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Relative Performance Indexes: replaced: cost effective → cost-effective using AWB

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In mathematics, the '''Laplacian of the indicator''' of the domain ''D'' is a generalisation of the Dirac {{mvar|δ'}}-function to higher dimensions, and is non-zero only on the ''surface'' of ''D''. It can be viewed as the ''surface delta prime function''. It is analogous to the second derivative of the [[Heaviside step function]] in one dimension. It can be obtained by letting the [[Laplace operator]] work on the [[indicator function]] of some domain ''D''.

The Laplacian of the indicator can be thought of as having infinitely positive and negative values when evaluated very near the boundary of the domain ''D''. From a mathematical viewpoint, it is not strictly a function but a [[generalized function]] or [[measure (mathematics)|measure]]. Similarly to the derivative of the [[Dirac delta function]] in one dimension, the Laplacian of the indicator only makes sense as a mathematical object when it appears under an integral sign; i.e. it is a distribution function. Just as in the formulation of distribution theory, it is in practice regarded as a limit of a sequence of smooth functions; one may meaningfully take the Laplacian of a [[bump function]], which is smooth by definition, and let the bump function approach the indicator in the limit.

==Introduction==
[[File:Laplacian of the indicator v2.jpg|thumb|right|An approximation of the negative indicator function of an ellipse in the plane (left), the derivative in the direction normal to the boundary (middle), and its Laplacian (right). In the limit, the right-most graph goes to the (negative) Laplacian of the indicator. Purely intuitively speaking, the right-most graph resembles an elliptic castle with a castle wall on the inside and a moat in front of it; in the limit, the wall and moat become infinitely high and deep (and narrow).]]

[[Paul Dirac]] introduced the [[Dirac delta function|Dirac {{mvar|δ}}-function]], as it has become known, as early as 1930.<ref>{{citation |last=Dirac|first=Paul | authorlink=Paul Dirac |year=1958|title=Principles of quantum mechanics|edition=4th|publisher=Oxford at the Clarendon Press|isbn=978-0-19-852011-5}}</ref> The one-dimensional Dirac {{mvar|δ}}-function is non-zero only at a single point. Likewise, the multidimensional generalisation, as it is usually made, is non-zero only at a single point. In Cartesian coordinates, the ''d''-dimensional Dirac {{mvar|δ}}-function is a product of ''d'' one-dimensional {{mvar|δ}}-functions; one for each Cartesian coordinate (see e.g. [[Dirac delta function#Generalizations|generalizations of the Dirac delta function]]).

However, a different generalisation is possible. The point zero, in one dimension, can be considered as the boundary of the positive halfline. The function '''1'''''x''>0 equals 1 on the positive halfline and zero otherwise, and is also known as the [[Heaviside step function]]. Formally, the Dirac {{mvar|δ}}-function and its derivative can be viewed as the first and second derivative of the Heaviside step function, i.e. ∂''x'''''1'''''x''>0 and <math>\partial_x^2 \mathbf{1}_{x>0}</math>.

The analogue of the step function in higher dimensions is the [[indicator function]], which can be written as '''1'''''x''∈''D'', where ''D'' is some domain. The indicator function is also known as the [[characteristic function]]. In analogy with the one-dimensional case, the following higher-dimensional generalisations of the Dirac {{mvar|δ}}-function and its derivative have been proposed:<ref name="Lange 2012">{{citation|last=Lange|first=Rutger-Jan|year=2012|publisher=Springer|title=Potential theory, path integrals and the Laplacian of the indicator|journal=Journal of High Energy Physics|volume=2012 |pages=1–49 |url=http://link.springer.com/article/10.1007%2FJHEP11(2012)032| issue=11 |bibcode=2012JHEP...11..032L |doi=10.1007/JHEP11(2012)032|arxiv = 1302.0864 }}</ref>

: <math>\begin{align}
&\delta(x) \to -n_x\cdot\nabla_x\mathbf{1}_{x\in D},\\
&\delta'(x) \to \nabla_x^2 \mathbf{1}_{x\in D}.
\end{align}</math>

Here ''n'' is the outward [[normal (geometry)|normal vector]]. Here the Dirac {{mvar|δ}}-function is generalised to a ''surface delta function'' on the boundary of some domain ''D'' in ''d'' ≥ 1 dimensions. This definition includes the usual one-dimensional case, when the domain is taken to be the positive halfline. It is zero except on the boundary of the domain ''D'' (where it is infinite), and it integrates to the total [[surface area]] enclosing ''D'', as shown [[#Normal derivative of the indicator|below]].

