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Let's consider the following Cauchy problem:

$$u_t=\alpha(x,t,u)u_{xx}+\beta(x,t,u)(u_x)^2+\gamma(x,t,u)u_x+\phi(x,t,u)$$

$$u(x,0)=u_0(x).$$

Assume that functions $\alpha,\beta,\gamma,\phi$ are regular (Holder Hölder continous or even smooth but may have some sort of singularity in $t=0$ like $1/t$ term for example). Suppose that we can prove that this problem has unique solution $u=u(x,t)$ in some domain $D\subset\mathbb{R}\times\mathbb{R}_+$.

Im interested in short time symptotic expansions of solutions, i.e. im looking for methods of showing that $$u(x,t)=\Sigma_{k=0}^mu^k(x)t^k+o(t^m)$$ as $t\rightarrow 0$. (With description of functions $u^k$ in terms of simpler PDE's of course).

My question is: are there any papers or monographs dealing with such asymptotic expansions in case of complicated nonlinear PDE's (with mathematical rigour, not only clever ansatz Ansatz coming out of the blue and with no proof)?

Another problem I'm concerned with:

In various works i saw asymptotic expansion technique was used in following manner:

1) Write down a series $\Sigma_{i=0}^{\infty}\lambda^iu_i(x,t)$, where $\lambda$ is time variable or some synthetic parameter

2) Plug that formal series into equation, perform differential operations term by term, group everything by powers of $\lambda$

3)Observe that coefficients standing by powers of $\lambda$ are differential operators applied to unknown functions $u_0,u_1,u_2,...$

4)Solve system of equations obtained by equating those coefficients to $0$

5)Be happy that you solved initial equation

So here is a basic question: how can we be sure without any further analysis between points 4) and 5) that obtained series is in any way related to actual solution?

In some papers i saw (physics and mathematical finance) authors just obtained that formal series and didn't even bother with proving that it has anythin to do with actual solution. Why?

Here is an example of a problem im trying to "assymptotically expand" in short time:

$$tu_t=Tr[\frac{u}{2}AD(\frac{x}{u})\otimes D(\frac{x}{u})]-t^2Tr[\frac{u^3}{8}ADu\otimes Du]+t<\alpha,Du>+t Tr[\frac{1}{2}AD^2u]-\frac{u}{2}$$ Where: $u=u(x,y,t)$ ($x,y>0$ $t\geq 0$) -solution (assume that it exists and that $u>0$), $A=A(x,y,t)$ -is $2\times 2$ matrix for all $x,y,t$ and its is Holder Hölder continous with exponent $\alpha$ in space and $\alpha/2$ in time, $\alpha=\alpha(x,y,t)\in \mathbb{R}^2$ also Holder Hölder continous with exponent $\alpha$ in space variables and $\alpha/2$ in time variable. $Tr$ stand for trace of a matrix and $\otimes$ for tensor product of vectors: $(a_{i})\otimes (b_{i})=(a_i b_j)_{i,j}$ ($a,b$ -vectors, $a\otimes b$ -matrix).

3 added 760 characters in body

Let's consider the following Cauchy problem:

$$u_t=\alpha(x,t,u)u_{xx}+\beta(x,t,u)(u_x)^2+\gamma(x,t,u)u_x+\phi(x,t,u)$$

$$u(x,0)=u_0(x).$$

Assume that functions $\alpha,\beta,\gamma,\phi$ are regular (Holder continous or even smooth but may have some sort of singularity in $t=0$ like $1/t$ term for example). Suppose that we can prove that this problem has unique solution $u=u(x,t)$ in some domain $D\subset\mathbb{R}\times\mathbb{R}_+$.

Im interested in short time symptotic expansions of solutions, i.e. im looking for methods of showing that $$u(x,t)=\Sigma_{k=0}^mu^k(x)t^k+o(t^m)$$ as $t\rightarrow 0$. (With description of functions $u^k$ in terms of simpler PDE's of course).

