Select Page

# Numerical Techniques-III

Numerical Techniques-III

Contents

Apply Least Square Approximation to fit the given set of data in the Polynomial / Exponential Function 2

Classify Partial Differential Equation 5

Show the relation of Partial Differential Equation 6

Describe and apply the Finite Difference Method to solve the Laplace Equation 8

Describe and apply the Finite Difference Method to solve the Poisson Equation 10

Describe the Bender-Schmidt Method to solve the Heat Equation 11

Apply the Bender-Schmidt Method to solve the Heat Equation 12

Describe the Crank-Nicolson Method to solve the Heat Equation 13

Apply the Crank-Nicholson Method to solve the Heat Equation 15

Describe the Implicit and Explicit Methods to solve the Wave Equation 17

Apply the Implicit and Explicit Methods to solve the Wave Equation 18

# Describe Least Square Approximation to the Function

The Least Square Approximation (LSA) is a mathematical method for finding the best fit line or curve that approximates a set of data points in a least squares sense. It is commonly used in regression analysis and data modeling.

Given a set of data points (x1, y1), (x2, y2), …, (xn, yn), the LSA method seeks to find a function y = f(x) that approximates the data points in a least squares sense. In other words, it finds the function that minimizes the sum of the squares of the residuals (i.e., the differences between the actual y-values and the predicted y-values from the function):minimize Σ(yi – f(xi))2

The LSA method typically involves finding the coefficients of a polynomial function of degree n-1 (where n is the number of data points) that minimizes the sum of the squared residuals. This polynomial function is often referred to as the “best fit” line or curve. The LSA method can be applied to a wide range of functions, including linear, quadratic, exponential, and logarithmic functions. The method can also be extended to multiple variables and higher dimensions.

# Apply Least Square Approximation to fit the given set of data in the Polynomial / Exponential Function

Sure, let’s go over how to apply Least Square Approximation to fit a given set of data to a polynomial or exponential function.

1. Polynomial Function:

Suppose we have a set of data points (x1, y1), (x2, y2), …, (xn, yn) that we want to fit to a polynomial function of degree k. To apply LSA, we can follow these steps:

Step 1: Set up the system of equations

We can write the polynomial function as:

y = a0 + a1x + a2x2 + … + ak*xk

where a0, a1, …, ak are the coefficients of the polynomial. We want to find the values of these coefficients that minimize the sum of the squared residuals:

Σ(yi – (a0 + a1xi + a2xi2 + … + ak*xik))2

Expanding this out and simplifying, we get a system of k+1 linear equations in k+1 unknowns:

Σyi = a0n + a1Σxi + a2Σxi^2 + … + akΣxi^k

Σyixi = a0Σxi + a1Σxi2 + a2Σxi^3 + … + akΣxi^(k+1)

Σyixi^k = a0Σxi^k + a1Σxi^(k+1) + a2Σxi^(k+2) + … + akΣxi^(2k)

where n is the number of data points.

Step 2: Solve the system of equations

We can solve this system of equations using matrix algebra or other numerical methods to obtain the values of a0, a1, …, ak.

Step 3: Evaluate the polynomial function

Once we have the coefficients of the polynomial function, we can evaluate the function at any point x to obtain the corresponding y-value.

Example:

Suppose we have the following set of data points:

(1, 2), (2, 3), (3, 5), (4, 7), (5, 11)

We want to fit these data points to a polynomial function of degree 2. Using the steps outlined above, we can set up the following system of equations:

5a0 + 15a1 + 55a2 = 28

15a0 + 55a1 + 225a2 = 89

55a0 + 225a1 + 979a2 = 391

Solving this system of equations, we get:

a0 = -0.6667, a1 = 3.5, a2 = -0.5

So the polynomial function that best fits the data is:

y = -0.6667 + 3.5x – 0.5x^2

We can evaluate this function at any point x to obtain the corresponding y-value.

1. Exponential Function:

Suppose we have a set of data points (x1, y1), (x2, y2), …, (xn, yn) that we want to fit to an exponential function of the form y = ae^(bx). To apply LSA, we can follow these steps:

Step 1: Take the logarithm of both sides

Taking the natural logarithm of both sides of the equation, we get:

ln(y) = ln(a) + bx

This transforms the exponential function into a linear function, which can be fitted using LSA.

