In applications in physics and engineering, the Dirac delta distribution (§1.16(iii)) is historically and customarily replaced by the Dirac delta (or Dirac delta function) . This is an operator with the properties:
subject to certain conditions on the function . From the mathematical standpoint the left-hand side of (1.17.2) can be interpreted as a generalized integral in the sense that
for a suitably chosen sequence of functions , . Such a sequence is called a delta sequence and we write, symbolically,
An example of a delta sequence is provided by
In this case
for all functions that are continuous when , and for each , converges absolutely for all sufficiently large values of . The last condition is satisfied, for example, when as , where is a real constant.
More generally, assume is piecewise continuous (§1.4(ii)) when for any finite positive real value of , and for each , converges absolutely for all sufficiently large values of . Then
Formal interchange of the order of integration in the Fourier integral formula ((1.14.1) and (1.14.4)):
The inner integral does not converge. However, for ,
Hence comparison with (1.17.5) shows that (1.17.9) can be interpreted as a generalized integral (1.17.3) with
provided that is continuous when , and for each , converges absolutely for all sufficiently large values of (as in the case of (1.17.6)). Then comparison of (1.17.2) and (1.17.9) yields the formal integral representation
Other similar integral representations of the Dirac delta that appear in the physics literature include the following:
See Seaton (2002a).
See Vallée and Soares (2010, §3.5.3).
Formal interchange of the order of summation and integration in the Fourier summation formula ((1.8.3) and (1.8.4)):
The sum does not converge, but (1.17.18) can be interpreted as a generalized integral in the sense that
provided that is continuous and of period ; see §1.8(ii).
By analogy with §1.17(ii) we have the formal series representation
Other similar series representations of the Dirac delta that appear in the physics literature include the following:
The references given in §§1.17(ii)–1.17(iii) are from the physics literature. For mathematical interpretations of (1.17.13), (1.17.15), (1.17.16) and (1.17.22)–(1.17.25) that resemble those given in §§1.17(ii) and 1.17(iii) for (1.17.12) and (1.17.21), see Li and Wong (2008). For (1.17.14) combine (1.17.13) and (10.47.3).