Prove that integral of $ 1-cosleft(frac{1}{x^2}right) $ is finite












1














I need to prove that
$$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right) dx < infty $$



My attempt:
$$ forall xin[0,infty] hspace{1cm} 0 < 1 - cosleft(frac{1}{x^2}right) < 2 tag{1}. $$
Using L'Hôpital's rule I can show that:
$$ 1 - cosleft(frac{1}{x^2}right) underset{x to infty}{sim} frac{1}{2x^4} tag{2} $$
Which means that $ 1 - cosleft(frac{1}{x^2}right)$ behaves like $ frac{1}{2x^4} $ when $ x to infty $.



So I think that, there exist $N in mathbb{N} $ such that:
$$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right)dx < int_0^{N} 2dx + int_N^{infty} frac{1}{x^4}dx < infty $$



but I'm not sure. I would appreciate any tips or hints.










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    1














    I need to prove that
    $$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right) dx < infty $$



    My attempt:
    $$ forall xin[0,infty] hspace{1cm} 0 < 1 - cosleft(frac{1}{x^2}right) < 2 tag{1}. $$
    Using L'Hôpital's rule I can show that:
    $$ 1 - cosleft(frac{1}{x^2}right) underset{x to infty}{sim} frac{1}{2x^4} tag{2} $$
    Which means that $ 1 - cosleft(frac{1}{x^2}right)$ behaves like $ frac{1}{2x^4} $ when $ x to infty $.



    So I think that, there exist $N in mathbb{N} $ such that:
    $$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right)dx < int_0^{N} 2dx + int_N^{infty} frac{1}{x^4}dx < infty $$



    but I'm not sure. I would appreciate any tips or hints.










    share|cite|improve this question



























      1












      1








      1







      I need to prove that
      $$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right) dx < infty $$



      My attempt:
      $$ forall xin[0,infty] hspace{1cm} 0 < 1 - cosleft(frac{1}{x^2}right) < 2 tag{1}. $$
      Using L'Hôpital's rule I can show that:
      $$ 1 - cosleft(frac{1}{x^2}right) underset{x to infty}{sim} frac{1}{2x^4} tag{2} $$
      Which means that $ 1 - cosleft(frac{1}{x^2}right)$ behaves like $ frac{1}{2x^4} $ when $ x to infty $.



      So I think that, there exist $N in mathbb{N} $ such that:
      $$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right)dx < int_0^{N} 2dx + int_N^{infty} frac{1}{x^4}dx < infty $$



      but I'm not sure. I would appreciate any tips or hints.










      share|cite|improve this question















      I need to prove that
      $$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right) dx < infty $$



      My attempt:
      $$ forall xin[0,infty] hspace{1cm} 0 < 1 - cosleft(frac{1}{x^2}right) < 2 tag{1}. $$
      Using L'Hôpital's rule I can show that:
      $$ 1 - cosleft(frac{1}{x^2}right) underset{x to infty}{sim} frac{1}{2x^4} tag{2} $$
      Which means that $ 1 - cosleft(frac{1}{x^2}right)$ behaves like $ frac{1}{2x^4} $ when $ x to infty $.



      So I think that, there exist $N in mathbb{N} $ such that:
      $$ int_0^{infty} left(1 - cosleft(frac{1}{x^2}right)right)dx < int_0^{N} 2dx + int_N^{infty} frac{1}{x^4}dx < infty $$



      but I'm not sure. I would appreciate any tips or hints.







      calculus integration proof-verification






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      edited Dec 2 '18 at 14:34









      Larry

      1,7232823




      1,7232823










      asked Dec 2 '18 at 14:01









      Wywana

      535




      535






















          4 Answers
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          active

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          2














          Use the substitution $1/x=t$, so the integral becomes
          $$
          int_0^{infty} frac{1-cos(t^2)}{t^2},dt
          $$

          Since
          $$
          lim_{tto0}frac{1-cos(t^2)}{t^2}=0
          $$

          convergence is not an issue at $0$. Since
          $$
          0le 1-cos(t^2)le 2
          $$

          we have that, for $tge1$,
          $$
          0lefrac{1-cos(t^2)}{t^2}lefrac{2}{t^2}
          $$

