arXiv subjects
Publications and source records attributed to Ariel Sánchez.
We consider the following quasilinear reaction-diffusion equation $$ \partial_tu=Δu^m+(1+|x|)^σu^p, \quad (x,t)\in\mathbb{R}^N\times(0,\infty), $$ in dimension $N\geq3$ and in the range of exponents $1 -2$, illustrating the character of threshold of the exponent $σ=-2$.
The large time behavior of solutions to the following generalized Burgers-Fisher-KPP equation $$ \partial_tu=u_{xx}+k(u^n)_x+u^p-u^q, \quad (x,t)\in\mathbb{R}\times(0,\infty), $$ with $n\geq2$, $p>q\geq1$ and $k\in\mathbb{R}$, is considered in this work. Denoting by $H(x,t)$, respectively $\widetilde{H}(x,t)$ the solutions having as initial condition the Heaviside, respectively the ``anti-Heaviside" functions $$ H_0(x)=\begin{cases} 0, & \mbox{if } x<0 1, & \mbox{if } x\geq0. \end{cases}, \quad \widetilde{H}_0(x)=1-H_0(x), $$ critical velocities $\overline{c}$, respectively $\widetilde{c}=kn+2\sqrt{p-q}$, are identified such that $H(x,t)$, respectively $\widetilde{H}(x,t)$ approach the unique traveling wave solution of the equation with these critical velocities as $t\to\infty$. The critical velocity $\overline{c}$ is \emph{anomalous}, that is, it cannot be made explicit by an algebraic expression. Assuming for simplicity $k>0$, a remarkable fact is that, while $\widetilde{H}(x,t)\to0$ as $t\to\infty$ uniformly on compact subsets of $\mathbb{R}$, the Heaviside solution $H$ might tend either to zero or to one as $t\to\infty$, depending on the sign of the critical velocity $\overline{c}$. This sign vary with respect to the exponents $n$, $p$, $q$ and the coefficient $k$ and, in fact, we prove that given $p$, $q$, $n$, there exists a critical coefficient $k^*(n,p,q)$ such that $\overline{c}>0$ if $k>k^*(n,p,q)$ and $\overline{c}<0$ if $k<k^*(n,p,q)$. The convergence to either zero or one reflects the sharp influence of the convection term, since in the absence of it (that is, $k=0$), $H(x,t)$ would always tend to zero as $t\to\infty$. The results include more general initial conditions than the Heaviside-type functions, and sharp estimates of the threshold coefficient $k^*(n,p,q)$ are also given.
Blow-up rates are established for general solutions to the quasilinear diffusion equation $$ \partial_tu=Δu^m+|x|^σu^p, \quad (x,t)\in\mathbb{R}^N\times(0,T), $$ in the range of exponents $1 0$. More precisely, if we consider a compactly supported solution $u(x,t)$ with blow-up time $T=T(u)\in(0,\infty)$, we derive the blow-up rate $$ C_1(T-t)^{-α}\leq \|u(x,t)\|_{\infty}\leq C_2(T-t)^{-α}, \quad t\in(0,T), $$ for some positive constants $C_1$, $C_2$, and the upper rate of expansion of the support $$ \sup\{|x|:u(x,t)>0\}\leq C_0(T-t)^{-β}, \quad t\in(0,T), $$ for some constant $C_0>0$, where $$ α=\frac{σ+2}{L}, \quad β=\frac{m-p}{L}, \quad L=σ(m-1)+2(p-1). $$ We also analyze the blow-up sets of solutions $u$, showing, under a suitable condition, that either $B(u)=\mathbb{R}^N$ or blow-up takes place only as $|x|\to\infty$.
