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that G is di¬erentiable in d and denoting formally by G (d) its derivative
with respect to d (if G : Rn ’ Rm , G (d) will be the Jacobian matrix of
G evaluated at the vector d), a Taylor™s expansion of G truncated at ¬rst
order ensures that

G(d + δd) ’ G(d) = G (d)δd + o( δd ) for δd ’ 0,

where · is a suitable norm for δd and o(·) is the classical in¬nitesimal
symbol denoting an in¬nitesimal term of higher order with respect to its
argument. Neglecting the in¬nitesimal of higher order with respect to δd ,
from (2.4) and (2.5) we respectively deduce that

K(d) G (d) , Kabs (d) G (d) , (2.7)

the symbol · denoting the matrix norm associated with the vector norm
(de¬ned in (1.19)). The estimates in (2.7) are of great practical usefulness
in the analysis of problems in the form (2.6), as shown in the forthcoming

Example 2.2 (Algebraic equations of second degree) The solutions to the
algebraic equation x2 ’ 2px + 1 = 0, with p ≥ 1, are x± = p ± p2 ’ 1. In this
case, F (x, p) = x2 ’ 2px + 1, the datum d is the coe¬cient p, while x is the vector
of components {x+ , x’ }. As for the condition number, we notice that (2.6) holds
36 2. Principles of Numerical Mathematics

by taking G : R ’ R2 , G(p) = {x+ , x’ }. Letting G± (p) = x± , it follows that
G± (p) = 1 ± p/ p2 ’ 1. Using (2.7) with · = · 2 we get

K(p) , p > 1. (2.8)
p2 ’ 1

From (2.8) it turns out that in the case of separated roots (say, if p ≥ 2)
problem F (x, p) = 0 is well conditioned. The behavior dramatically changes in
the case of multiple roots, that is when p = 1. First of all, one notices that the
function G± (p) = p ± p2 ’ 1 is no longer di¬erentiable for p = 1, which makes
(2.8) meaningless. On the other hand, equation (2.8) shows that, for p close to
1, the problem at hand is ill conditioned. However, the problem is not ill posed.
Indeed, following Remark 2.1, it is possible to reformulate it in an equivalent
manner as F (x, t) = x2 ’ ((1 + t2 )/t)x + 1 = 0, with t = p + p2 ’ 1, whose
roots x’ = t and x+ = 1/t coincide for t = 1. The change of parameter thus
removes the singularity that is present in the former representation of the roots
as functions of p. The two roots x’ = x’ (t) and x+ = x+ (t) are now indeed
regular functions of t in the neighborhood of t = 1 and evaluating the condition
number by (2.7) yields K(t) 1 for any value of t. The transformed problem is

thus well conditioned.

Example 2.3 (Systems of linear equations) Consider the linear system Ax
= b, where x and b are two vectors in Rn , while A is the matrix (n — n) of the
real coe¬cients of the system. Suppose that A is nonsingular; in such a case x
is the unknown solution x, while the data d are the right-hand side b and the
matrix A, that is, d = {bi , aij , 1 ¤ i, j ¤ n}.
Suppose now that we perturb only the right-hand side b. We have d = b,
x = G(b) = A’1 b so that, G (b) = A’1 , and (2.7) yields

A’1 b Ax
A’1 ¤ A A’1 = K(A),
K(d) = (2.9)
’1 b
A x

where K(A) is the condition number of matrix A (see Section 3.1.1) and the use
of a consistent matrix norm is understood. Therefore, if A is well conditioned,
solving the linear system Ax=b is a stable problem with respect to perturbations
of the right-hand side b. Stability with respect to perturbations on the entries of

A will be analyzed in Section 3.10.

Example 2.4 (Nonlinear equations) Let f : R ’ R be a function of class
C 1 and consider the nonlinear equation

F (x, d) = f (x) = •(x) ’ d = 0,

where • : R ’ R is a suitable function and d ∈ R a datum (possibly equal
to zero). The problem is well de¬ned only if • is invertible in a neighborhood
of d: in such a case, indeed, x = •’1 (d) and the resolvent is G = •’1 . Since
(•’1 ) (d) = [• (x)] , the ¬rst relation in (2.7) yields, for d = 0,

|[• (x)]’1 |,
K(d) (2.10)
2.2 Stability of Numerical Methods 37

while if d = 0 or x = 0 we have

|[• (x)]’1 |.
Kabs (d) (2.11)

The problem is thus ill posed if x is a multiple root of •(x)’d; it is ill conditioned
when • (x) is “small”, well conditioned when • (x) is “large”. We shall further

address this subject in Section 6.1.