          Now show convergence of
          $$
          int_1^infty frac{2}{t^2},dt
          $$






          share|cite|improve this answer





























            3














            $$I=int_{0}^{+infty}left[1-cosleft(tfrac{1}{x^2}right)right],dx stackrel{xmapsto 1/x}{=} int_{0}^{+infty}frac{1-cos(x^2)}{x^2},dx $$
            where the function $frac{1-cos(x^2)}{x^2}$ is continuous and bounded over $(0,1]$, non-negative and bounded by $frac{2}{x^2}$ over $[1,+infty)$. It follows that the above integral is finite. Its value can be found through the Laplace transform:



            $$int_{0}^{+infty}frac{1-cos x}{2xsqrt{x}},dx!stackrel{text{IBP}}{=}!int_{0}^{+infty}frac{sin x}{sqrt{x}},dx!stackrel{mathcal{L}}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{ds}{(s^2+1)sqrt{s}}!stackrel{smapsto t^2}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{2,dt}{t^4+1} $$
            leads to $I=color{red}{sqrt{frac{pi}{2}}}$.






            share|cite|improve this answer





























              1














              $int_0^infty(1-cos (1/x^2))dx= int_0^1(1-cos (1/x^2))dx+int_1^infty(1-cos (1/x^2))dxleq 2+int_1^infty2sin^2 left(frac{1}{2x^2}right)dxleq 2+int_1^infty2sin left(frac{1}{2x^2}right)dxleq2+int_1^infty2 frac{1}{2x^2}dx text{ (as } sin xleq x text{ forall } x>0) =2+int_1^infty frac{1}{x^2}dx .$






              share|cite|improve this answer





























                1














                One way is to use $ 1 - costheta = 2sin^2theta$ and $u = x^{-2}$ to get



                begin{multline}
                int_0^inftyleft[1-cos(x^{-2})right]dx = int_0^1 2sin^2(x^{-2})dx + int_1^infty 2sin^2(u)frac{du}{2u^{3/2}}
                \ = 2int_0^1sin^2(x^{-2})dx +int_0^1 u^{1/2}left[frac{sin(u)}{u}right]^2 du
                end{multline}



                Since both integrals are of bounded functions over a finite range, they must be finite.






                share|cite|improve this answer





















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                  4 Answers
                  4






                  active

                  oldest

                  votes








                  4 Answers
                  4






                  active

                  oldest

                  votes









                  active

                  oldest

                  votes






                  active

                  oldest

                  votes









                  2














                  Use the substitution $1/x=t$, so the integral becomes
                  $$
                  int_0^{infty} frac{1-cos(t^2)}{t^2},dt
                  $$

                  Since
                  $$
                  lim_{tto0}frac{1-cos(t^2)}{t^2}=0
                  $$

                  convergence is not an issue at $0$. Since
                  $$
                  0le 1-cos(t^2)le 2
                  $$

                  we have that, for $tge1$,
                  $$
                  0lefrac{1-cos(t^2)}{t^2}lefrac{2}{t^2}
                  $$

                  Now show convergence of
                  $$
                  int_1^infty frac{2}{t^2},dt
                  $$






                  share|cite|improve this answer


























                    2














                    Use the substitution $1/x=t$, so the integral becomes
                    $$
                    int_0^{infty} frac{1-cos(t^2)}{t^2},dt
                    $$

                    Since
                    $$
                    lim_{tto0}frac{1-cos(t^2)}{t^2}=0
                    $$

                    convergence is not an issue at $0$. Since
                    $$
                    0le 1-cos(t^2)le 2
                    $$

                    we have that, for $tge1$,
                    $$
                    0lefrac{1-cos(t^2)}{t^2}lefrac{2}{t^2}
                    $$

                    Now show convergence of
                    $$
                    int_1^infty frac{2}{t^2},dt
                    $$






                    share|cite|improve this answer
























                      2












                      2








                      2






                      Use the substitution $1/x=t$, so the integral becomes
                      $$
                      int_0^{infty} frac{1-cos(t^2)}{t^2},dt
                      $$

                      Since
                      $$
                      lim_{tto0}frac{1-cos(t^2)}{t^2}=0
                      $$

                      convergence is not an issue at $0$. Since
                      $$
                      0le 1-cos(t^2)le 2
                      $$

                      we have that, for $tge1$,
                      $$
                      0lefrac{1-cos(t^2)}{t^2}lefrac{2}{t^2}
                      $$

                      Now show convergence of
                      $$
                      int_1^infty frac{2}{t^2},dt
                      $$






                      share|cite|improve this answer












                      Use the substitution $1/x=t$, so the integral becomes
                      $$
                      int_0^{infty} frac{1-cos(t^2)}{t^2},dt
                      $$

                      Since
                      $$
                      lim_{tto0}frac{1-cos(t^2)}{t^2}=0
                      $$

                      convergence is not an issue at $0$. Since
                      $$
                      0le 1-cos(t^2)le 2
                      $$

                      we have that, for $tge1$,
                      $$
                      0lefrac{1-cos(t^2)}{t^2}lefrac{2}{t^2}
                      $$

