A quadratic form 
is indefinite if it is less than 0 for some values and
greater than 0 for others. The quadratic form,
written in the form 
, is indefinite if eigenvalues
of the matrix 
are of both signs.
Indefinite Quadratic Form
See also
Positive Definite Quadratic Form, Positive Semidefinite Quadratic FormExplore with Wolfram|Alpha
References
Gradshteyn, I. S. and Ryzhik, I. M. Tables of Integrals, Series, and Products, 6th ed. San Diego, CA: Academic Press, p. 1106, 2000.Referenced on Wolfram|Alpha
Indefinite Quadratic FormCite this as:
Weisstein, Eric W. "Indefinite Quadratic Form." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/IndefiniteQuadraticForm.html