# Homework Help: Diffusion equation PDE

1. Sep 12, 2010

### StewartHolmes

1. The problem statement, all variables and given/known data
Solve
$$u_{tt} - 4u_{xx} = 0$$, $$x \in \mathbb{R}, t > 0$$
$$u(x, 0) = e^{-x^2}$$, $$x \in \mathbb{R}$$

2. Relevant equations
General solution to the diffusion equation:
$$u(x, t) = \frac{1}{\sqrt{4\pi kt}} \int\limits_{-\infty}^{\infty} e^\frac{{-(x - y)^2}}{4kt} \varphi(y) \, dy$$

3. The attempt at a solution
$$u(x, t) = \frac{1}{\sqrt{4\pi kt}} \int\limits_{-\infty}^{\infty} e^\frac{{-(x - y)^2}}{4kt} e^{-y^2}$$

That's about as good as I've got. Integration by parts gets me no further. I've tried to combine the exponents in the integrand, but that leaves me with
$$- \frac{x^2 + y^2 - 2xy + 4kty^2}{4kt}$$
I have an example in a textbook where they do similar, then complete the square so that they can substitute $$p$$, then integrate $$\int\limits_{-\infty}^{\infty}e^{-p^2} \, dp$$ as $$\sqrt{\pi}$$... but I can't complete the square in this case.

2. Sep 12, 2010

### hunt_mat

Start from scratch, it'll be easier. Take Fourier transforms w.r.t x of the PDE and the initial condition, then look for the inverse transform.