The Dirac {{mvar|δ'}}-function is generalised to a ''surface delta prime function'' on the boundary of some domain ''D'' in ''d'' ≥ 1 dimensions. In one dimension and by taking ''D'' equal to the positive halfline, the usual one-dimensional {{mvar|δ'}}-function can be recovered.

Both the normal derivative of the indicator and the Laplacian of the indicator are supported by ''surfaces'' rather than ''points''. The generalisation is useful in e.g. quantum mechanics, as surface interactions can lead to boundary conditions in ''d > ''1, while point interactions cannot. Naturally, point and surface interactions coincide for ''d''=1. Both surface and point interactions have a long history in quantum mechanics, and there exists a sizeable literature on so-called surface delta potentials or delta-sphere interactions.<ref>{{citation|last1=Antoine|first1=J.P.|last2=Gesztesy|first2=F.|last3=Shabani|first3=J.|title=Exactly solvable models of sphere interactions in quantum mechanics|journal=Journal of Physics A: Mathematical and General |volume=20|number=12 |pages=3687 |year=1999 |url=http://iopscience.iop.org/0305-4470/20/12/022}}</ref> Surface delta functions use the one-dimensional Dirac {{mvar|δ}}-function, but as a function of the radial coordinate ''r'', e.g. δ(''r''−''R'') where ''R'' is the radius of the sphere.

Although seemingly ill-defined, derivatives of the indicator function can formally be defined using the [[Distribution (mathematics)|theory of distributions]] or [[generalized function]]s: one can obtain a well-defined prescription by postulating that the Laplacian of the indicator, for example, is defined by two [[integration by parts|integrations by parts]] when it appears under an integral sign. Alternatively, the indicator (and its derivatives) can be approximated using a [[bump function]] (and its derivatives). The limit, where the (smooth) bump function approaches the indicator function, must then be put outside of the integral.

==Surface Dirac delta prime function==


This section will prove that the Laplacian of the indicator is a ''surface delta prime function''. The ''surface delta function'' will be considered below.

First, for a function ''f'' in the interval (''a'',''b''), recall the [[fundamental theorem of calculus]]

:<math> \int_a^b \frac{\partial f(x)}{\partial x}\,dx=\underset{x \nearrow b}\lim f(x)-\underset{x \searrow a}\lim f(x),</math>

assuming that ''f'' is locally integrable. Now for ''a'' < ''b'' it follows, by proceeding heuristically, that

:<math>\begin{align}
\int_{-\infty}^{+\infty} \frac{\partial^2\mathbf{1}_{a<x<b}}{\partial x^2}\,f(x)\;dx&=\int_{-\infty}^{+\infty} \mathbf{1}_{a<x<b} \frac{\partial^2 f(x)}{\partial x^2}\;dx,\\
&=\displaystyle\int_a^b \frac{\partial^2 f(x)}{\partial x^2}\;dx,\\
&=\displaystyle\Big(\underset{ x \nearrow b}\lim -\underset{ x \searrow a}\lim\Big) \frac{\partial f(x)}{\partial x}.
\end{align}</math>

Here '''1'''''a''<''x''<''b'' is the [[indicator function]] of the domain ''a'' < ''x'' < ''b''. The indicator equals one when the condition in its subscript is satisfied, and zero otherwise. In this calculation, two [[integration by parts|integrations by parts]] show that the first equality holds; the boundary terms are zero when ''a'' and ''b'' are finite, or when ''f'' vanishes at infinity. The last equality shows a ''sum'' of outward normal derivatives, where the sum is over the boundary points ''a'' and ''b'', and where the signs follow from the outward direction (i.e. positive for ''b'' and negative for ''a''). Although derivatives of the indicator do not formally exist, following the usual rules of partial integration provides the 'correct' result. When considering a finite ''d''-dimensional domain ''D'', the sum over outward normal derivatives is expected to become an ''integral'', which can be confirmed as follows:

:<math> \begin{align}
\int _{\mathbf{R}^d}\nabla_x^2\mathbf{1}_{x\in D}\,f(x)\;dx&= \int _{\mathbf{R}^d}\mathbf{1}_{x\in D}\,\nabla_x^2 f(x)\;dx,\\
&= \int _{D}\,\nabla_x^2 f(x)\;dx,\\
&= \oint_{\partial D}\,\underset{\alpha \to \beta}\lim n_\beta \cdot \nabla_\alpha f(\alpha)\;d\beta.
\end{align} </math>

Again, the first equality follows by two integrations by parts (in higher dimensions this proceeds by [[Green's identities|Green's second identity]]) where the boundary terms disappear as long as the domain ''D'' is finite or if ''f'' vanishes at infinity; e.g. both '''1'''''x''∈''D'' and ∇''x'''''1'''''x''∈''D'' are zero when evaluated at the 'boundary' of '''R'''''d'' when the domain ''D'' is finite. The third equality follows by the [[divergence theorem]] and shows, again, a sum (or, in this case, an integral) of outward normal derivatives over all boundary locations. The divergence theorem is valid for piecewise smooth domains ''D'', and hence ''D'' needs to be piecewise smooth.

Thus the Dirac {{mvar|δ'}}-function can be generalised to exist on a piecewise smooth surface, by taking the Laplacian of the indicator of the domain ''D'' giving rise to that surface. Naturally, the difference between a point and a surface disappears in one dimension.

In electrostatics, a surface dipole (or [[Double layer potential]]) can be modelled by the limiting distribution of the Laplacian of the indicator.

The calculation above derives from research on path integrals in quantum physics.<ref name="Lange 2012"/>

==Surface Dirac delta function==

This section will prove that the (inward) normal derivative of the indicator is a ''surface delta function''.

For a finite domain ''D'' or when ''f'' vanishes at infinity, it follows by the [[divergence theorem]] that

:<math>\int _{\mathbf{R}^d}\nabla_x^2\left (\mathbf{1}_{x\in D}\,f(x)\right )\;dx= 0.</math>

By the [[product rule]], it follows that

:<math>\int _{\mathbf{R}^d}\,\nabla_x^2\mathbf{1}_{x\in D}\,f(x)\;dx+ \int_{\mathbf{R}^d}\mathbf{1}_{x\in D}\,\nabla_x^2 f(x)\;dx =-2 \int _{\mathbf{R}^d} \nabla_x \mathbf{1}_{x\in D}\cdot \nabla_x f(x)\;dx.</math>

Following from the analysis of the section [[#Surface Dirac delta prime function|above]], the two terms on the left-hand side are equal, and thus

:<math>\oint_{\partial D}\,\underset{\alpha \to \beta}\lim n_\beta \cdot \nabla_\alpha f(\alpha)\;d\beta =-\displaystyle \int _{\mathbf{R}^d}\nabla_x\mathbf{1}_{x\in D}\cdot \nabla_x f(x)\;dx.</math>

The gradient of the indicator vanishes everywhere, except near the boundary of ''D''. Suppose that, near the boundary, ∇''x''''f''(''x'') is equal to ''nxg''(''x''), where ''g'' is some other function. Then it follows that

:<math>\oint _{\partial D}\,g(\beta)\;d\beta=-\int_{\mathbf{R}^d}\,\nabla_x\mathbf{1}_{x\in D}\,\cdot\,n_x\,g(x)\;dx. </math>

The outward normal ''n''''x'' was originally only defined for ''x'' in the surface, but it can be defined to exist for all ''x''; for example by taking the outward normal of the boundary point nearest to ''x''.

The foregoing analysis shows that −''nx'' ⋅ ∇''x'''''1'''''x''∈''D'' can be regarded as the surface generalisation of the one-dimensional [[Dirac delta function]]. By setting the function ''g'' equal to one, it follows that the inward normal derivative of the indicator integrates to the [[surface area]] of ''D''.

In electrostatics, surface charge densities (or ''single boundary layers'') can be modelled using the surface delta function as above. The usual [[Dirac delta function]] be used in some cases, e.g. when the surface is spherical. In general, the surface delta function discussed here may be used to represent the surface charge density on a surface of any shape.

The calculation above derives from research on path integrals in quantum physics.<ref name="Lange 2012"/>

==Approximations by bump functions==

This section shows how derivatives of the indicator can be treated numerically under an integral sign.

