# Solving the heat equation

1. Jan 15, 2012

1. The problem statement, all variables and given/known data

Let $s = x/\sqrt{t}$ and look for a solution to the heat equation $u_{t} = u_{xx}$ which is of the form u(x,t) = f(s) and satisfies the IC u(x, 0) = 0 and the BC u(0, t) = 1 and u(∞, t) = 0.

2. Relevant equations

$∫e^{x^{2}} = \sqrt{\pi}$

3. The attempt at a solution
Let f(s) = u(x, t)
Substituting f(s) into the heat equation:
$\frac{-xf'(s)}{2t^{3/2}} = \frac{f''(s)}{t}$
$s = x/\sqrt{t}$
$-0.5sf'(s) = f''(s)$
It's now just an ODE.
$f'(s) = g(s)$
$-0.5sg(s) = g'(s)$
$ln(g(s)) = -0.25s^2 + constant$
$g(s) = Ke^{-0.25s^{2}} = f'(s)$
$f(s) = ∫f'(s)ds = Ke^{0.25}∫e^{-s^{2}}ds = Ke^{0.25}\pi^{1/2}$

So now I'm left with a trivial solution of f(s) = u(x,t) = constant. I don't think this is what u(x,t) is supposed to be. Which part did I do incorrectly?