Critical

In the theory of nonlinear PDE, we use the terms sub-critical, critical, and super-critical to denote a significant transition in the behaviour of a particular equation with respect to a specified regularity class (or conserved quantity). Typically, subcritical equations behave in an approximately linear manner, supercritical equation behave in a highly nonlinear manner, and critical equations are very finely balanced between the two.

This distinction into sub-critical, critical, and super-critical behaviour can be made in several contexts:

• Given a fixed equation with a scale invariance (or approximate scale invariance), one can classify various quantities (such as homogeneous Sobolev norms $\|u(0)\|_{{\dot {H}}_{x}^{s}(R^{d})}$, or various conserved quantities) as sub-critical, critical, or super-critical with respect to this scaling invariance. Example: In the cubic NLS on R^3, the energy or ${\dot {H}}_{x}^{1}(R^{3})$ norm is sub-critical, the momentum or ${\dot {H}}^{1/2}(R^{3})_{x}$ norm is critical, and the mass or $L_{x}^{2}(R^{3})$ norm is super-critical.

• Conversely, given a fixed regularity class or conserved quantity, one can classify an equation as sub-critical, critical, or super-critical with respect to that class or quantity. Example: the NLW and NLS on $R^{3}$ is energy-subcritical for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle p < 5} , energy-critical for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle p=5} , and energy-supercritical for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle p>5} .

• Occasionally one also discusses the sub-criticality, criticality, or super-criticality of regularities with respect to other symmetries than scaling, such as Galilean invariance or Lorentz invariance.

• When analyzing the long-time asymptotics of nonlinear equations with data which is small and highly

localized, one can distinguish equations into scattering-subcritical (short-range), scattering-critical (critical-range), and scattering-supercritical (long-range) classes; this measures the relative asymptotic strength of the nonlinear and linear components of the equation, and is generally unrelated to the distinction arising from scale invariance or other symmetries. Example: The NLS on Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle R^3} is scattering-subcritical for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle p > 5/3} , scattering-critical for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle p = 5/3} , and scattering-supercritical for Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle p < 5/3} .

• In focusing equations with a critical conserved quantity (e.g. mass), the behavior for small values of this quantity is often quite different from that at large values. In many equations there is a critical value of this quantity, below which linear-type behavior is expected, and above which nonlinear behavior can occur. (When one is exactly at the critical value, soliton-type behavior is very typical.) Example: in a mass critical NLS, masses less than the mass Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M(Q)} of the ground state are considered subcritical, masses greater than Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M(Q)} are supercritical, and the mass Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M(Q)} itself is critical.

Criticality with respect to scaling

A large number of equations studied here enjoy a scale invariance, which typically takes a form such as

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u^{(\lambda)}(t,x) := \frac{1}{\lambda^a} u( \frac{t}{\lambda^b}, \frac{x}{\lambda^c} )}

where the scaling parameter Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \lambda > 0} is arbitrary, and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle a,b,c} are constants (with Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle b,c} positive) which depend on the various parameters of the equation (the linear and nonlinear components of the equation, and the ambient dimension). Thus any given solution $u$ has fine-scale counterparts Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u^{(\lambda)}} with Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \lambda \ll 1} , as well as coarse-scale counterparts Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u^{(\lambda)}} with Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \lambda \gg 1} . Typically, the fine-scale counterparts are rapidly oscillating, highly concentrated, and can exhibit nonlinear behaviour in short amounts of time, whereas the coarse-scale counterparts are very smooth, spread out over large regions of space and time, and behave linearly for long periods of time.

Many quantities $Q(u)$ associated to a solution, such as a spatial norm (e.g. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q(u) = \|u(0)\|_{\dot H^s_x(\R^d)}} ), a spacetime norm (e.g. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q(u) = \|u\|_{L^q_t L^r_x(\R \times \R^d)}} ), or a conserved quantity (e.g. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} could be mass, momentum, or energy) transform in a simple manner under the above scale invariance. Indeed, if Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} is ``homogeneous, then we obtain a relationship of the form

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q(u^{(\lambda)}) = \lambda^{\alpha} Q(u).}

for some constant Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \alpha} which depends on the scaling symmetry and on the degree of homogeneity of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} ; from a dimensional analysis perspective, Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \alpha} measures the dimension of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} in terms of the unit of length (which is represented here by Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \lambda} ). For instance, for the NLS, the mass

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M(u) := \int_{\R^d} |u(0,x)|^2\ dx}

interacts with the scaling invariance via the formula

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle M(u^{(\lambda)}) = \lambda^{d - \frac{4}{p-1}} M(u)}

so in this case $\alpha =d-{\frac {4}{p-1}}$. The sign of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \alpha} is then used to classify the quantity Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} :

• If Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \alpha < 0} , then the fine-scaled copy Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u^{(\lambda)}} has a much larger value of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} than the original solution Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u} . Hence, if we are keeping the value of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} fixed, then we do not expect fine-scale behavior to occur, while conversely we expect plenty of coarse-scale behavior. In this case we say that the quantity Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} is sub-critical for this equation, or equivalently that the equation is sub-critical with respect to this quantity.

• If Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle \alpha > 0} , then the fine-scaled copy Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u^{(\lambda)}} has a much smaller value of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} than the original solution Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle u} . Hence, if we are keeping the value of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://en.wikipedia.org/api/rest_v1/":): {\displaystyle Q} fixed, then we expect a substantial amount of fine-scale behavior to occur, though conversely we do not expect much coarse-scale behavior. In this case we say that the quantity $Q$ is super-critical for this equation, or equivalently that the equation is super-critical with respect to this quantity.

• If $\alpha =0$, then $Q$ is scale-invariant, and so for a fixed value of $Q$ we expect the same behavior at all scales. In this case we say that the quantity $Q$ is critical for this equation, or equivalently that the equation is super-critical with respect to this quantity.

The behavior of an equation at short and long time scales, and at given levels of regularity, is often controlled by the distinction into sub-critical, critical, and super-critical quantities via the scaling heuristic.

The division of quantities into sub-critical, critical, and super-critical behavior can also be extended somewhat to inhomogeneous quantities (such as inhomogeneous Sobolev norms $H_{x}^{s}(\mathbb {R} ^{d})$), for instance by splitting such quantities into homogeneous components of differing degrees. However, the fine-scale and coarse-scale behavior can then become distinct. For instance, for the cubic NLS on R^3, the $H_{x}^{1}(\mathbb {R} ^{3})$ norm is sub-critical at fine scales and super-critical at coarse scales, thus excluding both fine-scale and coarse-scale behaviour simultaneously. A similar analysis allows one to extend these useful concepts to the case when the equation itself is inhomogeneous (as is for instance the case with the nonlinear Klein-Gordon equation. See also high-frequency limit and low-frequency limit.

Criticality with respect to other symmetries

To be written...

Criticality with respect to scattering

To be written...

Critical values of mass and energy

To be written...