In view of (2.8), the quantity G (d) is an approximation of Kabs (d) and
is sometimes called ¬rst order absolute condition number. This latter rep-
resents the limit of the Lipschitz constant of G (see Section 11.1) as the
perturbation on the data tends to zero.
Such a number does not always provide a sound estimate of the condition
number Kabs (d). This happens, for instance, when G vanishes at a point
whilst G is non null in a neighborhood of the same point. For example,
take x = G(d) = cos(d) ’ 1 for d ∈ (’π/2, π/2); we have G (0) = 0, while
Kabs (0) = 2/π.

2.2 Stability of Numerical Methods
We shall henceforth suppose the problem (2.1) to be well posed. A numer-
ical method for the approximate solution of (2.1) will consist, in general,
of a sequence of approximate problems

Fn (xn , dn ) = 0 (2.12)

depending on a certain parameter n (to be de¬ned case by case). The
understood expectation is that xn ’ x as n ’ ∞, i.e. that the numerical
solution converges to the exact solution. For that, it is necessary that dn ’
d and that Fn “approximates” F , as n ’ ∞. Precisely, if the datum d of
problem (2.1) is admissible for Fn , we say that (2.12) is consistent if

Fn (x, d) = Fn (x, d) ’ F (x, d) ’ 0 for n ’ ∞ (2.13)

where x is the solution to problem (2.1) corresponding to the datum d.
The meaning of this de¬nition will be made precise in the next chapters
for any single class of considered problems.
A method is said to be strongly consistent if Fn (x, d) = 0 for any value
of n and not only for n ’ ∞.
In some cases (e.g., when iterative methods are used) problem (2.12)
could take the following form

Fn (xn , xn’1 , . . . , xn’q , dn ) = 0 (2.14)

where x0 , x1 , . . . , xq’1 are given. In such a case, the property of strong
consistency becomes Fn (x, x, . . . , x, d) = 0 for all n ≥ q.
38 2. Principles of Numerical Mathematics

Example 2.5 Let us consider the following iterative method (known as New-
ton™s method and discussed in Section 6.2.2) for approximating a simple root ±
of a function f : R ’ R,

f (xn’1 )
xn = xn’1 ’ n ≥ 1.
given x0 , , (2.15)
f (xn’1 )

The method (2.15) can be written in the form (2.14) by setting Fn (xn , xn’1 , f ) =
xn ’ xn’1 + f (xn’1 )/f (xn’1 ) and is strongly consistent since Fn (±, ±, f ) = 0
for all n ≥ 1.
Consider now the following numerical method (known as the composite mid-
point rule discussed in Section 9.2) for approximating x = a f (t) dt,
tk + tk+1
xn = H f ,

where H = (b ’ a)/n and tk = a + (k ’ 1)H, k = 1, . . . , (n + 1). This method
is consistent; it is also strongly consistent provided thet f is a piecewise linear
More generally, all numerical methods obtained from the mathematical prob-
lem by truncation of limit operations (such as integrals, derivatives, series, . . . )

are not strongly consistent.

Recalling what has been previously stated about problem (2.1), in order
for the numerical method to be well posed (or stable) we require that for any
¬xed n, there exists a unique solution xn corresponding to the datum dn ,
that the computation of xn as a function of dn is unique and, furthermore,
that xn depends continuously on the data, i.e.

∀· > 0, ∃Kn (·, dn ) : δdn < · ’ δxn ¤ Kn (·, dn ) δdn . (2.16)

As done in (2.4), we introduce for each problem in the sequence (2.12) the

δxn / xn δxn
Kn (dn ) = sup , Kabs,n (dn ) = sup , (2.17)
δdn / dn δdn
δdn ∈Dn δdn ∈Dn

and then de¬ne

K num (dn ) = lim sup Kn (dn ), num
Kabs (dn ) = lim sup Kabs,n (dn ).
k’∞ n≥k k’∞ n≥k

We call K num (dn ) the relative asymptotic condition number of the numer-
ical method (2.12) and Kabs (dn ) absolute asymptotic condition number,
corresponding to the datum dn .
The numerical method is said to be well conditioned if K num is “small”
for any admissible datum dn , ill conditioned otherwise. As in (2.6), let us
2.2 Stability of Numerical Methods 39

consider the case where, for each n, the functional relation (2.1) de¬nes a
mapping Gn between the sets of the numerical data and the solutions
xn = Gn (dn ), that is Fn (Gn (dn ), dn ) = 0. (2.18)
Assuming that Gn is di¬erentiable, we can obtain from (2.17)
Kn (dn ) Gn (dn ) , Kabs,n (dn ) Gn (dn ) . (2.19)
Gn (dn )

