebra, Vol. 56, No. 3
References
- Beauregard, Raymond A.; Fraleigh, John B. (1973), A First Course In Linear Algebra: with Optional Introduction to Groups, Rings, and Fields, Boston: Houghton Mifflin Co., ISBN 0-395-14017-X
- Herstein, I. N. (1975), Topics In Algebra (2nd ed.), John Wiley & Sons
- Nering, Evar D. (1970), Linear Algebra and Matrix Theory (2nd ed.), New York: Wiley, LCCN 76091646