In mathematics, a definite bilinear form B is one for which
- B(v, v)
has a fixed sign (positive or negative) when it is not 0.
To give a formal definition, let K be one of the fields R (real numbers) or C (complex numbers). Suppose that V is a vector space over K, and
- B : V × V → K
is a bilinear map which is Hermitian in the sense that B(x, y) is always the complex conjugate of B(y, x). Then B is positive-definite if
- B(x, x) > 0
for every nonzero x in V. If it is greater than or equal to zero, we say B is positive semidefinite. Similarly for negative definite and negative semidefinite. If it is otherwise unconstrained, we say B is indefinite.
A self-adjoint operator A on an inner product space is positive-definite if
- (x, Ax) > 0 for every nonzero vector x.
See in particular positive-definite matrix.
See also