In principle, the indicator cannot be differentiated numerically, since its derivative is either zero or infinite. But, for practical purposes, the indicator can be approximated by a [[bump function]], indicated by ''I''ε(''x'') and approaching the indicator for ε → 0. Several options are possible, but it is convenient to let the bump function be non-negative and approach the indicator ''from below'', i.e.

: <math>\begin{align}
0 \leq I_\varepsilon(x)& \leq \mathbf{1}_{{x}\in D}\quad \forall \varepsilon >0\\
\underset{\varepsilon \searrow 0}\lim\; I_\varepsilon(x)&=\mathbf{1}_{x\in D}
\end{align}</math>

This ensures that the family of bump functions is identically zero outside of ''D''. This is convenient, since it is possible that the function ''f'' is only defined in the ''interior'' of ''D''. For ''f'' defined in ''D'', we thus obtain the following:

: <math>\begin{align}
- \underset{\varepsilon \searrow 0}\lim \int _{\mathbf{R}^d}\,f(x)\, n_x \cdot \nabla_x I_{\varepsilon}(x)\;dx &= \oint _{\partial D}\,\underset{\alpha \to \beta}\lim f(\alpha)\;d\beta, \\
\underset{\varepsilon \searrow 0}\lim\,\int _{\mathbf{R}^d}\nabla_x^2 I_{\varepsilon}(x)\,f(x)\;dx&= \oint_{\partial D}\,\underset{\alpha \to \beta}\lim n_\beta \cdot \nabla_\alpha f(\alpha)\;d\beta,
\end{align}</math>

where the interior coordinate α approaches the boundary coordinate β from the interior of ''D'', and where there is no requirement for ''f'' to exist outside of ''D''.

When ''f'' is defined on both sides of the boundary, and is furthermore differentiable across the boundary of ''D'', then it is less crucial how the bump function approaches the indicator.

==Applications==

===Quantum mechanics===
In [[quantum mechanics]], point interactions are well known and there is a large body of literature on the subject. A well-known example of a one-dimensional singular potential is the [[delta potential|Schrödinger equation with a Dirac delta potential]].<ref>{{citation|last1=Atkinson|first1=D.A.|last2=Crater|first2=H.W.|title=An exact treatment of the Dirac delta function potential in the Schrodinger equation|journal=American Journal of Physics|volume=43|pages=301–304|year=1975|url=http://ajp.aapt.org/resource/1/ajpias/v43/i4/p301_s1?isAuthorized=no|bibcode = 1975AmJPh..43..301A |doi = 10.1119/1.9857 }}</ref><ref>{{citation|last=Manoukian|first=E.B.|title=Explicit derivation of the propagator for a Dirac delta potential|journal=Journal of Physics A: Mathematical and General|volume=22|number=1|pages=67|year=1999|url=http://iopscience.iop.org/0305-4470/22/1/013}}</ref> The one-dimensional Dirac delta ''prime'' potential, on the other hand, has caused controversy.<ref>{{citation|last1=Albeverio|first1=S.|last2=Gesztesy|first2=F.|last3=Hoegh-Krohn|first3=R.|last4=Holden|first4=H.|title=Solvable models in quantum mechanics|year=1988|publisher=Springer-Verlag}}</ref><ref>{{citation|last=Zhao|first=B.H.|title=Comments on the Schr\"{o}dinger Equation with delta'-interaction in one dimension|journal=Journal of Physics A: Mathematical and General|year=1992|volume=25|pages=617|url=http://iopscience.iop.org/0305-4470/25/10/003}}</ref><ref>{{citation|last1=Albeverio|first1=S.|last2=Gesztesy|first2=F.|last3=Holden|first3=H.|title=Comments on a recent note on the Schrodinger equation with a delta'-interaction|journal=Journal of Physics A: Mathematical and General|year=1993|volume=26|pages=3903|url=http://iopscience.iop.org/0305-4470/26/15/037}}</ref> The controversy was seemingly settled by an independent paper,<ref>{{citation|last=Griffiths|first=D.J.|title=Boundary conditions at the derivative of a delta function|journal=Journal of Physics A: Mathematical and General|year=1993|volume=26|pages=2265|url=http://iopscience.iop.org/0305-4470/26/9/021/pdf/0305-4470_26_9_021.pdf}}</ref> although even this paper attracted later criticism.<ref name="Lange 2012"/><ref>{{citation|last1=Coutinho|first1=F.A.B.|last2=Nogami|first2=Y.|last3=Perez|first3=J.F.|title=Generalized point interactions in one-dimensional quantum mechanics|journal=Journal of Physics A: Mathematical and General|year=1997|volume=30|pages=3937|url=http://iopscience.iop.org/0305-4470/30/11/021}}</ref>

