1 Chapter2. Second-order differential Equations 1. Linear Differential Equations If we can express a second order differential equation in the form β²β² + β² + ()= () it is called linear. Otherwise, it is nonlinear. Consider a linear differential equation. If r(x) = 0 it is called homogeneous, otherwise it is called nonhomogeneous. Some examples are: Linear Combination: A linear combination of y 1 ; y 2 is y = a y 1 + b y 2 . For a homogeneous linear differential equation any linear combination of solutions is again a solution. The above result does NOT hold for nonhomogeneous equations. For example, both = and = are solutions ββ + = 0, so is = 2 + 5 . Both = + and = + are solutions to ββ + = , but = + + 2is not. This is a very important property of linear homogeneous equations, called superposition. It means we can multiply a solution by any number, or add two solutions, and obtain a new solution. Linear Independence: Two functions y 1 ; y 2 are linearly independent if a y 1 + b y 2 = 0 a = 0; b = 0. Otherwise they are linearly dependent. (One is a multiple of the other). For example, e x and e 2x are linearly independent. e x and 2e x are linearly dependent. General Solution and Basis: Given a second order, linear, homogeneous differential equation, the general solution is: = 1 + 2 where y 1 ; y 2 are linearly independent. The set y 1 ; y 2 is called a basis, or a fundamental set of the differential equation. As an illustration, consider the equation 2 ββ β 5β + 8= 0. You can easily check that = 2 is a solution. Therefore 22 ; 72 or β 2 are also solutions. But all these are linearly dependent. We expect a second, linearly independent solution, and this is y = x 4 . A combination of solutions is also a solution, so = 2 + 4 or = 102 β 5 4 are also solutions. Therefore the general solution is = 2 + 4 and the basis of solutions is 2 , 4 . 2. Reduction of Order If we know one solution of a second order homogeneous differential equation, we can find the second solution by the method of reduction of order. Consider the differential equation β²β² + β² + ()= 0 Suppose one solution y 1 is known, then set 2 = 1 and insert in the equation. The result will be
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Chapter2. Second-order differential Equations
1. Linear Differential Equations
If we can express a second order differential equation in the form
it is called linear. Otherwise, it is nonlinear. Consider a linear differential equation. If r(x) = 0 it is called
homogeneous, otherwise it is called nonhomogeneous. Some examples are:
Linear Combination: A linear combination of y1; y2 is y = a y1 + b y2.
For a homogeneous linear differential equation any linear combination of solutions is again a solution. The
above result does NOT hold for nonhomogeneous equations.
For example, both π¦ = π ππ π₯ and π¦ = πππ π₯ are solutions π‘π π¦ββ + π¦ = 0, so is π¦ = 2 π ππ π₯ + 5 πππ π₯.
Both π¦ = π ππ π₯ + π₯ and π¦ = πππ π₯ + π₯ are solutions to π¦ββ+ π¦ = π₯, but π¦ = π ππ π₯ + πππ π₯ + 2π₯ is
not.
This is a very important property of linear homogeneous equations, called superposition. It means we can
multiply a solution by any number, or add two solutions, and obtain a new solution.
Linear Independence: Two functions y1; y2 are linearly independent if a y1 + b y2 = 0 a = 0; b = 0.
Otherwise they are linearly dependent. (One is a multiple of the other).
For example, ex and e
2x are linearly independent. e
x and 2e
x are linearly dependent.
General Solution and Basis: Given a second order, linear, homogeneous differential equation, the general
solution is:
π¦ = π π¦1 + π π¦2
where y1; y2 are linearly independent. The set y1; y2 is called a basis, or a fundamental set of the
differential equation.
As an illustration, consider the equation π₯2π¦βββ 5π₯π¦β + 8π¦ = 0. You can easily check that π¦ = π₯2 is a
solution. Therefore 2π₯2; 7π₯2 or βπ₯2 are also solutions. But all these are linearly dependent.
We expect a second, linearly independent solution, and this is y = x4. A combination of solutions is also a
solution, so π¦ = π₯2 + π₯4 or π¦ = 10π₯2 β 5 π₯4 are also solutions.
Therefore the general solution is π¦ = π π₯2 + π π₯4 and the basis of solutions is π₯2 , π₯4 .
2. Reduction of Order
If we know one solution of a second order homogeneous differential equation, we can find the second
solution by the method of reduction of order. Consider the differential equation
Suppose one solution y1 is known, then set π¦2 = π’π¦1 and insert in the equation. The result will be
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π¦1π’β²β² + 2π¦ β²1
+ ππ¦ β²1
π’β² + (π¦ β²β²1
+ ππ¦ β²1
+ ππ¦1) = 0
But y1 is a solution, so the last term is canceled. So we have
π¦1π’β²β² + 2π¦β²
1+ ππ¦β²
1 π’β² = 0
This is still second order, but if we set w = uβ, we will obtain first order equation:
π¦1π€β² + 2π¦β²
1+ ππ¦1 = 0
Solving this, we can find w, then u and then π¦2.
Example: Legendre's equation. The equation
is known as Legendre's equation, after the French mathematician Adrien Marie Legendre
(1752-1833.
Example:
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3. Homogeneous Equations with Constant Coefficients
Up to now we have studied the theoretical aspects of the solution of linear homogeneous differential
equations. Now we will see how to solve the constant coefficient equation π¦ββ+ ππ¦β + ππ¦ = 0 in
practice.
We have the sum of a function and its derivatives equal to zero, so the derivatives must have the same form
as the function. Therefore we expect the function to be ex. If we insert this in the equation, we obtain:
2 + π + π = 0
This is called the characteristic equation of the homogeneous differential equation π¦ββ + ππ¦β + ππ¦ = 0. If we solve the characteristic equation, we will see three different possibilities: Two real roots, double real
root and complex conjugate roots.
Two Real Roots: The general solution is
π¦ = π1 π1π₯ + π2 π2π₯
Example:
Example:
Example:
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Double Real Root: One solution is ex but we know that a second order equation must have two independent
solutions. Let's use the method of reduction of order to find the second solution.
Let's insert y2 = u e
ax in the equation.
Obviously, uββ = 0 therefore u = c1 + c2 x. The general solution is
Example:
Example:
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π¦ = π1 ππ₯ + π2 π₯ ππ₯
Example: Solve π¦ββ + 2π¦β + π¦ = 0.
π¦ = ππ₯ . The characteristic equation is 2 + 2+ 1 = 0. Its solution is the double root = +,-1, therefore the
general solution is
π¦ = π1 πβπ₯ + π2 ππ₯
Complex Conjugate Roots: We need the complex exponentials for this case. Euler's formula is
πππ₯ = cos π₯ + π sin(π₯) This can be proved using Taylor series expansions. If the solution of the characteristic equation is
1 = a + i b, and 2 = a - i b
then the general solution of the differential equation will be