Is this Function Holomorphic (Riesz-Herglotz Representation)?

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Let $g(z)$ be a holomorphic function on the closed unit disc $overline mathbb D$



Is the function $ g_1(w) = 1/2pi int_0^2pi operatornameRe(g(e^itheta))(e^itheta + w)/(e^itheta - w) , dtheta $ holomorphic on the open unit disc $ mathbb D$ ?



From the context (see below) I presume that it is but I don't see how to prove this.




Context



This is part of a proof here on p.29 :
https://su.diva-portal.org/smash/get/diva2:1069992/FULLTEXT01.pdf

to establish the Riesz-Herglotz representation of a holomorphic function on $mathbb D$ with a non-negative real part.



I follow the proof up to establishing an expression for $w in mathbb D,$
$$ g(w) = frac 1 2pi int_0^2pi g(e^itheta) operatornameRe left( frace^itheta + we^itheta - wright) , dtheta $$



I can also see that $operatornameRe(g(w)) = operatornameRe(g_1(w))$ so if $g_1$ is holomorphic one can then say that $g, g_1$ differ only by a complex constant.



Then I could continue to follow the proof. Help would be appreciated.







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  • $dfracpartialpartialbarwg_1(w)=0$
    – Nosrati
    Jul 16 at 13:49















up vote
0
down vote

favorite












Let $g(z)$ be a holomorphic function on the closed unit disc $overline mathbb D$



Is the function $ g_1(w) = 1/2pi int_0^2pi operatornameRe(g(e^itheta))(e^itheta + w)/(e^itheta - w) , dtheta $ holomorphic on the open unit disc $ mathbb D$ ?



From the context (see below) I presume that it is but I don't see how to prove this.




Context



This is part of a proof here on p.29 :
https://su.diva-portal.org/smash/get/diva2:1069992/FULLTEXT01.pdf

to establish the Riesz-Herglotz representation of a holomorphic function on $mathbb D$ with a non-negative real part.



I follow the proof up to establishing an expression for $w in mathbb D,$
$$ g(w) = frac 1 2pi int_0^2pi g(e^itheta) operatornameRe left( frace^itheta + we^itheta - wright) , dtheta $$



I can also see that $operatornameRe(g(w)) = operatornameRe(g_1(w))$ so if $g_1$ is holomorphic one can then say that $g, g_1$ differ only by a complex constant.



Then I could continue to follow the proof. Help would be appreciated.







share|cite|improve this question





















  • $dfracpartialpartialbarwg_1(w)=0$
    – Nosrati
    Jul 16 at 13:49













up vote
0
down vote

favorite









up vote
0
down vote

favorite











Let $g(z)$ be a holomorphic function on the closed unit disc $overline mathbb D$



Is the function $ g_1(w) = 1/2pi int_0^2pi operatornameRe(g(e^itheta))(e^itheta + w)/(e^itheta - w) , dtheta $ holomorphic on the open unit disc $ mathbb D$ ?



From the context (see below) I presume that it is but I don't see how to prove this.




Context



This is part of a proof here on p.29 :
https://su.diva-portal.org/smash/get/diva2:1069992/FULLTEXT01.pdf

to establish the Riesz-Herglotz representation of a holomorphic function on $mathbb D$ with a non-negative real part.



I follow the proof up to establishing an expression for $w in mathbb D,$
$$ g(w) = frac 1 2pi int_0^2pi g(e^itheta) operatornameRe left( frace^itheta + we^itheta - wright) , dtheta $$



I can also see that $operatornameRe(g(w)) = operatornameRe(g_1(w))$ so if $g_1$ is holomorphic one can then say that $g, g_1$ differ only by a complex constant.



Then I could continue to follow the proof. Help would be appreciated.







share|cite|improve this question













Let $g(z)$ be a holomorphic function on the closed unit disc $overline mathbb D$



Is the function $ g_1(w) = 1/2pi int_0^2pi operatornameRe(g(e^itheta))(e^itheta + w)/(e^itheta - w) , dtheta $ holomorphic on the open unit disc $ mathbb D$ ?



From the context (see below) I presume that it is but I don't see how to prove this.




