# Ordinary Differential Equations Exercise 1

## Imports



In :

%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np
import seaborn as sns
from scipy.integrate import odeint
from IPython.html.widgets import interact, fixed
import math




:0: FutureWarning: IPython widgets are experimental and may change in the future.



## Euler's method

Euler's method is the simplest numerical approach for solving a first order ODE numerically. Given the differential equation

$$\frac{dy}{dx} = f(y(x), x)$$

with the initial condition:

$$y(x_0)=y_0$$

Euler's method performs updates using the equations:

$$y_{n+1} = y_n + h f(y_n,x_n)$$$$h = x_{n+1} - x_n$$

Write a function solve_euler that implements the Euler method for a 1d ODE and follows the specification described in the docstring:



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def solve_euler(derivs, y0, x):
"""Solve a 1d ODE using Euler's method.

Parameters
----------
derivs : function
The derivative of the diff-eq with the signature deriv(y,x) where
y and x are floats.
y0 : float
The initial condition y = y(x).
x : np.ndarray, list, tuple
The array of times at which of solve the diff-eq.

Returns
-------
y : np.ndarray
Array of solutions y[i] = y(x[i])
"""
h = x-x
y = [y0]
for i in range(len(x)-1):
y.append(y[i] + h*derivs(y[i],x[i]) )
return np.array(y)




In :

solve_euler(lambda y,x: 1, 0, [0,1,2])




Out:

array([0, 1, 2])




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assert np.allclose(solve_euler(lambda y, x: 1, 0, [0,1,2]), [0,1,2])



The midpoint method is another numerical method for solving the above differential equation. In general it is more accurate than the Euler method. It uses the update equation:

$$y_{n+1} = y_n + h f\left(y_n+\frac{h}{2}f(y_n,x_n),x_n+\frac{h}{2}\right)$$

Write a function solve_midpoint that implements the midpoint method for a 1d ODE and follows the specification described in the docstring:



In :

def solve_midpoint(derivs, y0, x):
"""Solve a 1d ODE using the Midpoint method.

Parameters
----------
derivs : function
The derivative of the diff-eq with the signature deriv(y,x) where y
and x are floats.
y0 : float
The initial condition y = y(x).
x : np.ndarray, list, tuple
The array of times at which of solve the diff-eq.

Returns
-------
y : np.ndarray
Array of solutions y[i] = y(x[i])
"""
h = x-x
y = [y0]
for i in range(len(x)-1):
y.append(y[i] + h*derivs( (y[i] + h/2 *derivs(y[i],x[i])) , (x[i] + h/2)))
return np.array(y)




In :

assert np.allclose(solve_midpoint(lambda y, x: 1, 0, [0,1,2]), [0,1,2])



You are now going to solve the following differential equation:

$$\frac{dy}{dx} = x + 2y$$

which has the analytical solution:

$$y(x) = 0.25 e^{2x} - 0.5 x - 0.25$$

First, write a solve_exact function that compute the exact solution and follows the specification described in the docstring:



In :

def solve_exact(x):
"""compute the exact solution to dy/dx = x + 2y.

Parameters
----------
x : np.ndarray
Array of x values to compute the solution at.

Returns
-------
y : np.ndarray
Array of solutions at y[i] = y(x[i]).
"""
h = x-x
y = []
for i in range(len(x)):
y.append(0.25 * np.exp(2* x[i]) - 0.5 * x[i] - 0.25)
return np.array(y)




In :

solve_exact(np.array([0,1,2]))




Out:

array([  0.        ,   1.09726402,  12.39953751])




In :

assert np.allclose(solve_exact(np.array([0,1,2])),np.array([0., 1.09726402, 12.39953751]))



In the following cell you are going to solve the above ODE using four different algorithms:

1. Euler's method
2. Midpoint method
3. odeint
4. Exact

Here are the details:

• Generate an array of x values with $N=11$ points over the interval $[0,1]$ ($h=0.1$).
• Define the derivs function for the above differential equation.
• Using the solve_euler, solve_midpoint, odeint and solve_exact functions to compute the solutions using the 4 approaches.

Visualize the solutions on a sigle figure with two subplots:

1. Plot the $y(x)$ versus $x$ for each of the 4 approaches.
2. Plot $\left|y(x)-y_{exact}(x)\right|$ versus $x$ for each of the 3 numerical approaches.

Your visualization should have legends, labeled axes, titles and be customized for beauty and effectiveness.

While your final plot will use $N=10$ points, first try making $N$ larger and smaller to see how that affects the errors of the different approaches.



In :

x = np.linspace(0,1,11)
h = x - x
derivs = lambda y,x : x + 2*y




In :

plt.plot(x, solve_euler(derivs, 0, x), label="Euler's Method", color="yellow")
plt.plot(x, solve_midpoint(derivs, 0, x), label ="Midpoint Method", color="blue")
plt.plot(x, solve_exact(x), label="Exact", color="red")
plt.plot(x, odeint(derivs, 0, x), label="ODEint", color="green")
plt.xlabel("x")
plt.ylabel("y(x)")
plt.title("Methods of Solving ODEs")
plt.legend()
ax = plt.gca()
ax.set_axis_bgcolor("#fcfcfc")







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assert True # leave this for grading the plots




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