Lab Sheet 09: Differential Equations
The final topic that we will consider is the study of differential equations.
Building blocks

Solving differential equations.
A video describing the concept.
We can use SymPy to solve differential equations. For example:
>>> import sympy as sym >>> y = sym.Function('y') >>> x = sym.symbols('x') >>> sol = sym.dsolve(sym.Derivative(y(x), x)  y(x), y(x)) >>> sol Eq(y(x), C1*exp(x))
The output given is an instance of the SymPy equation class that states:
Let us verify that the solution is correct:
>>> sym.diff(sol.rhs, x) == sol.rhs True
Here is the more general equation:
>>> k = sym.symbols('k') >>> sol = sym.dsolve(sym.Derivative(y(x), x)  k * y(x), y(x)) >>> sol Eq(y(x), C1*exp(k*x)) >>> sym.diff(sol.rhs, x) == k * sol.rhs True
Experiment with solving a modified version of the differential equation considered here.

Higher order differential equations.
A video describing the concept.
We can also solve higher order differential equations. For example, the following can be used to model the position of a mass on a spring:
>>> m, c, k, t = sym.symbols('m, c, k, t') >>> x = sym.Function('x') >>> sym.dsolve(m * sym.Derivative(x(t), t, 2) + c * sym.Derivative(x(t), t) + k * x(t), x(t)) Eq(x(t), C1*exp(t*(c  sqrt(c**2  4*k*m))/(2*m)) + C2*exp(t*(c + sqrt(c**2  4*k*m))/(2*m)))
Experiment with solving a modified version of the differential equation considered here.

Systems of differential equations.
A video describing the concept.
We can solve systems of differential equations like the following:
We still use
dsolve
we just pass it both equations as an argument:>>> eq1 = sym.Derivative(x(t), t)  1 + y(t) >>> eq2 = sym.Derivative(y(t), t)  1 + x(t) >>> sym.dsolve((eq1, eq2)) [Eq(x(t), C1*exp(t)  C2*exp(t) + 1), Eq(y(t), C1*exp(t) + C2*exp(t) + 1)]
The solution is given as:
Experiment with solving a modified version of the differential equations considered here.

Worked example: Solving and visualising the solution of a differential equation.
A video describing the concept.
Let us solve the following differential equation:
>>> y, x = sym.Function('y'), sym.symbols('x') >>> eq = sym.Derivative(y(x), x) + 4 * y(x)  5 * sym.exp(x) >>> sol = sym.dsolve(eq, y(x)) >>> sol Eq(y(x), (C1 + exp(5*x))*exp(4*x))
As before the right hand side of our equation has a constant of integration which defines a whole family of solutions.
However, because this is a first order differential equation, only one of those equations will go through any given point. So let us assume that \(y(0)=1)\) and find the particular solution of our differential equation.
>>> C1 = sym.symbols('C1') # We're going to use this as a symbolic variable >>> boundary_cond = sym.Eq(sol.rhs.subs({x: 0}), 1) >>> boundary_cond Eq(C1 + 1, 1) >>> particular_constant = sym.solveset(boundary_cond, C1) >>> particular_constant {0}
So we can now create the particular solution:
>>> particular_sol = sol.subs({C1: list(particular_constant)[0]}) >>> particular_sol Eq(y(x), exp(x))
We can plot this:
>>> sym.plot(particular_sol.rhs, (x, 0, 10)) <...
Further work
These questions aim to push a bit further.

Find the general solutions to the following 4 differential equations:
 \(\frac{dy}{dx}6y=3e^x\)
 \(\frac{dy}{dx}+\frac{x(2x3)}{x^2+1}=\sin(x)\)
 \(\frac{d^2y}{dx^2}y=\sin(5x)\)
 \(\frac{d^2y}{dx^2}+2\frac{dy}{dx}+2x=\cosh(x)\)

For each of the differential equations from question 5, obtain particular solutions with the corresponding conditions and plot the solutions.
 \(y(0)=3\)
 \(y(0)=4\)
 \(y(3)=1, y’(3)=0\)
 \(y(1)=2, y’(1)=72\)
(Hint: to be able to solve the systems of linear equations for 3 and 4 you can use linear algebra or perhaps look at the
sympy
function:linsolve
.) 
(Optional) The love story between Romeo and Juliet can be modelled with the following system of differential equations:
Where \(x(t)\) represents the affection of Juliet towards Romeo and \(y(t)\) the affection of Romeo towards Juliet (negative affection represents ‘hatred’).
Solve this system of equations and, assuming that Romeo is initially attracted to Juliet (\(y(0)=1\)) but that Juliet is initially indifferent to Romeo (\(x(0)=0\)), describe the long term relationship between the two characters.
Describe the behaviour of the system if Romeo and Juliet are initially indifferent to each other.

(Optional) A battle between two armies can be modelled with the following set of differential equations:
Obtain the solution to this system of equations. Assuming that \(x(0)=100\) and that \(y(0)=700\) plot the two solutions of the equations, which army wins this battle? When does the battle end?