A lot more attention has been focused on the one-dimensional Dirac delta prime potential recently. <ref>{{citation|last1=Kostenko|first1=A.|last2=Malamud|first2=M.|title=Spectral Theory of Semibounded Schr{\"o}dinger Operators with $\delta$′-Interactions|journal=Annales Henri Poincar{\'e}|year=2012|publisher=Springer|pages=617|url=http://arxiv.org/abs/1212.1691}}</ref><ref>{{citation|last1=Brasche|first1=J.F.|last2=Nizhnik|first2=L.|title=One-dimensional Schr{\"o}dinger operators with $\delta$′-interactions on a set of Lebesgue measure zero|journal=ArXiv|year=2012|url=http://arxiv.org/abs/1112.2545}}</ref><ref>{{citation|last1=Carreau|first1=M.|last2=Farhi|first2=E.|last3=Gutmann|first3=S.|title=Functional integral for a free particle in a box|journal=Physical Review D|volume=42|number=4|pages=1194|year=1990|publisher=APS|url=http://prd.aps.org/abstract/PRD/v42/i4/p1194_1}}</ref><ref>{{citation|last1=Carreau|first1=M.|title=Four-parameter point-interaction in 1D quantum systems|journal=Journal of Physics A: Mathematical and General|volume=26|number=2|pages=427|year=1993|publisher=IOP publishing|url=http://iopscience.iop.org/0305-4470/26/2/025}}</ref> <ref>{{citation|last1=Albeverio|first1=S.|last2=Dabrowski|first2=L.|last3=Kurasov|first3=P.|title=Symmetries of Schr{\"o}dinger operator with point interactions|journal=Letters in Mathematical Physics|volume=45|number=1|pages=33--47|year=1998|publisher=Springer|url=http://link.springer.com/article/10.1023/A:1007493325970}}</ref><ref>{{citation|last1=Araujo|first1=V.S.|last2=Coutinho|first2=F.A.B.|last3=Toyama|first3=F.M.|title=The time-dependent Schr{\"o}dinger equation: the need for the Hamiltonian to be self-adjoint|journal=Brazilian Journal of Physics|volume=38|number=1|pages=178--187|year=2008|publisher=SciELO Brasil|url=http://www.sbfisica.org.br/bjp/files/v38_178.pdf}}</ref><ref>{{citation|last1=Coutinho|first1=F.A.B.|last2=Nogami|first2=Y.|last3=Tomio|first3=L|last4=Toyama|first4=F.M.|title=Energy-dependent point interactions in one dimension|journal=Journal of Physics A: Mathematical and General|volume=38|number=22|pages=4989|year=2005|publisher=IOP publishing|url=http://iopscience.iop.org/0305-4470/38/22/020}}</ref><ref>{{citation|last1=Coutinho|first1=F.A.B.|last2=Nogami|first2=Y.|last3=Tomio|first3=L|last4=Toyama|first4=F.M.|title=The Fermi pseudo-potential in one dimension|journal=Journal of Physics A: Mathematical and General|volume=37|number=44|pages=10653|year=2004|publisher=IOP publishing|url=http://iopscience.iop.org/0305-4470/37/44/013/}}</ref><ref>{{citation|last1=Toyoma|first1=F.M.|last2=Nogami|first2=Y.|title=Transmission--reflection problem with a potential of the form of the derivative of the delta function|journal=Journal of Physics A: Mathematical and General|volume=40|number=29|pages=F685|year=2007|publisher=IOP publishing|url=http://iopscience.iop.org/1751-8121/40/29/F05}}</ref><ref>{{citation|last1=Golovaty|first1=Y.D.|last2=Man'ko|first2=S.S.|title=Solvable models for the Schrodinger operators with $$\backslash$ delta'$-like potentials|journal=arXiv preprint arXiv:0909.1034|year=2009|url=http://arxiv.org/abs/0909.1034}}</ref> <ref>{{citation|last1=Man'ko|first1=S.S.|title=On $\delta$'-like potential scattering on star graphs|journal=Journal of Physics A: Mathematical and General|volume=43|number=44|pages=445304|year=2010|publisher=IOP publishing|url=http://arxiv.org/abs/1007.0398}}</ref><ref>{{citation|last1=Golovaty|first1=Y.D.|last2=Hryniv|first2=R.O.|title=On norm resolvent convergence of Schr{\"o}dinger operators with $\delta$'-like potentials|journal=Journal of Physics A: Mathematical and Theoretical|volume=43|number=15|pages=155204|year=2010|publisher=IOP Publishing|url=http://arxiv.org/abs/1108.5345}}</ref><ref>{{citation|last1=Golovaty|first1=Y.D.|title=1D Schr{\"o}dinger operators with short range interactions: two-scale regularization of distributional potentials|journal=Integral Equations and Operator Theory|volume=75|number=3|pages=341--362|year=2013|publisher=Springe|url=http://arxiv.org/abs/1202.4711}}</ref><ref>{{citation|last1=Zolotaryuk|first1=A.V.|title=Boundary conditions for the states with resonant tunnelling across the $\delta$′-potential|journal=Physics Letters A|volume=374|number=15|pages=1636--1641|year=2010|publisher=Elsevier|url=http://arxiv.org/abs/0905.0974}}</ref> <ref>{{citation|last1=Zolotaryuk|first1=A.V.|title=Point interactions of the dipole type defined through a three-parametric power regularization|journal=Journal of Physics A: Mathematical and Theoretical|volume=43|number=10|pages=105302|year=2010|publisher=IOP Publishing|url=http://iopscience.iop.org/1751-8121/43/10/105302}}</ref><ref>{{citation|last1=Zolotaryuk|first1=A.V.|title=Single-point potentials with total resonant tunneling|journal=Physical Review A|volume=87|number=5|pages=052121|year=2013|publisher=APS|url=http://arxiv.org/abs/1303.4162}}</ref>