Step 2: Set up the system of equations

# Classify Partial Differential Equation

Partial Differential Equations (PDEs) are mathematical equations that involve partial derivatives of an unknown function with respect to two or more independent variables. They are widely used in physics, engineering, and other scientific fields to describe the behavior of complex systems.

There are several types of PDEs, and they can be classified based on their properties and characteristics. Here are some common types of PDEs and examples of each:

1. Elliptic PDEs

Elliptic PDEs describe systems where the solution at any point depends on the values of the function at all other points. They are typically used to model steady-state phenomena, such as electrostatics, fluid flow, and heat transfer. Elliptic PDEs have a unique solution that satisfies certain boundary conditions.

Examples of elliptic PDEs include:

• Laplace’s equation: ∇^2u = 0
• Poisson’s equation: ∇^2u = f(x,y,z)
1. Parabolic PDEs

Parabolic PDEs describe systems where the solution at any point depends on the values of the function and its first derivative at the same point and time. They are typically used to model transient phenomena, such as heat conduction and diffusion, where the solution evolves over time.

Examples of parabolic PDEs include:

• Heat equation: ∂u/∂t = k∇^2u
• Diffusion equation: ∂u/∂t = D∇^2u
1. Hyperbolic PDEs

Hyperbolic PDEs describe systems where the solution at any point depends on the values of the function and its first derivative in both space and time. They are typically used to model wave phenomena, such as sound waves and electromagnetic waves, where the solution propagates through space and time.

Examples of hyperbolic PDEs include:

• Wave equation: ∂^2u/∂t^2 = c^2∇^2u
• Telegraph equation: ∂^2u/∂t^2 + a^2∂^2u/∂x^2 = 0
1. Mixed PDEs

Mixed PDEs describe systems that exhibit properties of both elliptic and hyperbolic PDEs. They are typically used to model systems that involve both wave propagation and diffusion.

An example of a mixed PDE is the advection-diffusion equation:

• ∂u/∂t + v∂u/∂x = D∂^2u/∂x^2

where u is the unknown function, t is time, x is the spatial variable, v is the advection velocity, and D is the diffusion coefficient.

# Show the relation of Partial Differential Equation

Partial Differential Equations (PDEs) are mathematical equations that involve partial derivatives of an unknown function with respect to two or more independent variables. The solution to a PDE is a function that satisfies the equation and any boundary or initial conditions that are specified.

The relationship between the variables and their derivatives in a PDE can be expressed using different notations, such as:

1. Differential notation:

The differential notation expresses the relationship between the variables and their derivatives using differential operators. For example, the wave equation in differential notation is:

∂^2u/∂t^2 = c^2∇^2u

where u is the unknown function, t is time, and ∇^2 is the Laplacian operator.

1. Integral notation:

The integral notation expresses the relationship between the variables and their derivatives using integrals. For example, the heat equation in integral notation is:

u(x,t) = 1/√(4πkt) ∫exp(-(x-y)^2/4kt) u(y,0) dy

where u is the unknown function, x is the spatial variable, t is time, k is the thermal diffusivity, and the integral is taken over all values of y.

1. Operator notation:

The operator notation expresses the relationship between the variables and their derivatives using differential operators and algebraic symbols. For example, the diffusion equation in operator notation is:

(∂/∂t – D∇^2)u = 0

where u is the unknown function, t is time, D is the diffusion coefficient, and ∇^2 is the Laplacian operator.

In each notation, the PDE relates the values of the function u and its partial derivatives to the independent variables and their partial derivatives. The solution to the PDE is a function that satisfies this relationship and any boundary or initial conditions that are specified.

# Describe and apply the Finite Difference Method to solve the Laplace Equation

The Laplace equation is a second-order partial differential equation that appears in many areas of physics and engineering. It is given by:

∇^2u = 0

where ∇^2 is the Laplacian operator, and u is the unknown function.

The finite difference method is a numerical method for solving PDEs by discretizing the domain and approximating the partial derivatives with finite differences. The method involves dividing the domain into a grid of points and approximating the partial derivatives at each point using nearby values.

To apply the finite difference method to solve the Laplace equation, we can follow these steps:

1. Discretize the domain:

Divide the domain of the Laplace equation into a grid of points. For example, if the domain is a rectangular region, we can use a rectangular grid of points.

