Does an infinite dimensional operator $T$ having an eigenvalue of $0$ imply that $T^-1$ doesn't exist?

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I can't seem to find anywhere a discussion on whether or not the idea of a zero eigenvalue (value in the point spectrum) implies the inverse doesn't exists for an infinite dimensional operator.



We know for a finite operator, this is the case (a 0 eigenvalue means a matrix is not invertible). I am wondering if this same condition (or some other condition on the eigenvalues or spectrum) gives us that the inverse of an infinite dimensional operator doesn't exist.







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    I can't seem to find anywhere a discussion on whether or not the idea of a zero eigenvalue (value in the point spectrum) implies the inverse doesn't exists for an infinite dimensional operator.



    We know for a finite operator, this is the case (a 0 eigenvalue means a matrix is not invertible). I am wondering if this same condition (or some other condition on the eigenvalues or spectrum) gives us that the inverse of an infinite dimensional operator doesn't exist.







    share|cite|improve this question





















      up vote
      0
      down vote

      favorite









      up vote
      0
      down vote

      favorite











      I can't seem to find anywhere a discussion on whether or not the idea of a zero eigenvalue (value in the point spectrum) implies the inverse doesn't exists for an infinite dimensional operator.



      We know for a finite operator, this is the case (a 0 eigenvalue means a matrix is not invertible). I am wondering if this same condition (or some other condition on the eigenvalues or spectrum) gives us that the inverse of an infinite dimensional operator doesn't exist.







      share|cite|improve this question











      I can't seem to find anywhere a discussion on whether or not the idea of a zero eigenvalue (value in the point spectrum) implies the inverse doesn't exists for an infinite dimensional operator.



      We know for a finite operator, this is the case (a 0 eigenvalue means a matrix is not invertible). I am wondering if this same condition (or some other condition on the eigenvalues or spectrum) gives us that the inverse of an infinite dimensional operator doesn't exist.









      share|cite|improve this question










      share|cite|improve this question




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      asked Jul 30 at 4:10









      MathIsHard

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          Yes. Having an eigenvalue of $0$ is equivalent to $operatornameker (T - 0I)$ is non-trivial, which is equivalent to $T$ not being injective. What breaks down is that $0$ not being an eigenvalue only implies $T$ is injective, not that $T^-1$ exists, as injectivity and bijectivity are no longer equivalent for linear operators on infinite-dimensional spaces.






          share|cite|improve this answer





















          • Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
            – MathIsHard
            Jul 30 at 4:22











          • Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
            – Theo Bendit
            Jul 30 at 4:28










          • Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
            – MathIsHard
            Jul 30 at 4:34






          • 1




            The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
            – Theo Bendit
            Jul 30 at 4:38






          • 1




            I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
            – MathIsHard
            Jul 30 at 5:30










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          1 Answer
          1






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          accepted










          Yes. Having an eigenvalue of $0$ is equivalent to $operatornameker (T - 0I)$ is non-trivial, which is equivalent to $T$ not being injective. What breaks down is that $0$ not being an eigenvalue only implies $T$ is injective, not that $T^-1$ exists, as injectivity and bijectivity are no longer equivalent for linear operators on infinite-dimensional spaces.






          share|cite|improve this answer





















          • Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
            – MathIsHard
            Jul 30 at 4:22











          • Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
            – Theo Bendit
            Jul 30 at 4:28










          • Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
            – MathIsHard
            Jul 30 at 4:34






          • 1




            The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
            – Theo Bendit
            Jul 30 at 4:38






          • 1




            I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
            – MathIsHard
            Jul 30 at 5:30














          up vote
          2
          down vote



          accepted










          Yes. Having an eigenvalue of $0$ is equivalent to $operatornameker (T - 0I)$ is non-trivial, which is equivalent to $T$ not being injective. What breaks down is that $0$ not being an eigenvalue only implies $T$ is injective, not that $T^-1$ exists, as injectivity and bijectivity are no longer equivalent for linear operators on infinite-dimensional spaces.






          share|cite|improve this answer





















          • Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
            – MathIsHard
            Jul 30 at 4:22











          • Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
            – Theo Bendit
            Jul 30 at 4:28










          • Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
            – MathIsHard
            Jul 30 at 4:34






          • 1




            The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
            – Theo Bendit
            Jul 30 at 4:38