A point on the one-dimensional line can be considered both as a point and as surface; as a point marks the boundary between two regions. Two generalisations of the Dirac delta-function to higher dimensions have thus been made: the generalisation to a multidimensional point,<ref>{{citation|last1=Scarlatti|first1=S.|last2=Teta|first2=A.|title=Derivation of the time-dependent propagator for the three-dimensional Schrodinger equation with one point interaction|journal=Journal of Physics A: Mathematical and General|volume=23|number=19|pages=L1033|year=1990|url=http://iopscience.iop.org/0305-4470/23/19/003/pdf/0305-4470_23_19_003.pdf}}</ref><ref>{{citation|last=Grosche|first=C.|title=Path integrals for two-and three-dimensional $\delta$-function perturbations|journal=Annalen der Physik|volume=506|number=4|pages=283–312|year=1994|url=http://arxiv.org/abs/hep-th/9308082|arxiv = hep-th/9308082 |bibcode = 1994AnP...506..283G |doi = 10.1002/andp.19945060406 }}</ref> as well as the generalisation to a multidimensional surface.<ref name="Lange 2012"/><ref>{{citation|last1=Moszkowski|first1=S.A.|title=Derivation of the surface delta interaction|journal=Physical Review C|volume=19|number=6|pages=2344|year=1997|url=http://prc.aps.org/abstract/PRC/v19/i6/p2344_1}}</ref><ref>{{citation|last1=Antoine|first1=J.P.|last2=Gesztesy|first2=F.|last3=Shabani|first3=J.|title=Exactly solvable models of sphere interactions in quantum mechanics|journal=Journal of Physics A: Mathematical and General|volume=20|number=12|pages=3687|year=1999|url=http://iopscience.iop.org/0305-4470/20/12/022}}</ref><ref>{{citation|last1=Shabani|first1=J.|last2=Vyabandi|first2=A.|title=Exactly solvable models of delta-sphere interactions in relativistic quantum mechanics|journal=Journal of Mathematical Physics|volume=43|pages=6064|year=2002|url=http://jmp.aip.org/resource/1/jmapaq/v43/i12/p6064_s1?isAuthorized=no|bibcode = 2002JMP....43.6064S |doi = 10.1063/1.1518785 }}
</ref><ref>{{citation|last1=Hounkonnou|first1=M.N.|last2=Hounkpe|first2=M.|last3=Shabani|first3=J.|title=Exactly solvable models of δ′-sphere interactions in nonrelativistic quantum mechanics|journal=Journal of Mathematical Physics|volume=40|number=9|pages=4255–4273|year=1999|url=http://jmp.aip.org/resource/1/jmapaq/v40/i9/p4255_s1?isAuthorized=no|bibcode = 1999JMP....40.4255H |doi = 10.1063/1.532964 }}</ref>

The former generalisations are known as point interactions, whereas the latter are known under different names, e.g. "delta-sphere interactions" and "surface delta interactions". The latter generalisations may use derivatives of the indicator, as explained here, or the one-dimensional Dirac {{mvar|δ}}-function as a function of the radial coordinate ''r''.