Global self-similar solutions to the parabolic Hardy-Hénon equation $$ u_t=Δu^m+|x|^σu^p, \quad (x,t)\in\mathbb{R}^N\times(0,\infty), $$ are classified in the range of exponents $m\geq1$, $p>m$ and $σ>\max\{-2,-N\}$. The classification varies strongly with respect to the celebrated \emph{Fujita} and \emph{Sobolev critical exponents} $$ p_F(σ)=m+\frac{σ+2}{N}, \quad p_S(σ)= \begin{cases} \frac{m(N+2σ+2)}{N-2}, & \mbox{if } N\geq3, \\[1mm] \infty, & \mbox{if } N\in\{1,2\}. \end{cases} $$ Indeed, if $p\in(p_F(σ),p_S(σ))$, both equations admit self-similar solutions with either compact support (if $m>1$) or Gaussian-like tail as $|x|\to\infty$ (if $m=1$), as well as a one-parameter family satisfying $$ u(x,t)\sim C|x|^{-(σ+2)/(p-m)}, \quad {\rm as} \ |x|\to\infty. $$ If $p\geq p_S(σ)$, there are only self-similar solutions with the latter algebraic tail, while for $m<p\leq p_F(σ)$ no global solutions exist. The results open the way for a deeper study of the role of these solutions in the dynamics of the Hardy-Hénon equations.
We classify the self-similar solutions presenting finite time blow-up to the parabolic Hardy-Hénon equation $$ \partial_tu=Δu+|x|^σu^p, \quad (x,t)\in\mathbb{R}^N\times(0,\infty), $$ in dimension $N\geq3$ and the range of exponents $$ σ\in(-2,\infty), \quad p>p_S(σ):=\frac{N+2σ+2}{N-2}. $$ We establish the \emph{existence of self-similar blow-up solutions for any $p>p_S(σ)$}, provided $σ\geq2$. Moreover, we prove that, if $k$ is any natural number and $σ\geq 4k-2$, the parabolic Hardy-Hénon equation has at least $k$ different self-similar blow-up solutions for any $p>p_S(σ)$. These results are in a stark contrast with the standard reaction-diffusion equation $$ \partial_tu=Δu+u^p, \quad (x,t)\in\mathbb{R}^N\times(0,\infty), $$ for which non-existence of any self-similar solution has been established, provided $p$ overpasses the Lepin exponent $p_L:=1+\frac{6}{N-10}$, $N\geq11$. For $σ\in(-2,2)$, we derive the expression of generalized Lepin exponents $p_L(σ)$ for $σ\in(0,2)$, respectively $\overline{p_L}(σ)$ for $σ\in(-2,0)$, and prove existence of self-similar solutions with finite time blow-up for $p\in(p_S(σ),p_L(σ))$, respectively $p\in(p_S(σ),\overline{p_L}(σ))$. Numerical evidence of the optimality of these exponents is also included.
Traveling wave solutions, in the form $u(x,t)=f(x+ct)$, to the generalized Burgers-Fisher equation $$ \partial_tu=u_{xx}+k(u^n)_x+u^p-u^q, \quad (x,t)\in\mathbb{R}\times(0,\infty), $$ with $n\geq2$, $p>q\geq1$ and $k>0$, are classified with respect to their speed $c\in(-\infty,\infty)$ and the behavior at $\pm\infty$. The existence and uniqueness of traveling waves with any speed $c\in\mathbb{R}$ is established and their behavior as $x\to\pm\infty$ is described. In particular, it is shown that there exists a unique $c^*\in(0,\infty)$ such that there exists a unique soliton $f^*$ with speed $c^*$ and such that $$ \lim\limits_{ξ\to-\infty}f^*(ξ)=\lim\limits_{ξ\to\infty}f^*(ξ)=0, \quad ξ=x+ct. $$ Moreover, if $n p+q+1$ then $c^*>kn$. For $c<\min\{c^*,kn\}$, any traveling wave with speed $c$ satisfies $\lim\limits_{ξ\to-\infty}f(ξ)=0$ and $\lim\limits_{ξ\to\infty}f(ξ)=1$, while for $c>\max\{c^*,kn\}$ any traveling wave with speed $c$ satisfies $\lim\limits_{ξ\to-\infty}f(ξ)=1$ and $\lim\limits_{ξ\to\infty}f(ξ)=0$. In particular, for any speed $c\in(0,c^*)$, there are traveling wave solutions $u$ with speed $c$ such that $u(x,t)\to1$ as $t\to\infty$, in contrast to the non-convective case $k=0$.