Context



This is part of a proof here on p.29 :
https://su.diva-portal.org/smash/get/diva2:1069992/FULLTEXT01.pdf

to establish the Riesz-Herglotz representation of a holomorphic function on $mathbb D$ with a non-negative real part.



I follow the proof up to establishing an expression for $w in mathbb D,$
$$ g(w) = frac 1 2pi int_0^2pi g(e^itheta) operatornameRe left( frace^itheta + we^itheta - wright) , dtheta $$



I can also see that $operatornameRe(g(w)) = operatornameRe(g_1(w))$ so if $g_1$ is holomorphic one can then say that $g, g_1$ differ only by a complex constant.



Then I could continue to follow the proof. Help would be appreciated.









share|cite|improve this question












share|cite|improve this question




share|cite|improve this question








edited Jul 16 at 12:00









Michael Hardy

204k23186463




204k23186463









asked Jul 16 at 11:29









Tom Collinge

4,331932




4,331932











  • $dfracpartialpartialbarwg_1(w)=0$
    – Nosrati
    Jul 16 at 13:49

















  • $dfracpartialpartialbarwg_1(w)=0$
    – Nosrati
    Jul 16 at 13:49
















$dfracpartialpartialbarwg_1(w)=0$
– Nosrati
Jul 16 at 13:49





$dfracpartialpartialbarwg_1(w)=0$
– Nosrati
Jul 16 at 13:49











2 Answers
2






active

oldest

votes

















up vote
1
down vote













Theorem 7 in Rudin's RCA (it is actually Theorem 10.7 in my Indian edition of the book) immediately tells you that $g_1$ is holomorphic in $D$. Here are the details: let $g_2(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dmu (theta )$ where $dmu (theta ) =Re (g(e^itheta ))e^itheta , dtheta$ and $g_3(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dnu (theta )$ where $dnu (theta ) =Re (g(e^itheta )) , dtheta$. Then $g_1(w)=g_2(w)+wg_3(w)$. $g_2$ and $g_3$ are both holomorphic by the theorem in Rudin's book.






share|cite|improve this answer























  • Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
    – Tom Collinge
    Jul 16 at 11:52










  • @TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
    – Kavi Rama Murthy
    Jul 16 at 11:56










  • Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
    – Tom Collinge
    Jul 16 at 11:59










  • @TomCollinge I have provided the details now.
    – Kavi Rama Murthy
    Jul 16 at 23:24










  • Thanks. That helps a lot.
    – Tom Collinge
    Jul 17 at 7:27

















up vote
1
down vote













The answer goes along the lines of the answer to the question Analytic functions defined by integrals.



In our case, we begin by making two easy observations:



(1). The function $theta mapsto Re[g(e^i theta)]$ is bounded for $theta in [0,2pi)$. This is due to the assumption that $g$ is holomorphic in $overlinemathbbD$.



(2). The function $w mapsto (e^i theta + w)/(e^i theta - w)$ is holomorphic in $mathbbD$ for any fixed $theta in [0,2pi)$.



From (1) and (2), we quickly infer the following:



(3). Given a compact set $U subseteq mathbbD$, there exists a constant $C in mathbbR$, such that for any $w in U$, it holds that
$$int_0^2pileft|Re[g(e^i theta)]frace^i theta + we^i theta - wright|mathrmdtheta leq C.$$



Morera's theorem now gives that $g_1$ is holomorphic in $mathbbD$. Indeed, if $T subseteq mathbbD$ is any triangle, it holds that
beginalign
oint_Tg_1(w)mathrmdw & = frac12pioint_Tint_0^2piRe[g(e^i theta)]frace^i theta + we^i theta - wmathrmdtheta:mathrmdw \
& = frac12piint_0^2piRe[g(e^i theta)]underbraceoint_Tfrace^i theta + we^i theta - wmathrmdw_= 0:mathrmdue~to~ (2):mathrmdtheta \
& = 0.
endalign
Here, it was valid to change the order of integration as (3) allows us to apply Fubini's theorem and change the order of integration.






share|cite|improve this answer





















  • Very helpful. Nicely formatted.
    – Tom Collinge
    Jul 17 at 7:59










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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
1
down vote