===Fluid dynamics===
The Laplacian of the indicator has been used in fluid dyamics, e.g. to model the interfaces between different media.<ref>{{citation|last=Che|first=J.H.|title=Numerical simulations of complex multiphase flows: electrohydrodynamics and solidification of droplets|publisher=University of Michigan|year=1999|page=37}}</ref><ref>{{citation|last=Juric|first=D.|title=Computations of phase change|journal=PhD thesis|year=1996|publisher=NASA|page=150|url=http://alum.wpi.edu/~damir.juric/CompBoilingFlows_ThesisJuric_1996.pdf}}</ref><ref>{{citation|last1=Unverdi|first1=S.O.|last2=Tryggvason|first2=G.|title=A front-tracking method for viscous, incompressible, multi-fluid flows|journal=Journal of computational physics|volume=100|number=1|pages=29–30|year=1992|url=http://www.sciencedirect.com/science/article/pii/002199919290307K}}</ref><ref>{{citation|last1=Goz|first1=M.F.|last2=Bunner|first2=B.|last3=Sommerfeld|first3=M.|last4=Tryggvason|first4=G.|title=Direct numerical simulation of bubble swarms with a parallel front-tracking method|booktitle=High Performance Scientific and Engineering Computing: Proceedings of the 3rd International FORTWIHR Conference on HPSEC, Erlangen, March 12–14, 2001|volume=21|pages=97|year=2002|publisher=Springer|url=http://rd.springer.com/chapter/10.1007/978-3-642-55919-8_11}}</ref><ref>{{citation|last1=Juric|first1=D.|last2=Tryggvason|first2=G.|title=A front-tracking method for dendritic solidification|journal=Journal of Computational Physics|volume=123|number=1|pages=127–148|year=1996|url=http://www.sciencedirect.com/science/article/pii/S002199919690011X|bibcode = 1996JCoPh.123..127J |doi = 10.1006/jcph.1996.0011 }}</ref><ref>{{citation|last1=Uddin|first1=E.|last2=Sung|first2=H.J.|title=Simulation of flow-flexible body interactions with large deformation|journal=International Journal for Numerical Methods in Fluids|year=2011|publisher=Wiley Online Library|url=http://onlinelibrary.wiley.com/doi/10.1002/fld.2731/abstract}}</ref>

===Surface reconstruction===
The divergence of the indicator and the Laplacian of the indicator (or of the [[characteristic function]], as the indicator is also known) have been used as the sample information from which surfaces can be reconstructed.<ref>{{citation|last=Kazhdan|first=M.|title=Reconstruction of solid models from oriented point sets|booktitle=Proceedings of the third Eurographics symposium on Geometry processing|year=2005|pages=73|url=http://www.cs.jhu.edu/~misha/MyPapers/SGP05.pdf}}
</ref><ref>{{cite book|last1=Kazhdan|first1=M.|last2=Bolitho|first2=M.|last3=Hoppe|first3=H|year=2006|title=Proceedings of the fourth Eurographics symposium on Geometry processing|pages=1–3–4|url=http://faculty.cs.tamu.edu/schaefer/teaching/689_Fall2006/poissonrecon.pdf}}</ref>

==See also==
*[[Distribution (mathematics)]]
*[[Generalized function]]
*[[Dirac delta function]]
*[[Indicator function]]
*[[Delta potential]]
*[[Potential theory]]
*[[Electrostatics]]
*[[Double layer potential]]

==References==
{{Reflist|30em}}



[[Category:Mathematics of infinitesimals]]
[[Category:Generalized functions]]
[[Category:Measure theory]]

Alpha Indexes - Revision history

en>Chris the speller: /* Relative Performance Indexes */replaced: cost effective → cost-effective using AWB