In
linear algebra, the
adjugate of a
square matrix is a matrix which plays a role similar to the inverse of a matrix; it can however be defined for any square matrix without the need to perform any divisions.
The adjugate has sometimes been called the "adjoint", but that terminology is ambiguous and is not used in Wikipedia. Today, "adjoint" normally refers to the complex conjugate.
Suppose R is a commutative ring and A is an n-by-n matrix with entries from R. The adjugate of A, written as adj(A), is the n-by-n matrix defined by
- adj(A)[i,j] = (-1)^{i+j} det(A(j|i))
where
A(
j|
i) denotes the (
n-1)-by-(
n-1) matrix obtained from
A by deleting row
j and column
i, and det(
A(
j|
i)) is its
determinant (the determinant of the 0-by-0 matrix being defined as 1).
As a consequence of Laplace's formula for the computation of determinants, we have
- A · adj(A) = adj(A) · A = det(A) I_{n}
where
I_{n} denotes the
n-by-
n identity matrix. This formula is used to prove that
A is invertible as a matrix over
R if and only if det(
A) is invertible as an element of
R.
We have
- adj(I_{n}) = I_{n}
and
- adj(AB) = adj(B) adj(A)
for all
n-by-
n matrices
A and
B. The adjugate is also compatible with
transposition:
- adj(A^{T}) = (adj(A))^{T}.
Furthermore,
- det(adj(A)) = det(A)^{n-1}.
If
p(
t) = det(
A -
tI_{n}) is the
characteristic polynomial of
A
and we define the polynomial
q(
t) = (
p(0) -
p(
t))/
t, then
- adj(A) = q(A).
The adjugate appears in the formula of the
derivative of the
determinant.
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