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Blowup for Biharmonic NLS

We consider the Cauchy problem for the biharmonic (i.\,e.~fourth-order) NLS with focusing nonlinearity given by $i \partial_t u = Δ^2 u - μΔu -|u|^{2 σ} u$ for $(t,x) \in [0,T) \times \mathbb{R}^d$, where $0 < σ<\infty$ for $d \leq 4$ and $0 < σ\leq 4/(d-4)$ for $d \geq 5$; and $μ\in \mathbb{R}$ is some parameter to include a possible lower-order dispersion. In the mass-supercritical case $σ> 4/d$, we prove a general result on finite-time blowup for radial data in $H^2(\mathbb{R}^d)$ in any dimension $d \geq 2$. Moreover, we derive a universal upper bound for the blowup rate for suitable $4/d < σ< 4/(d-4)$. In the mass-critical case $σ=4/d$, we prove a general blowup result in finite or infinite time for radial data in $H^2(\mathbb{R}^d)$. As a key ingredient, we utilize the time evolution of a nonnegative quantity, which we call the (localized) Riesz bivariance for biharmonic NLS. This construction provides us with a suitable substitute for the variance used for classical NLS problems. In addition, we prove a radial symmetry result for ground states for the biharmonic NLS, which may be of some value for the related elliptic problem.