The large time behavior of general solutions to a class of quasilinear diffusion equations with a weighted source term $$ \partial_tu=Δu^m+\varrho(x)u^p, \quad (x,t)\in\mathbb{R}^N\times(0,\infty), $$ with $m>1$, $1<p<m$ and suitable functions $\varrho(x)$, is established. More precisely, we consider functions $\varrho\in C(\mathbb{R}^N)$ such that $$ \lim\limits_{|x|\to\infty}(1+|x|)^{-σ}\varrho(x)=A\in(0,\infty), $$ with $σ\in(\max\{-N,-2\},0)$ such that $L:=σ(m-1)+2(p-1)<0$. We show that, for all these choices of $\varrho$, solutions with initial conditions $u_0\in C(\mathbb{R}^N)\cap L^{\infty}(\mathbb{R}^N)\cap L^r(\mathbb{R}^N)$ for some $r\in[1,\infty)$ are global in time and, if $u_0$ is compactly supported, present the asymptotic behavior $$ \lim\limits_{t\to\infty}t^{-α}\|u(t)-V_*(t)\|_{\infty}=0, $$ where $V_*$ is a suitably rescaled version of the unique compactly supported self-similar solution to the equation with the singular weight $\varrho(x)=|x|^σ$: $$ U_*(x,t)=t^αf_*(|x|t^{-β}), \qquad α=-\frac{σ+2}{L}, \quad β=-\frac{m-p}{L}. $$ This behavior is an interesting example of \emph{asymptotic simplification} for the equation with a regular weight $\varrho(x)$ towards the singular one as $t\to\infty$.
A new transformation for radially symmetric solutions to the subcritical fast diffusion equation with spatially inhomogeneous source $$ \partial_tu=Δu^m+|x|^σu^p, $$ posed for $(x,t)\in\mathbb{R}^N\times(0,\infty)$ and with dimension and exponents $$ N\geq3, \quad 0 \max\{1,p_L(σ)\}$, having as starting point previous results by the authors.
Solutions in self-similar form, either global in time or presenting finite time blow-up, to the supercritical fast diffusion equation with spatially inhomogeneous source $$ \partial_tu=Δu^m+|x|^σu^p, \quad (x,t)\in\mathbb{R}^N\times(0,\infty) $$ with $$ m_c=\frac{(N-2)_+}{N}\leq m<1, \quad σ\in(\max\{-2,-N\},\infty), \quad p>\max\left\{1+\frac{σ(1-m)}{2},1\right\} $$ are considered. It is proved that global self-similar solutions with the specific tail behavior $$ u(x,t)\sim C(m)|x|^{-2/(1-m)}, \qquad {\rm as} \ |x|\to\infty $$ exist exactly for $p\in(p_F(σ),p_s(σ))$, where $$ p_F(σ)=m+\frac{σ+2}{N}, \qquad p_s(σ)=\left\{\begin{array}{ll}\frac{m(N+2σ+2)}{N-2}, & N\geq3,\\\infty, & N\in\{1,2\}, \end{array}\right. $$ are the renowned Fujita and Sobolev critical exponents. In contrast, it is shown that self-similar solutions presenting finite time blow-up exist for any $σ\in(-2,0)$ and $p$ as above, but do not exist for any $σ\geq0$ and $p\in(p_F(σ),p_s(σ))$. We stress that all these results are \emph{new also in the homogeneous case $σ=0$}.
The following Fisher-KPP type equation $$ u_t=Ku_{xx}-Bu^q+Au^p, \quad (x,t)\in\real\times(0,\infty), $$ with $p>q>0$ and $A$, $B$, $K$ positive coefficients, is considered. For both $p>q>1$ and $p>1$, $q=1$, we construct stationary solutions, establish their behavior as $|x|\to\infty$ and prove that they are separatrices between solutions decreasing to zero in infinite time and solutions presenting blow-up in finite time. We also establish decay rates for the solutions that decay to zero as $t\to\infty$.