Theorem 7 in Rudin's RCA (it is actually Theorem 10.7 in my Indian edition of the book) immediately tells you that $g_1$ is holomorphic in $D$. Here are the details: let $g_2(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dmu (theta )$ where $dmu (theta ) =Re (g(e^itheta ))e^itheta , dtheta$ and $g_3(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dnu (theta )$ where $dnu (theta ) =Re (g(e^itheta )) , dtheta$. Then $g_1(w)=g_2(w)+wg_3(w)$. $g_2$ and $g_3$ are both holomorphic by the theorem in Rudin's book.






share|cite|improve this answer























  • Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
    – Tom Collinge
    Jul 16 at 11:52










  • @TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
    – Kavi Rama Murthy
    Jul 16 at 11:56










  • Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
    – Tom Collinge
    Jul 16 at 11:59










  • @TomCollinge I have provided the details now.
    – Kavi Rama Murthy
    Jul 16 at 23:24










  • Thanks. That helps a lot.
    – Tom Collinge
    Jul 17 at 7:27














up vote
1
down vote













Theorem 7 in Rudin's RCA (it is actually Theorem 10.7 in my Indian edition of the book) immediately tells you that $g_1$ is holomorphic in $D$. Here are the details: let $g_2(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dmu (theta )$ where $dmu (theta ) =Re (g(e^itheta ))e^itheta , dtheta$ and $g_3(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dnu (theta )$ where $dnu (theta ) =Re (g(e^itheta )) , dtheta$. Then $g_1(w)=g_2(w)+wg_3(w)$. $g_2$ and $g_3$ are both holomorphic by the theorem in Rudin's book.






share|cite|improve this answer























  • Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
    – Tom Collinge
    Jul 16 at 11:52










  • @TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
    – Kavi Rama Murthy
    Jul 16 at 11:56










  • Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
    – Tom Collinge
    Jul 16 at 11:59










  • @TomCollinge I have provided the details now.
    – Kavi Rama Murthy
    Jul 16 at 23:24










  • Thanks. That helps a lot.
    – Tom Collinge
    Jul 17 at 7:27












up vote
1
down vote










up vote
1
down vote









Theorem 7 in Rudin's RCA (it is actually Theorem 10.7 in my Indian edition of the book) immediately tells you that $g_1$ is holomorphic in $D$. Here are the details: let $g_2(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dmu (theta )$ where $dmu (theta ) =Re (g(e^itheta ))e^itheta , dtheta$ and $g_3(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dnu (theta )$ where $dnu (theta ) =Re (g(e^itheta )) , dtheta$. Then $g_1(w)=g_2(w)+wg_3(w)$. $g_2$ and $g_3$ are both holomorphic by the theorem in Rudin's book.






share|cite|improve this answer















Theorem 7 in Rudin's RCA (it is actually Theorem 10.7 in my Indian edition of the book) immediately tells you that $g_1$ is holomorphic in $D$. Here are the details: let $g_2(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dmu (theta )$ where $dmu (theta ) =Re (g(e^itheta ))e^itheta , dtheta$ and $g_3(w)=frac 1 2pi int_0^2pi frac 1 e^itheta -w dnu (theta )$ where $dnu (theta ) =Re (g(e^itheta )) , dtheta$. Then $g_1(w)=g_2(w)+wg_3(w)$. $g_2$ and $g_3$ are both holomorphic by the theorem in Rudin's book.







share|cite|improve this answer















share|cite|improve this answer



share|cite|improve this answer








edited Jul 16 at 23:23


























answered Jul 16 at 11:46









Kavi Rama Murthy

21k2830




21k2830











  • Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
    – Tom Collinge
    Jul 16 at 11:52










  • @TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
    – Kavi Rama Murthy
    Jul 16 at 11:56










  • Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
    – Tom Collinge
    Jul 16 at 11:59










  • @TomCollinge I have provided the details now.
    – Kavi Rama Murthy
    Jul 16 at 23:24










  • Thanks. That helps a lot.
    – Tom Collinge
    Jul 17 at 7:27
















  • Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
    – Tom Collinge
    Jul 16 at 11:52










  • @TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
    – Kavi Rama Murthy
    Jul 16 at 11:56










  • Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
    – Tom Collinge
    Jul 16 at 11:59










  • @TomCollinge I have provided the details now.
    – Kavi Rama Murthy
    Jul 16 at 23:24










