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For $i{u}_t+\Delta u+|u{|}^{p-1}u=0$, a smooth real weight $\chi$ defines the localized mass $I_{\chi }(t)=\int \chi |u{|}^2$. For a sufficiently regular decaying solution and a weight with the required integrability,andThese formulas follow by substituting the nonlinear Schrödinger equation and integration by parts. The Hessian controls the gradient contribution; the fourth derivative term records variation of the cutoff.

Choose ${\chi }_R(x)=R^2\chi (x/R)$, with $\chi (x)=|x{|}^2$ near zero and constant outside a larger ball. The interior terms reproduce the virial identity, while the remaining terms are supported in the transition region or controlled by spatial tails. A bounded weight makes $I_{{\chi }_R}$ meaningful for finite mass without requiring finite variance. A blowup proof still needs estimates that control those tail errors.

For a finite-variance solution of the mass-critical focusing nonlinear Schrödinger equation, the virial identity givesIf $E(u_0)<0$, the right-hand side is a negative constant. The nonnegative variance would then become negative in finite time if the solution remained regular, so the solution blows up in finite time.

Nonlinear analysis studies equations and variational problems for which the superposition principle fails. If $u$ and $v$ solve an equation, their sum or a scalar multiple need not solve it. For the ordinary differential equation $u^{\prime }=u^2$ with $u(0)=1$, the solution $u(t)=1/(1-t)$ exists for $t<1$ and becomes unbounded at $t=1$. Multiplying this solution by two changes the two sides by different factors, so it does not produce another solution of the same equation.

The questions extend beyond finding a formula: does a solution exist, is it unique, how long does it remain regular, and does it stay near a stable shape? Nonlinear partial differential equations such as the Generalized Korteweg–De Vries equation and the mass-critical focusing nonlinear Schrödinger equation combine local evolution with conservation laws, compactness and functional inequalities.

A variational method can construct a ground state by minimizing an energy functional. A virial identity can instead show that a regular solution cannot persist indefinitely. Scaling and sharp constants identify the thresholds at which the same conserved quantity changes from controlling an evolution to permitting concentration.

Nonlinear Schrödinger blowup analysis asks whether a solution remains in its local well-posedness class or concentrates so strongly that continuation fails in finite time. For the mass-critical focusing nonlinear Schrödinger equation in the standard $H^1$ theory, finite-time failure forces the gradient L2 norm to diverge, even though Mass conservation for the nonlinear Schrödinger equation keeps the total squared L2 norm constant.

Concentration and total size are different. The mass-preserving scaling $u_{\lambda }(x)={\lambda }^{d/2}u(\lambda x)$ leaves $\Vert u_{\lambda }{\Vert }_2=\Vert u{\Vert }_2$ while $\Vert \nabla u_{\lambda }{\Vert }_2=\lambda \Vert \nabla u{\Vert }_2$. Thus a fixed amount of mass can occupy smaller spatial scales with larger gradients.

The Sharp Gagliardo-Nirenberg inequality gives coercivity below the NLS ground-state mass, while a virial identity can force finite-time failure for suitable negative-energy, finite-variance data. Near the threshold, profile decomposition modulo translations separates a concentrating profile from dispersive remainder. These arguments track the structure of a solution rather than merely the maximum of a plotted wave.

LetThe first virial identity, obtained from the equation by integration by parts, isDifferentiating once more givesWrite $\varphi =W\ast |u{|}^2$, where $W(x)=-1/(C_4|x{|}^2)$ is homogeneous of degree $-2$. Symmetrizing the double integral and applying Euler's identity $z\cdot \nabla W(z)=-2W(z)$ yieldsTherefore

Not all solutions are global. Choose smooth finite-variance data of negative energy, which is possible by multiplying any nonzero test function by a sufficiently large constant: the kinetic term is quadratic in the amplitude and the attractive potential term is quartic. If such a solution were global, the Virial identity for the four-dimensional gravitational Hartree equation would make the nonnegative function $I(t)$ strictly concave with constant negative second derivative, forcing it below zero in finite time. The solution must therefore blow up in finite time.

For a sufficiently regular solution of the mass-critical focusing nonlinear Schrödinger equation with finite variance, the variance and its first derivative areA second differentiation and integration by parts give the virial identityThese constants use Laplacian coefficient one in the evolution equation. Energy conservation makes the second derivative constant. If the energy is negative, the resulting quadratic expression for $I(t)$ eventually becomes negative, contradicting its definition as a nonnegative integral; this proves negative-energy blowup for the mass-critical focusing nonlinear Schrödinger equation under the finite-variance hypotheses.

The weighted moment condition is additional to an H1 norm bound. A localized virial identity replaces the unbounded weight by a smooth cutoff when only tail control is available.