My question is: are there any papers or monographs dealing with such asymptotic expansions in case of complicated nonlinear PDE's (with mathematical rigour, not only clever ansatz coming out of the blue and with no proof)?

Another problem I'm concerned with:

In various works i saw asymptotic expansion technique was used in following manner:

1) Write down a series $\Sigma_{i=0}^{\infty}\lambda^iu_i(x,t)$, where $\lambda$ is time variable or some synthetic parameter

2) Plug that formal series into equation, perform differential operations term by term, group everything by powers of $\lambda$

3)Observe that coefficients standing by powers of $\lambda$ are differential operators applied to unknown functions $u_0,u_1,u_2,...$

4)Solve system of equations obtained by equating those coefficients to $0$

5)Be happy that you solved initial equation

So here is a basic question: how can we be sure without any further analysis between points 4) and 5) that obtained series is in any way related to actual solution?

In some papers i saw (physics and mathematical finance) authors just obtained that formal series and didn't even bother with proving that it has anythin to do with actual solution. Why?

Here is an example of a problem im trying to "assymptotically expand" in short time:

$$tu_t=Tr[\frac{u}{2}AD(\frac{x}{u})\otimes D(\frac{x}{u})]-t^2Tr[\frac{u^3}{8}ADu\otimes Du]+t<\alpha,Du>+t Tr[\frac{1}{2}AD^2u]-\frac{u}{2}$$ Where: $u=u(x,y,t)$ ($x,y>0$ $t\geq 0$) -solution (assume that it exists and that $u>0$), $A=A(x,y,t)$ -is $2\times 2$ matrix for all $x,y,t$ and its is Holder continous with exponent $\alpha$ in space and $\alpha/2$ in time, $\alpha=\alpha(x,y,t)\in \mathbb{R}^2$ also Holder continous with exponent $\alpha$ in space variables and $\alpha/2$ in time variable. $Tr$ stand for trace of a matrix and $\otimes$ for tensor product of vectors: $(a_{i})\otimes (b_{i})=(a_i b_j)_{i,j}$ ($a,b$ -vectors, $a\otimes b$ -matrix).

2 added 1011 characters in body

Let's consider the following Cauchy problem:

$$u_t=\alpha(x,t,u)u_{xx}+\beta(x,t,u)(u_x)^2+\gamma(x,t,u)u_x+\phi(x,t,u)$$

$$u(x,0)=u_0(x).$$

Assume that functions $\alpha,\beta,\gamma,\phi$ are regular (Holder continous or even smooth but may have some sort of singularity in $t=0$ like $1/t$ term for example). Suppose that we can prove that this problem has unique solution $u=u(x,t)$ in some domain $D\subset\mathbb{R}\times\mathbb{R}_+$.

Im interested in short time symptotic expansions of solutions, i.e. im looking for methods of showing that $$u(x,t)=\Sigma_{k=0}^mu^k(x)t^k+o(t^m)$$ as $t\rightarrow 0$. (With description of functions $u^k$ in terms of simpler PDE's of course).

My question is: are there any papers or monographs dealing with such asymptotic expansions in case of complicated nonlinear PDE's (with mathematical rigour, not only clever ansatz coming out of the blue and with no proof)?

Another problem I'm concerned with:

In various works i saw asymptotic expansion technique was used in following manner:

1) Write down a series $\Sigma_{i=0}^{\infty}\lambda^iu_i(x,t)$, where $\lambda$ is time variable or some synthetic parameter

2) Plug that formal series into equation, perform differential operations term by term, group everything by powers of $\lambda$

3)Observe that coefficients standing by powers of $\lambda$ are differential operators applied to unknown functions $u_0,u_1,u_2,...$

4)Solve system of equations obtained by equating those coefficients to $0$

5)Be happy that you solved initial equation

So here is a basic question: how can we be sure without any further analysis between points 4) and 5) that obtained series is in any way related to actual solution?

In some papers i saw (physics and mathematical finance) authors just obtained that formal series and didn't even bother with proving that it has anythin to do with actual solution. Why?

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