Ch02 7

Ch 2.7: Numerical Approximations:
Euler’s Method
Recall that a first order initial value problem has the form
If f and ∂f/∂y are continuous, then this IVP has a unique
solution y = φ(t) in some interval about t0.
When the differential equation is linear, separable or exact,
we can find the solution by symbolic manipulations.
However, the solutions for most differential equations of
this form cannot be found by analytical means.
Therefore it is important to be able to approach the problem
in other ways.
00 )(),,( ytyytf
dt
dy
==

Direction Fields
For the first order initial value problem
we can sketch a direction field and visualize the behavior of
solutions. This has the advantage of being a relatively
simple process, even for complicated equations. However,
direction fields do not lend themselves to quantitative
computations or comparisons.
,)(),,( 00 ytyytfy ==′

Numerical Methods
For our first order initial value problem
an alternative is to compute approximate values of the
solution y = φ(t) at a selected set of t-values.
Ideally, the approximate solution values will be accompanied
by error bounds that ensure the level of accuracy.
There are many numerical methods that produce numerical
approximations to solutions of differential equations, some of
which are discussed in Chapter 8.
In this section, we examine the tangent line method, which is
also called Euler’s Method.
,)(),,( 00 ytyytfy ==′

Euler’s Method: Tangent Line Approximation
For the initial value problem
we begin by approximating solution y = φ(t) at initial point t0.
The solution passes through initial point (t0, y0) with slope
f(t0,y0). The line tangent to solution at initial point is thus
The tangent line is a good approximation to solution curve on
an interval short enough.
Thus if t1 is close enough to t0,
we can approximate φ(t1) by
( )( )0000 , ttytfyy −+=
,)(),,( 00 ytyytfy ==′
( )( )010001 , ttytfyy −+=

Euler’s Formula
For a point t2 close to t1, we approximate φ(t2) using the line
passing through (t1, y1) with slope f(t1,y1):
Thus we create a sequence yn of approximations to φ(tn):
where fn = f(tn,yn).
For a uniform step size h = tn – tn-1, Euler’s formula becomes
( )
( )
( )nnnnn ttfyy
ttfyy
ttfyy
−⋅+=
−⋅+=
−⋅+=
++ 11
12112
01001

( )( )121112 , ttytfyy −+=
,2,1,0,1 =+=+ nhfyy nnn

Euler Approximation
To graph an Euler approximation, we plot the points
(t0, y0), (t1, y1),…, (tn, yn), and then connect these points with
line segments.
( ) ( )nnnnnnnn ytffttfyy ,where,11 =−⋅+= ++

Example 1: Euler’s Method (1 of 3)
we can use Euler’s method with h = 0.1 to approximate the
solution at t = 0.1, 0.2, 0.3, 0.4, as shown below.
( )( )( )
( )( )( )
( )( )( ) 80.3)1.0(88.22.08.988.2
88.2)1.0(94.12.08.994.1
94.1)1.0(98.2.08.998.
98.)1.0(8.90
334
223
112
001
≈−+=⋅+=
≈−+=⋅+=
≈−+=⋅+=
=+=⋅+=
hfyy
hfyy
hfyy
hfyy
0)0(,2.08.9 =−=′ yyy

Example 1: Exact Solution (2 of 3)
We can find the exact solution to our IVP, as in Chapter 1.2:
( )
( )t
Ct
ey
ky
ekkey
Cty
dt
y
dy
yy
yyy
2.0
2.0
149
491)0(
,49
2.049ln
2.0
49
492.0
0)0(,2.08.9
−
−
−=⇒
−=⇒=
±=+=
+−=−
−=
−
−−=′
=−=′

Example 1: Error Analysis (3 of 3)
From table below, we see that the errors are small. This is
most likely due to round-off error and the fact that the
exact solution is approximately linear on [0, 0.4]. Note:
t Exact y Approx y Error % Rel Error
0.00 0 0.00 0.00 0.00
0.10 0.97 0.98 -0.01 -1.03
0.20 1.92 1.94 -0.02 -1.04
0.30 2.85 2.88 -0.03 -1.05
0.40 3.77 3.8 -0.03 -0.80
100ErrorRelativePercent ×
−
=
exact
approxexact
y
yy

Example 2: Euler’s Method (1 of 3)
we can use Euler’s method with h = 0.1 to approximate the
solution at t = 1, 2, 3, and 4, as shown below.
Exact solution (see Chapter 2.1):
( )
( )
( )
( )