  • Thanks. That helps a lot.
    – Tom Collinge
    Jul 17 at 7:27















Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
– Tom Collinge
Jul 16 at 11:52




Thanks: I have the INTERNATIONAL EDITION 1987. Could you give a little more detail to help me find it - chapter title, page number, opening words of the theorem ?
– Tom Collinge
Jul 16 at 11:52












@TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
– Kavi Rama Murthy
Jul 16 at 11:56




@TomCollinge It is in the chapter on Elementary Properties of Holomorphic Functions, bottom of 4-th page in that chapter. The theorem immediately precedes 'Integration over paths". I hope you can locate it. Otherwise, I will type out the exact statement of the theorem.
– Kavi Rama Murthy
Jul 16 at 11:56












Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
– Tom Collinge
Jul 16 at 11:59




Thanks. 10.7 in my book is "Suppose $mu$ is a complex (finite) measure .....", but I don;t see how to apply it. Note: I know that $g$ is holomorphic, it's $g_1$ that gives me a problem.
– Tom Collinge
Jul 16 at 11:59












@TomCollinge I have provided the details now.
– Kavi Rama Murthy
Jul 16 at 23:24




@TomCollinge I have provided the details now.
– Kavi Rama Murthy
Jul 16 at 23:24












Thanks. That helps a lot.
– Tom Collinge
Jul 17 at 7:27




Thanks. That helps a lot.
– Tom Collinge
Jul 17 at 7:27










up vote
1
down vote













The answer goes along the lines of the answer to the question Analytic functions defined by integrals.



In our case, we begin by making two easy observations:



(1). The function $theta mapsto Re[g(e^i theta)]$ is bounded for $theta in [0,2pi)$. This is due to the assumption that $g$ is holomorphic in $overlinemathbbD$.



(2). The function $w mapsto (e^i theta + w)/(e^i theta - w)$ is holomorphic in $mathbbD$ for any fixed $theta in [0,2pi)$.



From (1) and (2), we quickly infer the following:



(3). Given a compact set $U subseteq mathbbD$, there exists a constant $C in mathbbR$, such that for any $w in U$, it holds that
$$int_0^2pileft|Re[g(e^i theta)]frace^i theta + we^i theta - wright|mathrmdtheta leq C.$$



Morera's theorem now gives that $g_1$ is holomorphic in $mathbbD$. Indeed, if $T subseteq mathbbD$ is any triangle, it holds that
beginalign
oint_Tg_1(w)mathrmdw & = frac12pioint_Tint_0^2piRe[g(e^i theta)]frace^i theta + we^i theta - wmathrmdtheta:mathrmdw \
& = frac12piint_0^2piRe[g(e^i theta)]underbraceoint_Tfrace^i theta + we^i theta - wmathrmdw_= 0:mathrmdue~to~ (2):mathrmdtheta \
& = 0.
endalign
Here, it was valid to change the order of integration as (3) allows us to apply Fubini's theorem and change the order of integration.






share|cite|improve this answer





















  • Very helpful. Nicely formatted.
    – Tom Collinge
    Jul 17 at 7:59














up vote
1
down vote













The answer goes along the lines of the answer to the question Analytic functions defined by integrals.



In our case, we begin by making two easy observations:



(1). The function $theta mapsto Re[g(e^i theta)]$ is bounded for $theta in [0,2pi)$. This is due to the assumption that $g$ is holomorphic in $overlinemathbbD$.



(2). The function $w mapsto (e^i theta + w)/(e^i theta - w)$ is holomorphic in $mathbbD$ for any fixed $theta in [0,2pi)$.



From (1) and (2), we quickly infer the following:



(3). Given a compact set $U subseteq mathbbD$, there exists a constant $C in mathbbR$, such that for any $w in U$, it holds that
$$int_0^2pileft|Re[g(e^i theta)]frace^i theta + we^i theta - wright|mathrmdtheta leq C.$$



Morera's theorem now gives that $g_1$ is holomorphic in $mathbbD$. Indeed, if $T subseteq mathbbD$ is any triangle, it holds that
beginalign
oint_Tg_1(w)mathrmdw & = frac12pioint_Tint_0^2piRe[g(e^i theta)]frace^i theta + we^i theta - wmathrmdtheta:mathrmdw \
& = frac12piint_0^2piRe[g(e^i theta)]underbraceoint_Tfrace^i theta + we^i theta - wmathrmdw_= 0:mathrmdue~to~ (2):mathrmdtheta \
& = 0.
endalign
Here, it was valid to change the order of integration as (3) allows us to apply Fubini's theorem and change the order of integration.






share|cite|improve this answer





















  • Very helpful. Nicely formatted.
    – Tom Collinge
    Jul 17 at 7:59












up vote
1
down vote










up vote
1
down vote









The answer goes along the lines of the answer to the question Analytic functions defined by integrals.