1. Approximate the Laplacian operator:

Approximate the Laplacian operator at each point using the central difference formula:

∇^2u ≈ (u(i+1,j) + u(i-1,j) + u(i,j+1) + u(i,j-1) – 4u(i,j))/h^2

where u(i,j) is the value of the unknown function at the point (i,j) on the grid, and h is the grid spacing.

1. Solve the system of linear equations:

Substitute the approximated Laplacian operator into the Laplace equation and solve the resulting system of linear equations for the values of the unknown function at the grid points. The resulting system of linear equations will be a sparse system that can be solved using iterative or direct methods.

1. Apply boundary conditions:

Apply the boundary conditions to the solution obtained in step 3 to obtain a complete solution to the Laplace equation.

Here is an example of applying the finite difference method to solve the Laplace equation in a rectangular domain with Dirichlet boundary conditions:

Consider the Laplace equation in a rectangular domain with Dirichlet boundary conditions:

∇^2u = 0 in 0 ≤ x ≤ L, 0 ≤ y ≤ H

u(x,0) = g1(x) for 0 ≤ x ≤ L

u(x,H) = g2(x) for 0 ≤ x ≤ L

u(0,y) = g3(y) for 0 ≤ y ≤ H

u(L,y) = g4(y) for 0 ≤ y ≤ H

To apply the finite difference method, we divide the domain into a rectangular grid of points with spacing h. We can approximate the Laplacian operator using the central difference formula as:

∇^2u(i,j) ≈ (u(i+1,j) + u(i-1,j) + u(i,j+1) + u(i,j-1) – 4u(i,j))/h^2

Substituting this into the Laplace equation, we get:

(u(i+1,j) + u(i-1,j) + u(i,j+1) + u(i,j-1) – 4u(i,j))/h^2 = 0

Solving for u(i,j), we get:

u(i,j) = (u(i+1,j) + u(i-1,j) + u(i,j+1) + u(i,j-1))/4

We can use this equation to solve for the values of the unknown function at the interior points of the grid. For the boundary points, we can use the given boundary conditions to directly set the values of the unknown function.

Finally, we can use iterative or direct methods to solve the resulting system of linear equations and obtain a complete solution to the Laplace equation.

# Describe and apply the Finite Difference Method to solve the Poisson Equation

The Poisson equation is a second-order partial differential equation that describes the distribution of electric potential in a region of space. It is given by:

∇²ϕ = -ρ/ε₀

where ϕ is the electric potential, ρ is the charge density, and ε₀ is the permittivity of free space.

The finite difference method is a numerical technique for solving partial differential equations. It works by approximating the derivatives in the differential equation with finite differences, which are computed using values of the function at discrete points in space.

To apply the finite difference method to the Poisson equation, we first discretize the equation by dividing the region of interest into a grid of points. We then approximate the Laplacian operator (∇²) using finite differences. There are several ways to do this, but a common approach is to use the central difference formula:

∇²ϕ ≈ (ϕ(i+1,j) + ϕ(i-1,j) + ϕ(i,j+1) + ϕ(i,j-1) – 4ϕ(i,j))/Δx²

where ϕ(i,j) is the value of the electric potential at the grid point (i,j), and Δx is the grid spacing.

Substituting this approximation into the Poisson equation gives:

(ϕ(i+1,j) + ϕ(i-1,j) + ϕ(i,j+1) + ϕ(i,j-1) – 4ϕ(i,j))/Δx² = -ρ(i,j)/ε₀

We can rearrange this equation to solve for ϕ(i,j):

ϕ(i,j) = (1/4)(ϕ(i+1,j) + ϕ(i-1,j) + ϕ(i,j+1) + ϕ(i,j-1) + (Δx²/ε₀)ρ(i,j))

This equation provides a way to compute the electric potential at each grid point, given the charge density at that point and the values of ϕ at neighboring grid points.

To solve the Poisson equation using the finite difference method, we start with an initial guess for the electric potential at each grid point, and then iterate over the grid, updating the values of ϕ using the equation above until the solution converges to a desired accuracy.

There are many variations and refinements of the finite difference method, including different schemes for approximating the derivatives, different types of grids (e.g., Cartesian, cylindrical, or spherical), and different techniques for handling boundary conditions. However, the basic approach outlined above provides a general framework for using the finite difference method to solve partial differential equations like the Poisson equation.

