Level set

For the computational technique, see Level set method.

For level surfaces of force fields, see equipotential surface.

In mathematics, a level set of a real-valued function f of n variables is a set of the form

$L_c(f) = \left\{ (x_1, \cdots, x_n) \, \mid \, f(x_1, \cdots, x_n) = c \right\}~,$

that is, a set where the function takes on a given constant value c.

When the number of variables is two, a level set is generically a curve, called a level curve, contour line, or isoline. When n = 3, a level set is called a level surface (see also isosurface), and for higher values of n the level set is a level hypersurface.

A set of the form

$L_c^-(f) = \left\{ (x_1, \cdots, x_n) \, \mid \, f(x_1, \cdots, x_n) \leq c \right\}$

is called a sublevel set of f (or, alternatively, a lower level set or trench of f).

$L_c^%2B(f) = \left\{ (x_1, \cdots, x_n) \, \mid \, f(x_1, \cdots, x_n) \geq c \right\}$

is called a superlevel set of f.^[1]^[2]

A level set is a special case of a fiber.

Properties

The gradient of f at a point is perpendicular to the level set of f at that point.

The sublevel sets of a convex function are convex (the converse is however not generally true).

References

^ Voitsekhovskii, M.I. (2001), "Level set", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1556080104, http://www.encyclopediaofmath.org/index.php?title=L/l058220
^ Weisstein, Eric W., "Level Set" from MathWorld.

Level set

Properties

See also

References