          • 1




            I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
            – MathIsHard
            Jul 30 at 5:30












          up vote
          2
          down vote



          accepted







          up vote
          2
          down vote



          accepted






          Yes. Having an eigenvalue of $0$ is equivalent to $operatornameker (T - 0I)$ is non-trivial, which is equivalent to $T$ not being injective. What breaks down is that $0$ not being an eigenvalue only implies $T$ is injective, not that $T^-1$ exists, as injectivity and bijectivity are no longer equivalent for linear operators on infinite-dimensional spaces.






          share|cite|improve this answer













          Yes. Having an eigenvalue of $0$ is equivalent to $operatornameker (T - 0I)$ is non-trivial, which is equivalent to $T$ not being injective. What breaks down is that $0$ not being an eigenvalue only implies $T$ is injective, not that $T^-1$ exists, as injectivity and bijectivity are no longer equivalent for linear operators on infinite-dimensional spaces.







          share|cite|improve this answer













          share|cite|improve this answer



          share|cite|improve this answer











          answered Jul 30 at 4:15









          Theo Bendit

          11.8k1841




          11.8k1841











          • Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
            – MathIsHard
            Jul 30 at 4:22











          • Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
            – Theo Bendit
            Jul 30 at 4:28










          • Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
            – MathIsHard
            Jul 30 at 4:34






          • 1




            The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
            – Theo Bendit
            Jul 30 at 4:38






          • 1




            I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
            – MathIsHard
            Jul 30 at 5:30
















          • Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
            – MathIsHard
            Jul 30 at 4:22











          • Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
            – Theo Bendit
            Jul 30 at 4:28










          • Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
            – MathIsHard
            Jul 30 at 4:34






          • 1




            The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
            – Theo Bendit
            Jul 30 at 4:38






          • 1




            I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
            – MathIsHard
            Jul 30 at 5:30















          Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
          – MathIsHard
          Jul 30 at 4:22





          Thank you. I appreciate the help. Is there a condition that will make the operator invertible? I am dealing with this problem particularly, Let $(lambda_j)$ be a sequence of real numbers with $lambda_j neq 1$ for all $j$ and $lambda_j rightarrow 1$. Consider $T: ell^2 rightarrow ell^2$ defined for $xi_j in ell^2$ by $$T(xi_j)=(lambda_j xi_j)$$ I am trying to find a condition on the $lambda_j$ so that $T^-1$ exists. The $lambda_j$ are the equivalent of eigenvalues since they are in the point spectrum of T
          – MathIsHard
          Jul 30 at 4:22













          Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
          – Theo Bendit
          Jul 30 at 4:28




          Well, the obvious modified condition is that $0$ is not in the spectrum of the operator (by definition). My first potentially useful thought is that you might want to consider $|I - T|$, and show that it's strictly less than $1$. Then $sum_n=0^infty (I - T)^n$ converges to an inverse of $T$.
          – Theo Bendit
          Jul 30 at 4:28












          Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
          – MathIsHard
          Jul 30 at 4:34




          Wouldn't 0 not being in the spectrum not be enough from what you said above? I am confused what you mean by obvious modified condition... I was looking at that proof in my book that uses that sum, I will think about that some more, I am thinking that we need $||T||geq 1$ for the inverse to not exist. I think that is equivalent to one or more of the $lambda_j$ being larger than 1 in norm maybe?
          – MathIsHard
          Jul 30 at 4:34




          1




          1




          The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
          – Theo Bendit
          Jul 30 at 4:38




          The spectrum is the set of all scalars $lambda$ such that $T - lambda I$ is not invertible. This is larger than the set of eigenvalues: the subset of the spectrum such that $T - lambda I$ is not injective. So $0$ might be in the spectrum ($T$ is not invertible), but not be an eigenvalue ($T$ is injective). I call the modified condition "obvious" because saying $0$ is not in the spectrum is literally just re-stating the condition that $T$ is not invertible, without hinting at any way forward (except possibly armed with a term to Google).
          – Theo Bendit
          Jul 30 at 4:38




          1




          1




          I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
          – MathIsHard
          Jul 30 at 5:30




          I just figured out why you said $||I-T||<1$ because $(I-(I-T))^-1$ in that series will give the inverse of $T$ :D
          – MathIsHard
          Jul 30 at 5:30












           

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