In our case, we begin by making two easy observations:



(1). The function $theta mapsto Re[g(e^i theta)]$ is bounded for $theta in [0,2pi)$. This is due to the assumption that $g$ is holomorphic in $overlinemathbbD$.



(2). The function $w mapsto (e^i theta + w)/(e^i theta - w)$ is holomorphic in $mathbbD$ for any fixed $theta in [0,2pi)$.



From (1) and (2), we quickly infer the following:



(3). Given a compact set $U subseteq mathbbD$, there exists a constant $C in mathbbR$, such that for any $w in U$, it holds that
$$int_0^2pileft|Re[g(e^i theta)]frace^i theta + we^i theta - wright|mathrmdtheta leq C.$$



Morera's theorem now gives that $g_1$ is holomorphic in $mathbbD$. Indeed, if $T subseteq mathbbD$ is any triangle, it holds that
beginalign
oint_Tg_1(w)mathrmdw & = frac12pioint_Tint_0^2piRe[g(e^i theta)]frace^i theta + we^i theta - wmathrmdtheta:mathrmdw \
& = frac12piint_0^2piRe[g(e^i theta)]underbraceoint_Tfrace^i theta + we^i theta - wmathrmdw_= 0:mathrmdue~to~ (2):mathrmdtheta \
& = 0.
endalign
Here, it was valid to change the order of integration as (3) allows us to apply Fubini's theorem and change the order of integration.






share|cite|improve this answer













The answer goes along the lines of the answer to the question Analytic functions defined by integrals.



In our case, we begin by making two easy observations:



(1). The function $theta mapsto Re[g(e^i theta)]$ is bounded for $theta in [0,2pi)$. This is due to the assumption that $g$ is holomorphic in $overlinemathbbD$.



(2). The function $w mapsto (e^i theta + w)/(e^i theta - w)$ is holomorphic in $mathbbD$ for any fixed $theta in [0,2pi)$.



From (1) and (2), we quickly infer the following:



(3). Given a compact set $U subseteq mathbbD$, there exists a constant $C in mathbbR$, such that for any $w in U$, it holds that
$$int_0^2pileft|Re[g(e^i theta)]frace^i theta + we^i theta - wright|mathrmdtheta leq C.$$



Morera's theorem now gives that $g_1$ is holomorphic in $mathbbD$. Indeed, if $T subseteq mathbbD$ is any triangle, it holds that
beginalign
oint_Tg_1(w)mathrmdw & = frac12pioint_Tint_0^2piRe[g(e^i theta)]frace^i theta + we^i theta - wmathrmdtheta:mathrmdw \
& = frac12piint_0^2piRe[g(e^i theta)]underbraceoint_Tfrace^i theta + we^i theta - wmathrmdw_= 0:mathrmdue~to~ (2):mathrmdtheta \
& = 0.
endalign
Here, it was valid to change the order of integration as (3) allows us to apply Fubini's theorem and change the order of integration.







share|cite|improve this answer













share|cite|improve this answer



share|cite|improve this answer











answered Jul 17 at 7:41









nmbcktt

111




111











  • Very helpful. Nicely formatted.
    – Tom Collinge
    Jul 17 at 7:59
















  • Very helpful. Nicely formatted.
    – Tom Collinge
    Jul 17 at 7:59















Very helpful. Nicely formatted.
– Tom Collinge
Jul 17 at 7:59




Very helpful. Nicely formatted.
– Tom Collinge
Jul 17 at 7:59












 

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What is the equation of a 3D cone with generalised tilt?

Color the edges and diagonals of a regular polygon

Relationship between determinant of matrix and determinant of adjoint?