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In mathematics, a 'definite bilinear form' is a bilinear form ''B'' such that
:''B''(''x'', ''x'')
has a fixed sign (positive or negative) when ''x'' 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 form which is Hermitian in the sense that ''B''(''x'', ''y'') is always the complex conjugate of ''B''(''y'', ''x''). Then ''B'' is called 'positive definite' if
:''B''(''x'', ''x'') > 0
for every nonzero ''x'' in ''V''. If ''B''(''x'', ''x'') ≥ 0 for all ''x'', ''B'' is said to be 'positive semidefinite'. 'Negative definite' and 'negative semidefinite' bilinear forms are defined similarly. If ''B''(''x'', ''x'') takes both positive and negative values, it is called 'indefinite'.
As an example, let ''V''='R'², and consider the bilinear form
:$B(x,\; y)=c\_1x\_1y\_1+c\_2x\_2y\_2$
where $x=(x\_1,\; x\_2)$, $y=(y\_1,\; y\_2)$, and $c\_1$ and $c\_2$ are constants. If $c\_1>0$ and $c\_2>0$, the bilinear form $B$ is positive definite. If one of the constants is positive and the other is zero, then $B$ is positive semidefinite. If $c\_1>0$ and , then $B$ is indefinite.
Given a Hermitian bilinear form $B$, the function
: $Q(x)=B(x,\; x)$
is a quadratic form. The definitions of definiteness for $B$ are then transferred to corresponding definitions for $Q.$
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.

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See also

See also

★ positive definite function

★ positive definite matrix