The derivatives of the Dirac delta ``function'' are defined in a way which
is similar to that of the delta ``function'' itself. The first condition
is the same, and the second and third conditions are not really required.
The crucial difference is that the fourth condition in the definition of
the Dirac delta ``function'' is replaced by the second condition in the
list that follows. The ``function''
is the
derivative of
with respect
to
if its defining limit
satisfies the two
conditions that follow.
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(32) |
has the value shown, for any open interval which contains the
point
, where
is the
derivative of
with respect to
.
This is the usual form of this condition, when it is formulated in
strictly real terms. However, we will impose a slight additional
restriction on the real functions , by assuming that the
limit to the point
on the unit circle that corresponds to
, of the
angular derivative of the
corresponding inner analytic function
, exists and is
finite. Since these proper inner analytic functions are all in the same
integral-differential chain, this implies that the limits to
of
all the inner analytic functions
exist, for all
.
The second condition above is, in fact, the fundamental property of each
derivative of the delta ``function'', including the ``function'' itself in
the case . Just as in the case of the delta ``function'' itself, the
additional part of the second condition, involving the inner analytic
function
, consists of a weak limitation on the test
functions
, and does not affect the definition of the singular
distributions themselves. This is certainly the case for our definitions
here, since we define each one of these objects through a definite and
unique inner analytic function.
In this section we will prove the following theorem.
Before we attempt to prove this theorem, let us note that the proof relies
on a property of angular differentiation, which was established
in [#!CAoRFI!#], namely that angular differentiation is equivalent to
partial differentiation with respect to , at constant
. When
we take the
limit, this translates to the fact that
taking the angular derivative of the inner analytic function
within
the open unit disk corresponds to taking the derivative with respect to
, on the unit circle, of the corresponding real object.
If this derivative cannot be taken directly on the unit circle, then one
can define it by taking the angular derivative of the corresponding
inner analytic function and then considering the
limit
of the real part of the resulting function. Since analytic functions can
be differentiated any number of times, the procedure can then be iterated
in order to define all the higher-order derivatives with respect to
on the unit circle. Equivalently, one can just consider traveling
along the integral-differential chain indefinitely in the differentiation
direction.
Consider therefore the integral-differential chain of proper inner
analytic functions that is obtained from the proper inner analytic
function associated to
, that is, the unique
integral-differential chain to which the proper inner analytic function
belongs. Consider in particular the set of proper inner analytic functions
which is obtained from
in the
differentiation direction along this chain, for which we have
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|
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(34) |
for all strictly positive , where we recall that
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(35) |
We will now prove that in the
limit we have
for
, or, equivalently, that we have for the
inner analytic function
associated to the
derivative
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(37) |
for
. We are now ready to prove the theorem,
as stated in Equation (36). Let us first prove, however, that
the first condition holds for all the derivatives of the delta
``function''.
Since
has a single singular point at
,
the same is true for all its angular derivatives. Therefore the
limit of all the angular derivatives exists everywhere within the
open interval of the unit circle that excludes the point
. Since
is identically zero
within this interval, and since angular differentiation within the open
unit disk corresponds to differentiation with respect to
on the
unit circle, so that we have
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(38) |
for
, it follows at once that
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|
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(39) |
for
, everywhere but at the singular point
, for all values of
. This establishes that the first
condition holds.
Let us now prove that the second condition, which relates directly to the singular point, holds, leading to the result as stated in Equation (36).
In order to do this, we start with the case , and consider the
following real integral on the circle of radius
, which we
integrate by parts, noting that the integrated term is zero because we are
integrating on a circle,
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(40) |
where
is the inner analytic function associated to
. Note that the
partial derivatives involved certainly exist, since both
and
are the
real parts of inner analytic functions. If we now take the
limit, on the right-hand side we recover the Dirac delta
``function'' on the unit circle, and therefore we have
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|
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(41) |
so long as is differentiable, were we used the fundamental
property of the Dirac delta ``function''. We thus obtain the relation for
the derivative of the delta ``function'',
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(42) |
where
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(43) |
We may therefore write that
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(44) |
In this way we have obtained the result for
by using the known result for
. We may now
repeat this procedure to obtain the result for
from the result for
, and therefore from the result for
. We simply consider the following real
integral on the circle of radius
, which we again integrate by
parts, recalling that the integrated term is zero because we are
integrating on a circle,
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(45) |
If we now take the
limit, on the right-hand side we
recover the first derivative of the Dirac delta ``function'' on the unit
circle, and therefore we have
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|
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(46) |
so long as is differentiable to second order, were we used the
fundamental property of the first derivative of the Dirac delta
``function''. We thus obtain the relation for the second derivative of the
delta ``function'',
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(47) |
where
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(48) |
We may therefore write that
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(49) |
Clearly, this procedure can be iterated times, thus resulting in the
relation
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(50) |
for
. Note that all the derivatives with
respect to
involved in the argument exist, for arbitrarily high
orders, since both
and
are the real parts of inner analytic functions,
and thus are infinitely differentiable on both arguments.
