Question

Find the Wronskian of two solutions of the given differential equation without solving the equation. \(t^{2} y^{\prime \prime}-t(t+2) y^{\prime}+(t+2) y=0\)

Short answer

Question: Find the Wronskian of two solutions of the given second-order differential equation without actually solving the equation: \(t^{2} y^{\prime \prime}-t(t+2) y^{\prime}+(t+2) y=0\) Answer: The Wronskian of two solutions for the given differential equation is: \(W(t) = ce^{(t + 2\ln |t| - C)}\), where c is a constant.

Step-by-step solution

Write down the given differential equation

The given differential equation is: \[ t^{2} y^{\prime \prime}-t(t+2) y^{\prime}+(t+2) y=0 \]

Rewrite the differential equation into a normal form

Divide by \(t^2\) to rewrite the given differential equation in the normal (standard) form: \[ y^{\prime \prime}-\frac{t+2}{t} y^{\prime}+\frac{t+2}{t^2} y=0 \]

Identify the coefficients in the normal form

The coefficients in the normal form are: - \(p(t) = -\frac{t+2}{t}\) - \(q(t) = \frac{t+2}{t^2}\)

Use Abel's formula

Recall Abel's formula for the Wronskian of two solutions of a second-order linear homogeneous differential equation with variable coefficients: \[ W(t) = ce^{-\int p(t) \mathrm{d}t} \] where \(c\) is a constant. Find the integral of \(p(t)\): \[ \int p(t) \mathrm{d}t = \int -\frac{t+2}{t} \mathrm{d}t \]

Evaluate the integral

To evaluate the integral, split the fraction into two parts: \[ \int -\frac{t+2}{t} \mathrm{d}t = -\int \left(\frac{t}{t} + \frac{2}{t}\right) \mathrm{d}t \] Now integrate each part: \[ - \int (1+\frac{2}{t}) \mathrm{d}t = - \left(t + 2\ln |t|\right) + C \] where \(C\) is the constant of integration.

Apply Abel's formula

Now apply Abel's formula to find the Wronskian: \[ W(t) = ce^{-(- \left(t + 2\ln |t|\right) + C)} \] Simplify to obtain the final expression for the Wronskian: \[ W(t) = ce^{(t + 2\ln |t| - C)} \] Since we don't need to compute the actual solutions for the given differential equation, the expression for the Wronskian is complete.

Key concepts

Second-order Linear Homogeneous Differential Equations

In the study of differential equations, second-order linear homogeneous differential equations hold particular significance. They're written in the form \( y^{{\tt\prime\prime}} + p(t)y^{{\tt\prime}} + q(t)y = 0 \) where \( p(t) \) and \( q(t) \) are functions of the independent variable \( t \) and \( y \) is the dependent variable. Homogeneity implies that there is no term without the dependent variable. Such equations often arise in physics and engineering to describe phenomena such as harmonic oscillations and wave propagation.

For instance, the differential equation from our example, \( t^{2} y^{{\tt\prime\prime}}-t(t+2) y^{{\tt\prime}}+(t+2) y=0 \), is of this type, once we divide through by \( t^{2} \) to express it in standard form. The solutions to these equations can reveal valuable physical behaviors and in the instance of no variable coefficient \( t \) present, the solutions could even potentially form a basis for a solution space—characteristic of linear homogeneous differential equations.

Understanding the nature of the solutions to these equations, the conditions under which they exist, and the mathematical methods for finding them, like the method of undetermined coefficients or variation of parameters, is critical for students tackling complex systems in their studies.

Abel's Formula

One of the powerful tools for analyzing second-order linear homogeneous differential equations with variable coefficients is Abel's formula, also known as Abel's identity. It provides us with an expression to directly compute the Wronskian of two solutions without finding the solutions themselves. The Wronskian is a determinant used to establish the linear independence of a set of functions—crucial in proving that we have a full set of solutions.

Abel's formula states that for a differential equation of the form \( y^{{\tt\prime\prime}} + p(t)y^{{\tt\prime}} + q(t)y = 0 \), the Wronskian \( W(t) \) can be found using \( W(t) = ce^{-\int p(t) \mathrm{d}t} \), where \( c \) is an arbitrary constant. This form is valuable because it bypasses the need to derive the actual functional solutions, which can be quite cumbersome with variable coefficients. In our example problem, Abel’s formula swiftly takes us to the expression for the Wronskian after substituting in and integrating the coefficient \( p(t) \).

Employing Abel's formula s requires one to recognize the structure of a homogeneous differential equation and perform the necessary algebraic manipulations to bring it to standard form, as displayed in the solution steps. Given its efficiency and utility, Abel’s formula is a must-know for students delving into differential equations.

Variable Coefficients in Differential Equations

A major complicating factor in differential equations occurs when coefficients are not constants but rather functions of the independent variable, termed variable coefficients. The equation from our example features variable coefficients, and transforming it into its standard form involves dividing the entire equation by \( t^{2} \) to isolate the \( y^{{\tt\prime\prime}} \) term.

Differential equations with variable coefficients, such as \( y^{{\tt\prime\prime}} - p(t)y^{{\tt\prime}} + q(t)y = 0 \) are typically more challenging to solve analytically because standard methods for constant coefficients don’t apply. Techniques like power series or Frobenius methods may be used alongside numerical methods for more complex cases.

In our exercise, by transforming the differential equation to have variable coefficients distinctly represented as functions \( p(t) \) and \( q(t) \) in the normal form, we set up all necessary parameters to calculate the Wronskian via Abel's formula, which circumvents directly solving the differential equation. This process highlights why understanding the manipulation and application of variable coefficients is imperative when facing a broad range of differential equations. It equips students with the adaptability needed to approach these often daunting equations.