# Describe the Bender-Schmidt Method to solve the Heat Equation

The Bender-Schmidt method is a numerical technique for solving the one-dimensional heat equation, which describes the time evolution of the temperature distribution in a thin rod of length L. The heat equation is given by:

∂u/∂t = α ∂²u/∂x²

where u(x,t) is the temperature at position x and time t, α is the thermal diffusivity of the rod, and L is the length of the rod.

The Bender-Schmidt method works by discretizing the rod into N equally spaced points, and approximating the temperature u(x,t) at each point using a linear combination of basis functions. The basis functions used in the method are piecewise linear functions that are zero at all but two adjacent grid points. The coefficients of these basis functions are determined by solving a system of linear equations that arise from imposing the initial and boundary conditions.

The time evolution of the temperature distribution is then approximated using an iterative scheme that involves solving a system of linear equations at each time step. This is done by first discretizing the time variable into M equally spaced intervals, and using a forward difference scheme to approximate the time derivative. The resulting equation is then rearranged to obtain an expression for u(x,t+Δt) in terms of the temperature values at the previous time step:

u(i,t+Δt) = u(i,t) + (αΔt/Δx²) [u(i+1,t) – 2u(i,t) + u(i-1,t)]

where i is the index of the grid point, and Δx and Δt are the grid spacing in space and time, respectively.

The Bender-Schmidt method is a relatively simple and efficient technique for solving the one-dimensional heat equation, and can be extended to higher dimensions using finite element methods. However, it is important to ensure that the grid spacing is sufficiently small to ensure accurate results, and to use appropriate boundary conditions to model the physical situation being studied.

# Apply the Bender-Schmidt Method to solve the Heat Equation

To apply the Bender-Schmidt method to solve the one-dimensional heat equation, we first discretize the rod into N equally spaced grid points, with spacing Δx = L/(N-1). We then approximate the temperature u(x,t) at each grid point using a linear combination of basis functions. In particular, we use the following piecewise linear basis functions:
φ(i-1/2)(x) = (x(i) – x)/(Δx) for x(i-1) <= x <= x(i)
φ(i+1/2)(x) = (x – x(i))/(Δx) for x(i) <= x <= x(i+1)
where x(i) is the coordinate of the ith grid point.
Using these basis functions, we can express the temperature u(x,t) at any point in the rod as:
u(x,t) = ∑(i=1 to N) u(i,t)φ(i-1/2)(x)
where u(i,t) is the temperature at the ith grid point at time t.
Next, we impose the initial and boundary conditions of the heat equation. For example, if the rod is initially at a uniform temperature of u0, we have:
u(i,0) = u0 for 1 <= i <= N
If the rod is insulated at both ends, we have:
u(1,t) = u(N,t) = 0 for 0 <= t <= T
where T is the final time of the simulation.
To solve for the temperature at each time step, we use the Bender-Schmidt method. This involves first approximating the time derivative using a forward difference scheme:

∂u/∂t ≈ (u(i,t+Δt) – u(i,t))/Δt
Substituting this approximation into the heat equation, and using the approximation for u(x,t) above, we obtain:

(u(i,t+Δt) – u(i,t))/Δt = α (u(i+1,t) – 2u(i,t) + u(i-1,t))/(Δx²)

Rearranging this equation to solve for u(i,t+Δt), we get:

u(i,t+Δt) = u(i,t) + (αΔt/Δx²) [u(i+1,t) – 2u(i,t) + u(i-1,t)]

This equation can be solved at each time step using a system of linear equations. The coefficients of the basis functions are stored in a matrix A, which depends on the boundary conditions, and the temperature values at the previous time step are stored in a vector b. Solving the system Ax = b gives the temperature values at the current time step.

We can then iterate this process until the simulation reaches the final time T, and obtain the temperature distribution at each time step. The Bender-Schmidt method is an efficient and accurate technique for solving the one-dimensional heat equation, and can be extended to higher dimensions using finite element methods.

# Describe the Crank-Nicolson Method to solve the Heat Equation

The Crank-Nicholson method is a numerical technique for solving partial differential equations, including the one-dimensional heat equation. It is a popular method because it is unconditionally stable and second-order accurate in both space and time.

To apply the Crank-Nicholson method to solve the heat equation, we first discretize the rod into N equally spaced grid points, with spacing Δx. We then approximate the temperature u(x,t) at each grid point using a linear combination of basis functions, as in the Bender-Schmidt method. In particular, we use the same piecewise linear basis functions:

φ(i-1/2)(x) = (x(i) – x)/(Δx) for x(i-1) <= x <= x(i)

φ(i+1/2)(x) = (x – x(i))/(Δx) for x(i) <= x <= x(i+1)

where x(i) is the coordinate of the ith grid point.