We may now formalize the proof using finite induction. We thus assume the
results for the case ,
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|
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(51) |
and proceed to examine the next case. We consider therefore the following
real integral on the circle of radius , which we integrate by
parts, recalling once more that the integrated term is zero because we are
integrating on a circle,
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(52) |
If we now take the
limit, on the right-hand side we
recover the
derivative of the Dirac delta ``function'' on
the unit circle, and therefore we have
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|
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(53) |
so long as is differentiable to order
, were we used the
fundamental property of the
derivative of the Dirac delta
``function''. We thus obtain the relation for the
derivative
of the delta ``function'',
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(54) |
where
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(55) |
We may therefore write that, by finite induction,
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(56) |
for
. We have therefore completed the proof
of Theorem 2.
It is important to note that, just as in the case of the Dirac delta
``function'', when we adopt as the definition of the
derivative of the delta ``function'' the
limit of the
real part of the inner analytic function
,
for
, the limitations imposed on the test
functions
and on the corresponding inner analytic functions
become irrelevant. In fact, these definitions stand by
themselves, and are independent of any set of test functions. Not only one
can use them for any inner analytic functions derived from integrable real
functions, but one can do this for any inner analytic function
, regardless of whether or not it corresponds to an integrable real
function, so long as the
limit of the corresponding real
part
exists almost everywhere. Just as in the case of the
Dirac delta ``function'', whenever the
limit of the real
integral
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(57) |
exists, it defines the action of the derivative of the delta
``function'' on that particular real object.
It is also interesting to observe that, just as in the case of the Dirac
delta ``function'', it is true that its derivatives of all orders,
although they are not simply integrable real functions, are in fact
integrable real objects, even if they are related to inner analytic
functions with non-integrable hard singularities. Just as is the case for
the inner analytic function associated to the delta ``function'' itself,
the poles of the proper inner analytic functions associated to the
derivatives are always oriented in such a way that one can approach the
singularities along the unit circle while keeping the real parts of the
functions equal to zero, a fact that allows one to define the integrals on
of the real parts via the
limit4. Just as in the case of the
delta ``function'', the Fourier-conjugate functions of the derivatives are
simply non-integrable real functions. This fact provides the first hint
that there must be some relation of such non-integrable real functions
with corresponding inner analytic functions.
In the development presented in [#!CAoRFI!#] the real functions were
represented by their Fourier coefficients, and the inner analytic
functions by their Taylor coefficients. The same can be done in our case
here. Starting from the power series for
given in Equation (28), we can see that the definition of the
angular derivative implies that we have for the inner analytic functions
associated to the derivatives of the delta ``function'',
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(58) |
for
, so that the corresponding Taylor
coefficients are given by
and
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(59) |
for
, and where
. The identification of the Fourier
coefficients
and
will now depend on
the parity of
.
Once we have the Dirac delta ``function'' and all its derivatives, both as
inner analytic functions and as the corresponding real objects, we may
consider collections of such singular objects, with their singularities
located at all the possible points of the periodic interval ,
as well as arbitrary linear combinations of some or all of them. There is
a well-known theorem of the Schwartz theory of
distributions [#!DTSchwartz!#,#!DTLighthill!#] which states that any
distribution which is singular at a given point
can be
expressed as a linear combination of the Dirac delta ``function''
and its derivatives of arbitrarily high orders
.
Since, as was observed in [#!CAoRFI!#], the set of all inner analytic functions forms a vector space over the field of complex numbers, it is immediately apparent that we may assemble such linear combinations within the space of inner analytic functions. Therefore, the set of distributions formed by the delta ``functions'' and all their derivatives, as defined here, with their singularities located at all possible points of the unit circle, constitutes a complete basis that spans the space of all possible singular Schwartz distributions defined in a compact domain. We may conclude therefore that the whole space of Schwartz distributions in a compact domain is contained within the set of inner analytic functions.
It is interesting to note that, since we have the inner analytic function that corresponds to the delta ``function'' in explicit form, we are in a position to perform simple calculations in order to obtain in explicit form the inner analytic functions that correspond to the first few derivatives of the delta ``function''. For example, a few simple and straightforward calculations lead to the following proper inner analytic functions,
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|
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|
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(60) |
These proper inner analytic functions are all very simple rational
functions of the complex variable , which can be written as functions
of only
, and hence as functions of only
and
. Note that we can induce from these examples that the
derivative of the delta ``function'' is indeed represented by
an inner analytic function with a pole of order
on the unit circle,
which is thus a hard singularity with degree of hardness
, as one
would expect from the structure of the corresponding integral-differential
chain.