Stochastic Heat Equation with general noise

In this paper, we study a nonlinear one spatial dimensional stochastic heat equations driven by Gaussian noise: $\frac{\partial u }{\partial t}=\frac{\partial^2 u }{\partial x^2}+\sigma(u )\dot{W}$, where $\dot{W}$ is white in time and has the covariance of a fractional Brownian motion with Hurst parameter $H\in(\frac 14,\frac 12)$. We remove a critical and unnatural condition $\sigma(0)=0$ previously imposed in a recent paper by Hu, Huang, L\^{e}, Nualart and Tindel. The idea is to work on a weighted space $\mathcal{Z}_{\lambda,T}^p$ for some power decay weight $\lambda(x)=c_H(1+|x|^2)^{H-1}$. We obtain the weak existence of solution. With additional decay conditions on $\sigma$ we obtain the existence of strong solution and the pathwise uniqueness of the strong solution. The reason to introduce the weight function is that the solution $u(t,x)$ may explode as $|x|\rightarrow \infty$ when the "diffusion coefficient" $\sigma(u)$ does not satisfy $\sigma(0)=0$ regardless of the initial condition. This motivates us to study the exact asympotics of the solution $u_{{\rm add}}(t,x)$ as $t$ and $x$ go to infinity when $\sigma(u)=1$ and when the initial condition $u_0(x)\equiv 0$. In particular, we find the exact growth of $\sup_{|x|\leq L}{|u_{{\rm add}}(t,x)|}$. Furthermore, we find the sharp growth rate for the H\"older coefficients, namely, $\sup_{|x|\leq L} \frac{| u_{{\rm add}}(t,x+h)-u_{{\rm add}}(t,x)|}{|h|^\beta}$ and $\sup_{|x|\leq L} \frac{| u_{{\rm add}}(t+\tau,x)-u_{{\rm add}}(t,x)|}{\tau^\alpha}$. These results are interesting and fundamental themselves.