Large Deviations rate function and Cramérs Theorem












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$begingroup$


Given $X_1,...$ of iid random variables. We know that if the moment generating function $M(theta) < infty, forall theta $ from Cramérs Theorem we get:



$lim_{nto infty} frac{1}{n}log mathbb{P}(S_n ge na) = -I(a)$ where



$I(a) = sup_theta(atheta - log M(theta))$.



Question: What happens if $M(theta)$ isn't finite for all values of $theta$? Namely, is there another version of Cramérs theorem to help when calculating the rate function $I$ when this situation comes up.










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$endgroup$

















    1












    $begingroup$


    Given $X_1,...$ of iid random variables. We know that if the moment generating function $M(theta) < infty, forall theta $ from Cramérs Theorem we get:



    $lim_{nto infty} frac{1}{n}log mathbb{P}(S_n ge na) = -I(a)$ where



    $I(a) = sup_theta(atheta - log M(theta))$.



    Question: What happens if $M(theta)$ isn't finite for all values of $theta$? Namely, is there another version of Cramérs theorem to help when calculating the rate function $I$ when this situation comes up.










    share|cite|improve this question











    $endgroup$















      1












      1








      1


      1



      $begingroup$


      Given $X_1,...$ of iid random variables. We know that if the moment generating function $M(theta) < infty, forall theta $ from Cramérs Theorem we get:



      $lim_{nto infty} frac{1}{n}log mathbb{P}(S_n ge na) = -I(a)$ where



      $I(a) = sup_theta(atheta - log M(theta))$.



      Question: What happens if $M(theta)$ isn't finite for all values of $theta$? Namely, is there another version of Cramérs theorem to help when calculating the rate function $I$ when this situation comes up.










      share|cite|improve this question











      $endgroup$




      Given $X_1,...$ of iid random variables. We know that if the moment generating function $M(theta) < infty, forall theta $ from Cramérs Theorem we get:



      $lim_{nto infty} frac{1}{n}log mathbb{P}(S_n ge na) = -I(a)$ where



      $I(a) = sup_theta(atheta - log M(theta))$.



      Question: What happens if $M(theta)$ isn't finite for all values of $theta$? Namely, is there another version of Cramérs theorem to help when calculating the rate function $I$ when this situation comes up.







      large-deviation-theory






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      edited Jan 22 at 20:47







      all.over

















      asked Jan 22 at 20:24









      all.overall.over

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          $begingroup$

          Yes. Let $Theta$ be the set of $theta$ such that $M(theta)<infty$. It is easy to check that $Theta$ is convex, that is, an interval. As long as $theta$ is in the interior of $Theta$, and solves $M'(theta)/M(theta)=a$, that is, the maximization problem defining $I(a)$ is solved the calculus way, the conclusion holds for that value of $a$. The condition that you cite, that $Theta=mathbb R$, is introduced for convenience of exposition.



          I'm away from my books at the moment, so I can't give a reference to this version.






          share|cite|improve this answer









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            $begingroup$

            Yes. Let $Theta$ be the set of $theta$ such that $M(theta)<infty$. It is easy to check that $Theta$ is convex, that is, an interval. As long as $theta$ is in the interior of $Theta$, and solves $M'(theta)/M(theta)=a$, that is, the maximization problem defining $I(a)$ is solved the calculus way, the conclusion holds for that value of $a$. The condition that you cite, that $Theta=mathbb R$, is introduced for convenience of exposition.



            I'm away from my books at the moment, so I can't give a reference to this version.






            share|cite|improve this answer









            $endgroup$


















              0












              $begingroup$

              Yes. Let $Theta$ be the set of $theta$ such that $M(theta)<infty$. It is easy to check that $Theta$ is convex, that is, an interval. As long as $theta$ is in the interior of $Theta$, and solves $M'(theta)/M(theta)=a$, that is, the maximization problem defining $I(a)$ is solved the calculus way, the conclusion holds for that value of $a$. The condition that you cite, that $Theta=mathbb R$, is introduced for convenience of exposition.



              I'm away from my books at the moment, so I can't give a reference to this version.






              share|cite|improve this answer









              $endgroup$
















                0












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                $begingroup$

                Yes. Let $Theta$ be the set of $theta$ such that $M(theta)<infty$. It is easy to check that $Theta$ is convex, that is, an interval. As long as $theta$ is in the interior of $Theta$, and solves $M'(theta)/M(theta)=a$, that is, the maximization problem defining $I(a)$ is solved the calculus way, the conclusion holds for that value of $a$. The condition that you cite, that $Theta=mathbb R$, is introduced for convenience of exposition.



                I'm away from my books at the moment, so I can't give a reference to this version.






                share|cite|improve this answer









                $endgroup$



                Yes. Let $Theta$ be the set of $theta$ such that $M(theta)<infty$. It is easy to check that $Theta$ is convex, that is, an interval. As long as $theta$ is in the interior of $Theta$, and solves $M'(theta)/M(theta)=a$, that is, the maximization problem defining $I(a)$ is solved the calculus way, the conclusion holds for that value of $a$. The condition that you cite, that $Theta=mathbb R$, is introduced for convenience of exposition.



                I'm away from my books at the moment, so I can't give a reference to this version.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Jan 23 at 23:50









                kimchi loverkimchi lover

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                11.1k31229






























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