15.4)1.0()15.3)(2(3.0415.3
15.3)1.0()31.2)(2(2.0431.2
31.2)1.0()6.1)(2(1.046.1
6.1)1.0()1)(2(041
334
223
112
001
≈+−+=⋅+=
≈+−+=⋅+=
=+−+=⋅+=
=+−+=⋅+=
hfyy
hfyy
hfyy
hfyy
1)0(,24 =+−=′ yyty
t
ety 2
4
11
2
1
4
7
++−=

Example 2: Error Analysis (2 of 3)
The first ten Euler approxs are given in table below on left.
A table of approximations for t = 0, 1, 2, 3 is given on right.
See text for numerical results with h = 0.05, 0.025, 0.01.
The errors are small initially, but quickly reach an
unacceptable level. This suggests a nonlinear solution.
0.00 1.00 1.00 0.00 0.00
0.10 1.66 1.60 0.06 3.55
0.20 2.45 2.31 0.14 5.81
0.30 3.41 3.15 0.26 7.59
0.40 4.57 4.15 0.42 9.14
0.50 5.98 5.34 0.63 10.58
0.60 7.68 6.76 0.92 11.96
0.70 9.75 8.45 1.30 13.31
0.80 12.27 10.47 1.80 14.64
0.90 15.34 12.89 2.45 15.96
1.00 19.07 15.78 3.29 17.27
0.00 1.00 1.00 0.00 0.00
1.00 19.07 15.78 3.29 17.27
2.00 149.39 104.68 44.72 29.93
3.00 1109.18 652.53 456.64 41.17
4.00 8197.88 4042.12 4155.76 50.69
t
ety 2
4
11
2
1
4
7
:SolutionExact
++−=

Example 2: Error Analysis & Graphs (3 of 3)
Given below are graphs showing the exact solution (red)
plotted together with the Euler approximation (blue).
0.00 1.00 1.00 0.00 0.00
1.00 19.07 15.78 3.29 17.27
2.00 149.39 104.68 44.72 29.93
3.00 1109.18 652.53 456.64 41.17
4.00 8197.88 4042.12 4155.76 50.69
t
ety 2
4
11
2
1
4
7
:SolutionExact
++−=

General Error Analysis Discussion (1 of 4)
Recall that if f and ∂f/∂y are continuous, then our first order
initial value problem
has a solution y = φ(t) in some interval about t0.
In fact, the equation has infinitely many solutions, each one
indexed by a constant c determined by the initial condition.
Thus φ is the member of an infinite family of solutions that
satisfies φ(t0) = y0.
00 )(),,( ytyytfy ==′

General Error Analysis Discussion (2 of 4)
The first step of Euler’s method uses the tangent line to φ at
the point (t0, y0) in order to estimate φ(t1) with y1.
The point (t1, y1) is typically not on the graph of φ, because y1
is an approximation of φ(t1).
Thus the next iteration of Euler’s method does not use a
tangent line approximation to φ, but rather to a nearby
solution φ1 that passes through the point (t1, y1).
Thus Euler’s method uses a
succession of tangent lines
to a sequence of different
solutions φ, φ1, φ2,… of the
differential equation.

Error Analysis Example:
Converging Family of Solutions (3 of 4)
Since Euler’s method uses tangent lines to a sequence of
different solutions, the accuracy after many steps depends on
behavior of solutions passing through (tn, yn), n = 1, 2, 3, …
For example, consider the following initial value problem:
The direction field and graphs of a few solution curves are
given below. Note that it doesn’t matter which solutions we
are approximating with tangent lines, as all solutions get closer
to each other as t increases.
Results of using Euler’s method
for this equation are given in text.
2/
326)(1)0(,23 ttt
eetyyyey −−−
−−==⇒=−+=′ φ

Error Analysis Example:
Divergent Family of Solutions (4 of 4)
Now consider the initial value problem for Example 2:
The direction field and graphs of solution curves are given
below. Since the family of solutions is divergent, at each step
of Euler’s method we are following a different solution than
the previous step, with each solution separating from the
desired one more and more as t increases.
4112471)0(,24 2t
etyyyty ++−=⇒=+−=′

Error Bounds and Numerical Methods
In using a numerical procedure, keep in mind the question of
whether the results are accurate enough to be useful.
In our examples, we compared approximations with exact
solutions. However, numerical procedures are usually used
when an exact solution is not available. What is needed are
bounds for (or estimates of) errors, which do not require
knowledge of exact solution. More discussion on these issues
and other numerical methods is given in Chapter 8.
Since numerical approximations ideally reflect behavior of
solution, a member of a diverging family of solutions is harder
to approximate than a member of a converging family.
Also, direction fields are often a relatively easy first step in
understanding behavior of solutions.

Ch02 7

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