Using these basis functions, we can express the temperature u(x,t) at any point in the rod as:

u(x,t) = ∑(i=1 to N) u(i,t)φ(i-1/2)(x)

where u(i,t) is the temperature at the ith grid point at time t.

Next, we impose the initial and boundary conditions of the heat equation, as in the Bender-Schmidt method.

To solve for the temperature at each time step, we use the Crank-Nicholson method. This involves approximating the heat equation using a central difference scheme:

(∂u/∂t) = α (∂²u/∂x²)

where both the time and space derivatives are approximated using central differences. This gives us:

(u(i,t+Δt) – u(i,t))/Δt = (α/2) [(u(i+1,t) – 2u(i,t) + u(i-1,t))/(Δx²) + (u(i+1,t+Δt) – 2u(i,t+Δt) + u(i-1,t+Δt))/(Δx²)]

Rearranging this equation, we obtain:

(-αΔt/(2Δx²)) u(i-1,t+Δt) + (1 + αΔt/Δx²) u(i,t+Δt) + (-αΔt/(2Δx²)) u(i+1,t+Δt) = (αΔt/(2Δx²)) u(i-1,t) + (1 – αΔt/Δx²) u(i,t) + (αΔt/(2Δx²)) u(i+1,t)

This equation is a linear system of equations, which can be written as:

Au(t+Δt) = Bu(t)

where A and B are matrices that depend on the grid spacing, and u(t) is the vector of temperature values at time t. We can solve this system of equations using standard techniques, such as LU decomposition, to obtain the temperature values at the next time step.

We can then iterate this process until the simulation reaches the final time T, and obtain the temperature distribution at each time step. The Crank-Nicholson method is an efficient and accurate technique for solving the one-dimensional heat equation, and can be extended to higher dimensions using finite element methods.

# Apply the Crank-Nicholson Method to solve the Heat Equation

To apply the Crank-Nicholson method to solve the one-dimensional heat equation, we first discretize the rod into N equally spaced grid points, with spacing Δx. We then approximate the temperature u(x,t) at each grid point using a linear combination of basis functions, as described in ALO:

∂u/∂t = α ∂²u/∂x², 0 < x < L, 0 < t < T

u(x,0) = f(x), 0 ≤ x ≤ L

u(0,t) = g1(t), u(L,t) = g2(t), 0 ≤ t ≤ T

where α is the thermal diffusivity constant, L is the length of the rod, and T is the final time.

We start by discretizing the heat equation using the central difference scheme and approximating the temperature at each grid point using the linear basis functions as described in ALO:

(-αΔt/(2Δx²)) u(i-1,t+Δt) + (1 + αΔt/Δx²) u(i,t+Δt) + (-αΔt/(2Δx²)) u(i+1,t+Δt) = (αΔt/(2Δx²)) u(i-1,t) + (1 – αΔt/Δx²) u(i,t) + (αΔt/(2Δx²)) u(i+1,t)

where u(i,t) is the temperature at the ith grid point at time t, and Δt is the time step.

We can write this equation in matrix form as:

Au(t+Δt) = Bu(t)

where A and B are matrices that depend on the grid spacing and the thermal diffusivity constant α.

To apply the Crank-Nicholson method, we need to solve this system of equations at each time step. We can do this using standard numerical techniques, such as LU decomposition or iterative methods like the Gauss-Seidel method. In particular, we can use the Thomas algorithm to efficiently solve the tridiagonal system of equations that arise from the Crank-Nicholson method.

Here’s an outline of the algorithm to solve the one-dimensional heat equation using the Crank-Nicholson method:

1. Discretize the rod into N equally spaced grid points, with spacing Δx.
2. Approximate the temperature u(x,t) at each grid point using the linear basis functions.
3. Set the initial and boundary conditions of the heat equation.
4. Choose the time step Δt and the final time T.
5. Define the matrices A and B based on the grid spacing and the thermal diffusivity constant α.
6. Initialize the temperature vector u(t=0) to the initial condition f(x).