Some qualitative properties of radially symmetric solutions to the non-homogeneous heat equation with critical density and weighted source $$ |x|^{-2}\partial_tu=Δu+|x|^σu^p, \quad (x,t)\in\mathbb{R}^N\times(0,T), $$ are obtained, in the range of exponents $p>1$, $σ\ge-2$. More precisely, we establish conditions fulfilled by the initial data in order for the solutions to either blow-up in finite time or decay to zero as $t\to\infty$ and, in the latter case, we also deduce decay rates and large time behavior. In the limiting case $σ=-2$ we prove the existence of non-trivial, non-negative solutions, in stark contrast to the homogeneous case. A transformation to a generalized Fisher-KPP equation is derived and employed in order to deduce these properties.
Existence of a specific family of \emph{eternal solutions} in exponential self-similar form is proved for the following porous medium equation with strong absorption $$\partial_t u-Δu^m+|x|^σu^q = 0 \;\;\text{ in }\;\; (0,\infty)\times\mathbb{R}^N,$$ with $m>1$, $q\in(0,1)$ and $σ=2(1-q)/(m-1)$. Looking for solutions of the form $$ u(t,x)=e^{-αt}f(|x|e^{βt}), \qquad α=\frac{2}{m-1}β,$$ it is shown that, for $m+q>2$, there exists a unique exponent $β_*\in(0,\infty)$ for which there exists a one-parameter family of compactly supported profiles presenting a \emph{dead core}. The precise behavior of the solutions at their interface is also determined. Moreover, these solutions show the optimal limitations for the finite time extinction property of genuine non-negative solutions to the Cauchy problem, studied in previous works.
We classify radially symmetric self-similar profiles presenting finite time blow-up to the quasilinear diffusion equation with weighted source $$ u_t=Δu^m+|x|^σu^p, $$ posed for $(x,t)\in\real^N\times(0,T)$, $T>0$, in dimension $N\geq1$ and in the range of exponents $-2<σ<\infty$, $1<m<p<p_s(σ)$, where $$ p_s(σ)=\left\{\begin{array}{ll}\frac{m(N+2σ+2)}{N-2}, & N\geq3,\\ +\infty, & N\in\{1,2\},\end{array}\right. $$ is the renowned Sobolev critical exponent. The most interesting result is the \emph{multiplicity of two different types} of self-similar profiles for $p$ sufficiently close to $m$ and $σ$ sufficiently close to zero in dimension $N\geq2$, including \emph{dead-core profiles}. For $σ=0$, this answers in dimension $N\geq2$ a question still left open in \cite[Section IV.1.4, pp. 195-196]{S4}, where only multiplicity in dimension $N=1$ had been established. Besides this result, we also prove that, for any $σ\in(-2,0)$, $N\geq1$ and $m<p<p_s(σ)$ \emph{existence} of at least a self-similar blow-up profile is granted. In strong contrast with the previous results, given any $N\geq1$, $σ\geqσ^*=(mN+2)/(m-1)$ and $p\in(m,p_s(σ))$, \emph{non-existence} of any radially symmetric self-similar profile is proved.
We establish the existence of self-similar solutions presenting finite time blow-up to the quasilinear reaction-diffusion equation $$ u_t=Δu^m + u^p, $$ posed in dimension $N\geq3$, $m>1$. More precisely, we show that there is always at least one solution in backward self-similar form if $p>p_s=m(N+2)/(N-2)$. In particular, this establishes \emph{non-optimality of the Lepin critical exponent} introduced in \cite{Le90} in the semilinear case $m=1$ and extended for $m>1$ in \cite{GV97, GV02}, for the existence of self-similar blow-up solutions. We also prove that there are multiple solutions in the same range, provided $N$ is sufficiently large. This is in strong contrast with the semilinear case, where the Lepin critical exponent has been proved to be optimal.
