Assumption on traveling wave solutions of Fisher's equation












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I have a question about Fisher's equation in a biology context.
For example, in Fisher's equation $u_{t} = Du_{xx} + u(1-u)$, where $u$ is a density of cell, the logistic term explains that the capacity is 1.



When we look for a traveling wave solution $U(x-ct)$ (or a heteroclinic orbit), $c$ is a speed, connecting $u=0$ and $u=1,$ do we assume that a traveling wave ansatz must satisfy $0leq U(x-ct) leq 1?$ Or, could it be $U(x-ct)>1?$



Thank you so much!










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    $begingroup$


    I have a question about Fisher's equation in a biology context.
    For example, in Fisher's equation $u_{t} = Du_{xx} + u(1-u)$, where $u$ is a density of cell, the logistic term explains that the capacity is 1.



    When we look for a traveling wave solution $U(x-ct)$ (or a heteroclinic orbit), $c$ is a speed, connecting $u=0$ and $u=1,$ do we assume that a traveling wave ansatz must satisfy $0leq U(x-ct) leq 1?$ Or, could it be $U(x-ct)>1?$



    Thank you so much!










    share|cite|improve this question









    $endgroup$















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      $begingroup$


      I have a question about Fisher's equation in a biology context.
      For example, in Fisher's equation $u_{t} = Du_{xx} + u(1-u)$, where $u$ is a density of cell, the logistic term explains that the capacity is 1.



      When we look for a traveling wave solution $U(x-ct)$ (or a heteroclinic orbit), $c$ is a speed, connecting $u=0$ and $u=1,$ do we assume that a traveling wave ansatz must satisfy $0leq U(x-ct) leq 1?$ Or, could it be $U(x-ct)>1?$



      Thank you so much!










      share|cite|improve this question









      $endgroup$




      I have a question about Fisher's equation in a biology context.
      For example, in Fisher's equation $u_{t} = Du_{xx} + u(1-u)$, where $u$ is a density of cell, the logistic term explains that the capacity is 1.



      When we look for a traveling wave solution $U(x-ct)$ (or a heteroclinic orbit), $c$ is a speed, connecting $u=0$ and $u=1,$ do we assume that a traveling wave ansatz must satisfy $0leq U(x-ct) leq 1?$ Or, could it be $U(x-ct)>1?$



      Thank you so much!







      ordinary-differential-equations pde mathematical-modeling biology






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      asked Aug 18 '18 at 22:42









      jp31433jp31433

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          $begingroup$

          Assuming this is a biological problem and the domain is infinite, then the form of the model that you used indicates a re-scale version, where carrying capacity $u$ is normalized to 1. If the initial profile $u(x,0) leq 1$ (biological), then $U(x-ct) leq 1$ for all $tgeq 0$. This is because $u(1-u)leq 1$ and $Du_{xx}$ ($D>0$) contributes to the diffusion process (e.g. diffusing $u$ from high to lower value across space). If the initial profile $u(x,0) > 1$ (not biological), then it is possible that $U(x-ct) > 1$ for small $t$. However, asymptotically, $U(x-ct)$ will still be bounded by $0$ and $1$, e.g. $0 leq liminf U(x-ct) leq limsup U(x-ct) leq 1$.



          If the boundary is finite, then the answer depends on the specific boundary conditions and the question needs to be examined case by case.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
            $endgroup$
            – jp31433
            Dec 4 '18 at 15:50











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          1 Answer
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          1 Answer
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          active

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          1












          $begingroup$

          Assuming this is a biological problem and the domain is infinite, then the form of the model that you used indicates a re-scale version, where carrying capacity $u$ is normalized to 1. If the initial profile $u(x,0) leq 1$ (biological), then $U(x-ct) leq 1$ for all $tgeq 0$. This is because $u(1-u)leq 1$ and $Du_{xx}$ ($D>0$) contributes to the diffusion process (e.g. diffusing $u$ from high to lower value across space). If the initial profile $u(x,0) > 1$ (not biological), then it is possible that $U(x-ct) > 1$ for small $t$. However, asymptotically, $U(x-ct)$ will still be bounded by $0$ and $1$, e.g. $0 leq liminf U(x-ct) leq limsup U(x-ct) leq 1$.