Example 2.6 (Sum and subtraction) The function f : R2 ’ R, f (a, b) =
a + b, is a linear mapping whose gradient is the vector f (a, b) = (1, 1)T . Using
the vector norm · 1 de¬ned in (1.13) yields K(a, b) (|a| + |b|)/(|a + b|), from
which it follows that summing two numbers of the same sign is a well conditioned
operation, being K(a, b) 1. On the other hand, subtracting two numbers almost
equal is ill conditioned, since |a + b| |a| + |b|. This fact, already pointed out in
Example 2.2, leads to the cancellation of signi¬cant digits whenever numbers can
be represented using only a ¬nite number of digits (as in ¬‚oating-point arithmetic,

see Section 2.5).

Example 2.7 Consider again the problem of computing the roots of a polyno-
mial of second degree analyzed in Example 2.2. When p > 1 (separated roots),
such a problem is well conditioned. However, we generate an unstable algorithm
if we evaluate the root x’ by the formula x’ = p ’ p2 ’ 1. This formula is
indeed subject to errors due to numerical cancellation of signi¬cant digits (see
Section 2.4) that are introduced by the ¬nite arithmetic of the computer. A pos-
sible remedy to this trouble consists of computing x+ = p + p2 ’ 1 at ¬rst,
then x’ = 1/x+ . Alternatively, one can solve F (x, p) = x2 ’ 2px + 1 = 0 using
Newton™s method (proposed in Example 2.5)
xn = xn’1 ’ (x2 ’ 2pxn’1 + 1)/(2xn’1 ’ 2p) = fn (p), n ≥ 1, x0 given.

|p|/|xn ’ p|. To compute K num (p)
Applying (2.19) for p > 1 yields Kn (p)
we notice that, in the case when the algorithm converges, the solution xn would
converge to one of the roots x+ or x’ ; therefore, |xn ’ p| ’ p2 ’ 1 and thus
Kn (p) ’ K num (p) |p|/ p2 ’ 1, in perfect agreement with the value (2.8) of
the condition number of the exact problem.
We can conclude that Newton™s method for the search of simple roots of a
second order algebraic equation is ill conditioned if |p| is very close to 1, while it

is well conditioned in the other cases.

The ¬nal goal of numerical approximation is, of course, to build, through
numerical problems of the type (2.12), solutions xn that “get closer” to the
solution of problem (2.1) as much as n gets larger. This concept is made
precise in the next de¬nition.

De¬nition 2.2 The numerical method (2.12) is convergent i¬
∀µ > 0 ∃n0 (µ), ∃δ(n0 , µ) > 0 :
∀n > n0 (µ), ∀ δdn < δ(n0 , µ) ’ x(d) ’ xn (d + δdn ) ¤ µ,
40 2. Principles of Numerical Mathematics

where d is an admissible datum for the problem (2.1), x(d) is the corre-
sponding solution and xn (d + δdn ) is the solution of the numerical problem
(2.12) with datum d + δdn .

To verify the implication (2.20) it su¬ces to check that under the same
x(d + δdn ) ’ xn (d + δdn ) ¤ . (2.21)
Indeed, thanks to (2.3) we have

x(d) ’ xn (d + δdn ) ¤ x(d) ’ x(d + δdn )

+ x(d + δdn ) ’ xn (d + δdn ) ¤ K(δ(n0 , µ), d) δdn + 2 .

Choosing δdn such that K(δ(n0 , µ), d) δdn < 2 , one obtains (2.20).

Measures of the convergence of xn to x are given by the absolute error
or the relative error, respectively de¬ned as

|x ’ xn |
E(xn ) = |x ’ xn |, Erel (xn ) = , (if x = 0). (2.22)

In the cases where x and xn are matrix or vector quantities, in addition
to the de¬nitions in (2.22) (where the absolute values are substituted by
suitable norms) it is sometimes useful to introduce the error by component
de¬ned as
|(x ’ xn )ij |
Erel (xn ) = max . (2.23)
|xij |

2.2.1 Relations between Stability and Convergence
The concepts of stability and convergence are strongly connected.
First of all, if problem (2.1) is well posed, a necessary condition in order
for the numerical problem (2.12) to be convergent is that it is stable.
Let us thus assume that the method is convergent, and prove that it is
stable by ¬nding a bound for δxn . We have

xn (d + δdn ) ’ xn (d) ¤ xn (d) ’ x(d)
δxn =

x(d) ’ x(d + δdn ) + x(d + δdn ) ’ xn (d + δdn )
+ (2.24)

