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I'm trying to solve the 2D heat equation in cartesian coordinates with initial condition of the form $1/r$. The problem is as follows:

$$ \begin{equation} \begin{cases} \frac{\partial u}{\partial t} = \alpha \left( \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} \right), & x \in \mathbb{R}, y \in \mathbb{R}, t > 0 \\ u(x, y, 0) = \frac{-y}{x^2 + y^2}, & x \in \mathbb{R}, y \in \mathbb{R}. \end{cases} \end{equation} $$

Separating variables doesn't seem to be the best approach here and I couldn't find the Fourier transform of this singular initial condition. So I'm trying to solve it via convolution:

$u(x, y, t) = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} G(x - \xi, y - \eta, t) \frac{-\eta}{\xi^2 + \eta^2} \, d\xi \, d\eta$

where: $G(x, y, t) = \frac{1}{4 \pi \alpha t} e^{-\frac{x^2 + y^2}{4 \alpha t}}$

The problem is that the integral for $u(x,y,t)$ doesn't seem to have an analytical solution. Is that really that case? Can this PDE only be solved numerically?

Edit: The Fourier Transform of the initial condition can be found explicitly with Mathematica:

initialFourierTransform = -((I ky (HeavisideTheta[-kx] + HeavisideTheta[kx]))/(kx^2 + ky^2))

However the evaluation of the inverse Fourier transform to find the solution is taking a lifetime and I suspect there might be an error.

uHat[kx_, ky_, t_] := 
 initialFourierTransform*Exp[-alpha*(kx^2 + ky^2)*t]

(Compute the inverse Fourier transform to find u(x,y,t)) u[x_, y_, t_] := InverseFourierTransform[uHat[kx, ky, t], {kx, ky}, {x, y}]

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    Perhaps polar coordinates may be more fruitful? – whpowell96 Mar 26 '24 at 20:10
  • Alternatively, the Fourier transform of a very similar function is given here https://math.stackexchange.com/questions/2973394/fourier-transform-of-frac-x-x2y2 – whpowell96 Mar 27 '24 at 17:46
  • I can evaluate the Fourier transform explicitly on Mathematica, the result is -((I ky (HeavisideTheta[-kx] + HeavisideTheta[kx]))/(kx^2 + ky^2)) however evaluating the inverse Fourier transform to find the solution is taking a lifetime and I suspect an error. – justsome1 Mar 29 '24 at 18:22

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