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In [[mathematics]], a '''positive harmonic function''' on the [[unit disc]] in the [[complex numbers]] is characterized as the [[Poisson integral]] of a finite [[positive measure]] on the circle. This result, the ''Herglotz representation theorem'', was proved by [[Gustav Herglotz]] in 1911. It can be used to give a related formula and characterization for any [[holomorphic function]] on the unit disc with positive real part. Such functions had already been characterized in 1907 by [[Constantin Carathéodory]] in terms of the [[Positive definite function on a group|positive definiteness]] of their [[Taylor coefficient]]s. | |||
==Herglotz representation theorem for harmonic functions== | |||
A positive function ''f'' on the unit disk with ''f''(0) = 1 is harmonic if and only if there is a [[probability measure]] μ on the unit circle such that | |||
:<math> f(re^{i\theta})=\int_0^{2\pi} {1-r^2\over 1-2r\cos (\theta-\varphi) + r^2} \, d\mu(\varphi).</math> | |||
The formula clearly defines a positive harmonic function with ''f''(0) = 1. | |||
Conversely if ''f'' is positive and harmonic and ''r''<sub>''n''</sub> increases to 1, define | |||
:<math> f_n(z)=f(r_nz). \, </math> | |||
Then | |||
:<math> f_n(re^{i\theta}) = {1\over 2\pi}\int_0^{2\pi} {1-r^2\over 1-2r\cos(\theta-\varphi) + r^2 }\, f_n(\varphi)\,d\phi =\int_0^{2\pi}{1-r^2\over 1-2r\cos(\theta-\varphi) + r^2 } d\mu_n(\varphi)</math> | |||
where | |||
:<math> d\mu_n(\varphi)={1\over 2\pi} f(r_n e^{i\varphi})\,d\varphi</math> | |||
is a probability measure. | |||
By a compactness argument (or equivalently in this case | |||
[[Helly's selection theorem]] for [[Stieltjes integral]]s), a subsequence of these probability measures has a weak limit which is also a probability measure μ. | |||
Since ''r''<sub>''n''</sub> increases to 1, so that ''f''<sub>''n''</sub>(''z'') tends to ''f''(''z''), the Herglotz formula follows. | |||
==Herglotz representation theorem for holomorphic functions== | |||
A holomorphic function ''f'' on the unit disk with ''f''(0) = 1 has positive real part if and only if there is a probability measure μ on the unit circle such that | |||
:<math> f(z) =\int_0^{2\pi} {1 + e^{-i\theta}z\over 1 -e^{-i\theta}z} \, d\mu(\theta).</math> | |||
This follows from the previous theorem because: | |||
* the Poisson kernel is the real part of the integrand above | |||
* the real part of a holomorphic function is harmonic and determines the holomorphic function up to addition of a scalar | |||
* the above formula defines a holomorphic function, the real part of which is given by the previous theorem | |||
==Carathéodory's positivity criterion for holomorphic functions== | |||
Let | |||
:<math> f(z)=1 + a_1 z + a_2 z^2 + \cdots</math> | |||
be a holomorphic function on the unit disk. Then ''f''(''z'') has positive real part on the disk | |||
if and only if | |||
:<math> \sum_m\sum_n a_{m-n} \lambda_m\overline{\lambda_n} \ge 0</math> | |||
for any complex numbers λ<sub>0</sub>, λ<sub>1</sub>, ..., λ<sub>''N''</sub>, where | |||
:<math> a_0=2,\,\,\, a_{-m} =\overline{a_m}</math> | |||
for ''m'' > 0. | |||
In fact from the Herglotz representation for ''n'' > 0 | |||
:<math> a_n =2\int_0^{2\pi} e^{-in\theta}\, d\mu(\theta).</math> | |||
Hence | |||
:<math>\sum_m\sum_n a_{m-n} \lambda_m\overline{\lambda_n} =\int_0^{2\pi} \left|\sum_{n=}^N \lambda_n e^{-in\theta}\right|^2 \, d\mu(\theta) \ge 0.</math> | |||
Conversely, setting λ<sub>''n''</sub> = ''z''<sup>''n''</sup>, | |||
:<math>\sum_{m=0}^\infty\sum_{n=0}^\infty a_{m-n} \lambda_m\overline{\lambda_n} = 2(1-|z|^2) \,\Re\, f(z).</math> | |||
==See also== | |||
*[[Bochner's theorem]] | |||
==References== | |||
*{{citation|title=Über den Variabilitatsbereich der Koeffizienten von Potenzreflien, die gegebene Werte nicht annehmen|journal= Math. Ann.|year=1907|volume= 64|pages=95–115|first=C.|last=Carathéodory}} | |||
*{{citation|last=Duren|first=P. L.|title= | |||
Univalent functions|series=Grundlehren der Mathematischen Wissenschaften|volume= 259|publisher= Springer-Verlag|year= 1983|isbn= 0-387-90795-5}} | |||
*{{citation|last=Herglotz|first=G.|title=Über Potenzreihen mit positivem, reellen Teil im Einheitskreis|journal=Ber. Verh. Sachs. Akad. Wiss. Leipzig|volume=63|pages= 501–511|year=1911}} | |||
*{{citation|last=Pommerenke|first= C.|authorlink=Christian Pommerenke|title=Univalent functions, with a chapter on quadratic differentials by Gerd Jensen|series= Studia Mathematica/Mathematische Lehrbücher|volume=15|publisher= Vandenhoeck & Ruprecht|year= 1975}} | |||
[[Category:Harmonic analysis]] | |||
[[Category:Complex analysis]] |
Revision as of 14:22, 27 July 2013
In mathematics, a positive harmonic function on the unit disc in the complex numbers is characterized as the Poisson integral of a finite positive measure on the circle. This result, the Herglotz representation theorem, was proved by Gustav Herglotz in 1911. It can be used to give a related formula and characterization for any holomorphic function on the unit disc with positive real part. Such functions had already been characterized in 1907 by Constantin Carathéodory in terms of the positive definiteness of their Taylor coefficients.