          If the boundary is finite, then the answer depends on the specific boundary conditions and the question needs to be examined case by case.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
            $endgroup$
            – jp31433
            Dec 4 '18 at 15:50
















          1












          $begingroup$

          Assuming this is a biological problem and the domain is infinite, then the form of the model that you used indicates a re-scale version, where carrying capacity $u$ is normalized to 1. If the initial profile $u(x,0) leq 1$ (biological), then $U(x-ct) leq 1$ for all $tgeq 0$. This is because $u(1-u)leq 1$ and $Du_{xx}$ ($D>0$) contributes to the diffusion process (e.g. diffusing $u$ from high to lower value across space). If the initial profile $u(x,0) > 1$ (not biological), then it is possible that $U(x-ct) > 1$ for small $t$. However, asymptotically, $U(x-ct)$ will still be bounded by $0$ and $1$, e.g. $0 leq liminf U(x-ct) leq limsup U(x-ct) leq 1$.



          If the boundary is finite, then the answer depends on the specific boundary conditions and the question needs to be examined case by case.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
            $endgroup$
            – jp31433
            Dec 4 '18 at 15:50














          1












          1








          1





          $begingroup$

          Assuming this is a biological problem and the domain is infinite, then the form of the model that you used indicates a re-scale version, where carrying capacity $u$ is normalized to 1. If the initial profile $u(x,0) leq 1$ (biological), then $U(x-ct) leq 1$ for all $tgeq 0$. This is because $u(1-u)leq 1$ and $Du_{xx}$ ($D>0$) contributes to the diffusion process (e.g. diffusing $u$ from high to lower value across space). If the initial profile $u(x,0) > 1$ (not biological), then it is possible that $U(x-ct) > 1$ for small $t$. However, asymptotically, $U(x-ct)$ will still be bounded by $0$ and $1$, e.g. $0 leq liminf U(x-ct) leq limsup U(x-ct) leq 1$.



          If the boundary is finite, then the answer depends on the specific boundary conditions and the question needs to be examined case by case.






          share|cite|improve this answer









          $endgroup$



          Assuming this is a biological problem and the domain is infinite, then the form of the model that you used indicates a re-scale version, where carrying capacity $u$ is normalized to 1. If the initial profile $u(x,0) leq 1$ (biological), then $U(x-ct) leq 1$ for all $tgeq 0$. This is because $u(1-u)leq 1$ and $Du_{xx}$ ($D>0$) contributes to the diffusion process (e.g. diffusing $u$ from high to lower value across space). If the initial profile $u(x,0) > 1$ (not biological), then it is possible that $U(x-ct) > 1$ for small $t$. However, asymptotically, $U(x-ct)$ will still be bounded by $0$ and $1$, e.g. $0 leq liminf U(x-ct) leq limsup U(x-ct) leq 1$.



          If the boundary is finite, then the answer depends on the specific boundary conditions and the question needs to be examined case by case.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Dec 3 '18 at 8:39









          PaichuPaichu

          751616




          751616












          • $begingroup$
            Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
            $endgroup$
            – jp31433
            Dec 4 '18 at 15:50


















          • $begingroup$
            Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
            $endgroup$
            – jp31433
            Dec 4 '18 at 15:50
















          $begingroup$
          Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
          $endgroup$
          – jp31433
          Dec 4 '18 at 15:50




          $begingroup$
          Thank you for your answer! It totally makes sense! I haven't connected it with the initial profile. You are right. I've seen some cases where the boundary is finite. That's why I was wondering about the question! Again, thank you for your help!
          $endgroup$
          – jp31433
          Dec 4 '18 at 15:50


















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