Iterate over time steps t = Δt, 2Δt, …, T:

a. Compute the right-hand side vector Bu(t) based on the previous time step.

b. Solve the linear system of equations Au(t+Δt) = Bu(t) using the Thomas algorithm.

c. Store the new temperature vector u(t+Δt).

1. Output the temperature vector u at each time step.

By following this algorithm, we can obtain an approximate solution to the one-dimensional heat equation using the Crank-Nicholson method.

# Describe the Implicit and Explicit Methods to solve the Wave Equation

The wave equation is a second-order partial differential equation that describes the propagation of waves in space and time. It has many physical applications, such as the propagation of sound waves, electromagnetic waves, and seismic waves. The general form of the wave equation is:

∂²u/∂t² = c² ∂²u/∂x²

where u(x,t) is the displacement of the wave at position x and time t, and c is the wave speed.

To solve the wave equation numerically, we need to discretize the space and time variables. There are two main methods for discretizing the wave equation: the explicit method and the implicit method.

The explicit method is based on approximating the second-order time derivative using a forward difference, and the second-order spatial derivative using a central difference. This gives us the following equation:

u(i,j+1) = 2u(i,j) – u(i,j-1) + (cΔt/Δx)² (u(i+1,j) – 2u(i,j) + u(i-1,j))

where u(i,j) is the displacement of the wave at grid point i and time step j, Δx is the grid spacing, Δt is the time step, and c is the wave speed.

This method is called explicit because we can solve for the value of u(i,j+1) explicitly in terms of the previous time steps and grid points. However, this method is subject to stability constraints, which limit the time step size we can use. Specifically, the time step size must be less than a certain value for the method to be stable. This condition is known as the CFL condition and depends on the wave speed and grid spacing.

The implicit method, on the other hand, is based on approximating the second-order time derivative using a backward difference, and the second-order spatial derivative using a central difference. This gives us the following equation:

u(i,j+1) = 2u(i,j) – u(i,j-1) + (cΔt/Δx)² (u(i+1,j+1) – 2u(i,j+1) + u(i-1,j+1))

where u(i,j+1) is the displacement of the wave at grid point i and time step j+1.

This method is called implicit because the value of u(i,j+1) depends on the value of u(i,j+1) itself, as well as the values of u(i+1,j+1) and u(i-1,j+1). Therefore, we need to solve a system of equations at each time step to obtain the values of u(i,j+1). This method is unconditionally stable, which means that we can use arbitrarily large time step sizes without causing instability.

# Apply the Implicit and Explicit Methods to solve the Wave Equation

The Wave Equation is a partial differential equation that describes the propagation of waves. It is given by:

∂²u/∂t² = c²(∂²u/∂x²)

where u(x,t) is the wave function, c is the speed of the wave, x is the position, and t is time.

1. Explicit Method:

The Explicit Method is a numerical method that approximates the solution of the Wave Equation at each time step using the values of the wave function at the previous time step. The method is called “explicit” because the value of the wave function at the next time step is explicitly computed from the values at the previous time step. The explicit method can be written as:

u(i,j+1) = 2u(i,j) – u(i,j-1) + r²(u(i+1,j) – 2u(i,j) + u(i-1,j))

where u(i,j) represents the value of the wave function at position i and time j, r = cΔt/Δx is the Courant-Friedrichs-Lewy (CFL) number, Δt is the time step, and Δx is the spatial step.

The explicit method is simple and easy to implement, but it is conditionally stable, meaning that the time step size must be small enough to ensure numerical stability.

1. Implicit Method:

The Implicit Method is a numerical method that approximates the solution of the Wave Equation at each time step using a system of linear equations. The method is called “implicit” because the value of the wave function at the next time step is implicitly defined by the solution of the linear equations. The implicit method can be written as:

u(i,j+1) – 2u(i,j) + u(i,j-1) = r²(u(i+1,j+1) – 2u(i,j+1) + u(i-1,j+1))

To solve this equation, we need to set up a system of linear equations in the form of:

Au(j+1) = Bu(j)

where A is a matrix, u(j+1) is the vector of the wave function at time j+1, B is a matrix, and u(j) is the vector of the wave function at time j.

The matrix A and B can be constructed using the boundary conditions and the Wave Equation. The system of linear equations can then be solved using any numerical method for solving systems of linear equations.

The implicit method is unconditionally stable, meaning that it is stable for any time step size. However, it is more computationally expensive than the explicit method because it involves solving a system of linear equations at each time step.