Herglotz representation theorem for harmonic functions
A positive function f on the unit disk with f(0) = 1 is harmonic if and only if there is a probability measure μ on the unit circle such that
The formula clearly defines a positive harmonic function with f(0) = 1.
Conversely if f is positive and harmonic and rn increases to 1, define
Then
where
is a probability measure.
By a compactness argument (or equivalently in this case Helly's selection theorem for Stieltjes integrals), a subsequence of these probability measures has a weak limit which is also a probability measure μ.
Since rn increases to 1, so that fn(z) tends to f(z), the Herglotz formula follows.
Herglotz representation theorem for holomorphic functions
A holomorphic function f on the unit disk with f(0) = 1 has positive real part if and only if there is a probability measure μ on the unit circle such that
This follows from the previous theorem because:
- the Poisson kernel is the real part of the integrand above
- the real part of a holomorphic function is harmonic and determines the holomorphic function up to addition of a scalar
- the above formula defines a holomorphic function, the real part of which is given by the previous theorem
Carathéodory's positivity criterion for holomorphic functions
Let
be a holomorphic function on the unit disk. Then f(z) has positive real part on the disk if and only if
for any complex numbers λ0, λ1, ..., λN, where
for m > 0.
In fact from the Herglotz representation for n > 0
Hence
Conversely, setting λn = zn,
See also
References
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In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
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15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
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Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010 - Many property agents need to declare for the PIC grant in Singapore. However, not all of them know find out how to do the correct process for getting this PIC scheme from the IRAS. There are a number of steps that you need to do before your software can be approved.
Naturally, you will have to pay a safety deposit and that is usually one month rent for annually of the settlement. That is the place your good religion deposit will likely be taken into account and will kind part or all of your security deposit. Anticipate to have a proportionate amount deducted out of your deposit if something is discovered to be damaged if you move out. It's best to you'll want to test the inventory drawn up by the owner, which can detail all objects in the property and their condition. If you happen to fail to notice any harm not already mentioned within the inventory before transferring in, you danger having to pay for it yourself.
In case you are in search of an actual estate or Singapore property agent on-line, you simply should belief your intuition. It's because you do not know which agent is nice and which agent will not be. Carry out research on several brokers by looking out the internet. As soon as if you end up positive that a selected agent is dependable and reliable, you can choose to utilize his partnerise in finding you a home in Singapore. Most of the time, a property agent is taken into account to be good if he or she locations the contact data on his website. This may mean that the agent does not mind you calling them and asking them any questions relating to new properties in singapore in Singapore. After chatting with them you too can see them in their office after taking an appointment.
Have handed an trade examination i.e Widespread Examination for House Brokers (CEHA) or Actual Property Agency (REA) examination, or equal; Exclusive brokers are extra keen to share listing information thus making certain the widest doable coverage inside the real estate community via Multiple Listings and Networking. Accepting a severe provide is simpler since your agent is totally conscious of all advertising activity related with your property. This reduces your having to check with a number of agents for some other offers. Price control is easily achieved. Paint work in good restore-discuss with your Property Marketing consultant if main works are still to be done. Softening in residential property prices proceed, led by 2.8 per cent decline within the index for Remainder of Central Region
Once you place down the one per cent choice price to carry down a non-public property, it's important to accept its situation as it is whenever you move in – faulty air-con, choked rest room and all. Get round this by asking your agent to incorporate a ultimate inspection clause within the possibility-to-buy letter. HDB flat patrons routinely take pleasure in this security net. "There's a ultimate inspection of the property two days before the completion of all HDB transactions. If the air-con is defective, you can request the seller to repair it," says Kelvin.
15.6.1 As the agent is an intermediary, generally, as soon as the principal and third party are introduced right into a contractual relationship, the agent drops out of the image, subject to any problems with remuneration or indemnification that he could have against the principal, and extra exceptionally, against the third occasion. Generally, agents are entitled to be indemnified for all liabilities reasonably incurred within the execution of the brokers´ authority.
To achieve the very best outcomes, you must be always updated on market situations, including past transaction information and reliable projections. You could review and examine comparable homes that are currently available in the market, especially these which have been sold or not bought up to now six months. You'll be able to see a pattern of such report by clicking here It's essential to defend yourself in opposition to unscrupulous patrons. They are often very skilled in using highly unethical and manipulative techniques to try and lure you into a lure. That you must also protect your self, your loved ones, and personal belongings as you'll be serving many strangers in your home. Sign a listing itemizing of all of the objects provided by the proprietor, together with their situation. HSR Prime Recruiter 2010