Well-posedness and a number of qualitative properties for solutions to the Cauchy problem for the following nonlinear diffusion equation with a spatially inhomogeneous source $$ \partial_tu=Δu^m+|x|^σu^p, $$ posed for $(x,t)\in\mathbb{R}^N\times(0,T)$, with exponents $1 0$, are established. More precisely, we identify the \emph{optimal class of initial conditions} $u_0$ for which (local in time) existence is ensured and prove \emph{non-existence of solutions} for the complementary set of data. We establish then (local in time) \emph{uniqueness and a comparison principle} for this class of data. We furthermore prove that any non-trivial solution to the Cauchy problem \emph{blows up in a finite time} $T\in(0,\infty)$ and \emph{finite speed of propagation} holds true for $t\in(0,T)$: if $u_0\in L^{\infty}(\mathbb{R}^N)$ is an initial condition with compact support and blow-up time $T>0$, then $u(t)$ is compactly supported for $t\in(0,T)$. We also establish in this work the \emph{absence of localization at the blow-up time} $T$ for solutions stemming from compactly supported data.
We study the existence and qualitative properties of solutions to the Cauchy problem associated to the quasilinear reaction-diffusion equation $$ \partial_tu=Δu^m+(1+|x|)^σu^p, $$ posed for $(x,t)\in\real^N\times(0,\infty)$, where $m>1$, $p\in(0,1)$ and $σ>0$. Initial data are taken to be bounded, non-negative and compactly supported. In the range when $m+p\geq2$, we prove \emph{local existence of solutions} together with a \emph{finite speed of propagation} of their supports for compactly supported initial conditions. We also show in this case that, for a given compactly supported initial condition, there exist \emph{infinitely many solutions} to the Cauchy problem, by prescribing the evolution of their interface. In the complementary range $m+p<2$, we establish new \emph{Aronson-Bénilan estimates} satisfied by solutions to the Cauchy problem, which are of independent interest as a priori bounds for the solutions. We apply these estimates to establish \emph{infinite speed of propagation} of the supports of solutions if $m+p<2$, that is, $u(x,t)>0$ for any $x\in\real^N$, $t>0$, even in the case when the initial condition $u_0$ was compactly supported.
We establish both extinction and non-extinction self-similar profiles for the following fast diffusion equation with a weighted source term $$ \partial_tu=Δu^m+|x|^σu^p, $$ posed for $(x,t)\in\real^N\times(0,\infty)$, $N\geq3$, in the sub-critical range of the fast diffusion equation $0 0$ and $\max\{p_c(σ),1\} 0$, in the form $$ u(x,t)=t^αf(|x|t^β), \qquad f(ξ)\sim Cξ^{-(N-2)/m}, \qquad α>0, \ β>0 $$ exist, provided $0<m<m_s=(N-2)/(N+2)$ and $p_s(σ)=m(N+2σ+2)/(N-2)<p<p_L(σ)$. On the other hand, we prove that there exists $p_0(σ)\in(p_c(σ),p_s(σ))$ such that self-similar solutions presenting finite time extinction are established both for $p\in(p_0(σ),p_s(σ))$ and for $p\in(p_s(σ),p_L(σ))$, but with profiles $f(ξ)$ having different spatially decreasing tails as $|x|\to\infty$. We also prove non-existence of self-similar solutions in complementary ranges of exponents to the ones described above or if $m\geq m_c$.
Some transformations acting on radially symmetric solutions to the following class of non-homogeneous reaction-diffusion equations $$ |x|^{σ_1}\partial_tu=Δu^m+|x|^{σ_2}u^p, \qquad (x,t)\in\real^N\times(0,\infty), $$ which has been proposed in a number of previous mathematical works as well as in several physical models, are introduced. We consider here $m\geq1$, $p\geq1$, $N\geq1$ and $σ_1$, $σ_2$ real exponents. We apply these transformations in connection to previous results on the one hand to deduce general qualitative properties of radially symmetric solutions and on the other hand to construct self-similar solutions which are expected to be patterns for the dynamics of the equations, strongly improving the existing theory. We also introduce mappings between solutions which work in the semilinear case $m=1$.