Solution Of PDE by Laplace Transform

Solution of Partial Differential Equations by Laplace Transform

The Laplace Transform is a powerful integral transform used to convert partial differential equations (PDEs) into algebraic equations, which are often easier to solve. This method is particularly useful for solving linear PDEs with constant coefficients and specific boundary conditions.

1. Definition of the Laplace Transform

The Laplace Transform of a function \(f(t)\) is defined as:

\(F(s) = \mathcal{L}\{f(t)\} = \int_{0}^{\infty} e^{-st} f(t) dt\)

where \(s\) is a complex number frequency parameter.

2. Applying the Laplace Transform to PDEs

To solve a PDE using the Laplace Transform, we follow these steps:

Take the Laplace Transform of both sides of the PDE with respect to time variable \(t\).
Solve the resulting algebraic equation in the Laplace domain.
Apply the inverse Laplace Transform to obtain the solution in the time domain.

3. Example: Solving the Heat Equation

Consider the one-dimensional heat equation:

\(\frac{\partial u}{\partial t} = \alpha \frac{\partial^2 u}{\partial x^2}\)

with initial condition \(u(x,0) = \sin(x)\) and boundary conditions \(u(0,t) = u(\pi,t) = 0\).

Solution Steps:

Step 1: Take the Laplace Transform \(\L{\frac{\partial u}{\partial t}} = sU(x,s) - u(x,0)\) :math:` L{alpha frac{partial^2 u}{partial x^2}} = alpha frac{partial^2 U}{partial x^2}`

Step 2: Solve the algebraic equation \(\sU(x,s) - f(x) = \alpha \frac{\partial^2 U}{\partial x^2}\)

Rearranging gives: \(\alpha \frac{\partial^2 U}{\partial x^2} - sU(x,s) = -f(x)\)

Homogeneous solution and particular solution methods can be applied here. \(\alpha \frac{\partial^2 U}{\partial x^2} - sU(x,s) = 0\)

General solution: \(U(x,s) = A(s) \sinh(\sqrt{(s/α)}x) + B(s) \cosh(\sqrt{(s/α)}x)\)

Particular solution can be found using methods like undetermined coefficients or variation of parameters.

Step 3: Apply the inverse Laplace Transform to find u(x,t)

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