¤ K(δ(n0 , µ), d) δdn + µ,

having used (2.3) and (2.21) twice. From (2.24) we can conclude that, for n
su¬ciently large, δxn / δdn can be bounded by a constant of the order
2.3 A priori and a posteriori Analysis 41

of K(δ(n0 , µ), d), so that the method is stable. Thus, we are interested in
stable numerical methods since only these can be convergent.
The stability of a numerical method becomes a su¬cient condition for
the numerical problem (2.12) to converge if this latter is also consistent
with problem (2.1). Indeed, under these assumptions we have
x(d + δdn ) ’ xn (d + δdn ) ¤ x(d + δdn ) ’ x(d)

x(d) ’ xn (d) + xn (d) ’ xn (d + δdn ) .
Thanks to (2.3), the ¬rst term at right-hand side can be bounded by δdn
(up to a multiplicative constant independent of δdn ). A similar bound holds
for the third term, due to the stability property (2.16). Finally, concerning
the remaining term, if Fn is di¬erentiable with respect to the variable x,
an expansion in a Taylor series gives
Fn (x(d), d) ’ Fn (xn (d), d) = |(x,d) (x(d) ’ xn (d)),
for a suitable x “between” x(d) and xn (d). Assuming also that ‚Fn /‚x is
invertible, we get
x(d) ’ xn (d) = [Fn (x(d), d) ’ Fn (xn (d), d)]. (2.25)
‚x |(x,d)

On the other hand, replacing Fn (xn (d), d) with F (x(d), d) (since both terms
are equal to zero) and passing to the norms, we ¬nd
x(d) ’ xn (d) ¤ Fn (x(d), d) ’ F (x(d), d) .
‚x |(x,d)

Thanks to (2.13) we can thus conclude that x(d)’xn (d) ’ 0 for n ’ ∞.
The result that has just been proved, although stated in qualitative terms,
is a milestone in numerical analysis, known as equivalence theorem (or
Lax-Richtmyer theorem): “for a consistent numerical method, stability is
equivalent to convergence”. A rigorous proof of this theorem is available in
[Dah56] for the case of linear Cauchy problems, or in [Lax65] and in [RM67]
for linear well-posed initial value problems.

2.3 A priori and a posteriori Analysis
The stability analysis of a numerical method can be carried out following
di¬erent strategies:
1. forward analysis, which provides a bound to the variations δxn on
the solution due to both perturbations in the data and to errors that
are intrinsic to the numerical method;
42 2. Principles of Numerical Mathematics

2. backward analysis, which aims at estimating the perturbations that
should be “impressed” to the data of a given problem in order to
obtain the results actually computed under the assumption of working
in exact arithmetic. Equivalently, given a certain computed solution
xn , backward analysis looks for the perturbations δdn on the data
such that Fn (xn , dn + δdn ) = 0. Notice that, when performing such
an estimate, no account at all is taken into the way xn has been
obtained (that is, which method has been employed to generate it).

Forward and backward analyses are two di¬erent instances of the so
called a priori analysis. This latter can be applied to investigate not only
the stability of a numerical method, but also its convergence. In this case
it is referred to as a priori error analysis, which can again be performed
using either a forward or a backward technique.
A priori error analysis is distincted from the so called a posteriori error
analysis, which aims at producing an estimate of the error on the grounds
of quantities that are actually computed by a speci¬c numerical method.
Typically, denoting by xn the computed numerical solution, approximation
to the solution x of problem (2.1), the a posteriori error analysis aims at
evaluating the error x ’ xn as a function of the residual rn = F (xn , d) by
means of constants that are called stability factors (see [EEHJ96]).

Example 2.8 For the sake of illustration, consider the problem of ¬nding the
zeros ±1 , . . . , ±n of a polynomial pn (x) = n ak xk of degree n.
Denoting by pn (x) = n ak xk a perturbed polynomial whose zeros are ±i ,
˜ ˜ ˜
forward analysis aims at estimating the error between two corresponding zeros
±i and ±i , in terms of the variations on the coe¬cients ak ’ ak , k = 0, 1, . . . , n.
˜ ˜
On the other hand, let {±i } be the approximate zeros of pn (computed some-
how). Backward analysis provides an estimate of the perturbations δak which
should be impressed to the coe¬cients so that n (ak + δak )±i = 0, for a ¬xed
±i . The goal of a posteriori error analysis would rather be to provide an estimate
of the error ±i ’ ±i as a function of the residual value pn (±i ).
ˆ ˆ

This analysis will be carried out in Section 6.1.

Example 2.9 Consider the linear system Ax=b, where A∈ Rn—n is a nonsin-
gular matrix.

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