                      Now show convergence of
                      $$
                      int_1^infty frac{2}{t^2},dt
                      $$







                      share|cite|improve this answer












                      share|cite|improve this answer



                      share|cite|improve this answer










                      answered Dec 2 '18 at 15:22









                      egreg

                      178k1484201




                      178k1484201























                          3














                          $$I=int_{0}^{+infty}left[1-cosleft(tfrac{1}{x^2}right)right],dx stackrel{xmapsto 1/x}{=} int_{0}^{+infty}frac{1-cos(x^2)}{x^2},dx $$
                          where the function $frac{1-cos(x^2)}{x^2}$ is continuous and bounded over $(0,1]$, non-negative and bounded by $frac{2}{x^2}$ over $[1,+infty)$. It follows that the above integral is finite. Its value can be found through the Laplace transform:



                          $$int_{0}^{+infty}frac{1-cos x}{2xsqrt{x}},dx!stackrel{text{IBP}}{=}!int_{0}^{+infty}frac{sin x}{sqrt{x}},dx!stackrel{mathcal{L}}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{ds}{(s^2+1)sqrt{s}}!stackrel{smapsto t^2}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{2,dt}{t^4+1} $$
                          leads to $I=color{red}{sqrt{frac{pi}{2}}}$.






                          share|cite|improve this answer


























                            3














                            $$I=int_{0}^{+infty}left[1-cosleft(tfrac{1}{x^2}right)right],dx stackrel{xmapsto 1/x}{=} int_{0}^{+infty}frac{1-cos(x^2)}{x^2},dx $$
                            where the function $frac{1-cos(x^2)}{x^2}$ is continuous and bounded over $(0,1]$, non-negative and bounded by $frac{2}{x^2}$ over $[1,+infty)$. It follows that the above integral is finite. Its value can be found through the Laplace transform:



                            $$int_{0}^{+infty}frac{1-cos x}{2xsqrt{x}},dx!stackrel{text{IBP}}{=}!int_{0}^{+infty}frac{sin x}{sqrt{x}},dx!stackrel{mathcal{L}}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{ds}{(s^2+1)sqrt{s}}!stackrel{smapsto t^2}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{2,dt}{t^4+1} $$
                            leads to $I=color{red}{sqrt{frac{pi}{2}}}$.






                            share|cite|improve this answer
























                              3












                              3








                              3






                              $$I=int_{0}^{+infty}left[1-cosleft(tfrac{1}{x^2}right)right],dx stackrel{xmapsto 1/x}{=} int_{0}^{+infty}frac{1-cos(x^2)}{x^2},dx $$
                              where the function $frac{1-cos(x^2)}{x^2}$ is continuous and bounded over $(0,1]$, non-negative and bounded by $frac{2}{x^2}$ over $[1,+infty)$. It follows that the above integral is finite. Its value can be found through the Laplace transform:



                              $$int_{0}^{+infty}frac{1-cos x}{2xsqrt{x}},dx!stackrel{text{IBP}}{=}!int_{0}^{+infty}frac{sin x}{sqrt{x}},dx!stackrel{mathcal{L}}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{ds}{(s^2+1)sqrt{s}}!stackrel{smapsto t^2}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{2,dt}{t^4+1} $$
                              leads to $I=color{red}{sqrt{frac{pi}{2}}}$.






                              share|cite|improve this answer












                              $$I=int_{0}^{+infty}left[1-cosleft(tfrac{1}{x^2}right)right],dx stackrel{xmapsto 1/x}{=} int_{0}^{+infty}frac{1-cos(x^2)}{x^2},dx $$
                              where the function $frac{1-cos(x^2)}{x^2}$ is continuous and bounded over $(0,1]$, non-negative and bounded by $frac{2}{x^2}$ over $[1,+infty)$. It follows that the above integral is finite. Its value can be found through the Laplace transform:



                              $$int_{0}^{+infty}frac{1-cos x}{2xsqrt{x}},dx!stackrel{text{IBP}}{=}!int_{0}^{+infty}frac{sin x}{sqrt{x}},dx!stackrel{mathcal{L}}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{ds}{(s^2+1)sqrt{s}}!stackrel{smapsto t^2}{=}!frac{1}{sqrt{pi}}!int_{0}^{+infty}frac{2,dt}{t^4+1} $$
                              leads to $I=color{red}{sqrt{frac{pi}{2}}}$.







                              share|cite|improve this answer












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                              answered Dec 2 '18 at 17:24









                              Jack D'Aurizio

                              287k33280657




                              287k33280657























                                  1














                                  $int_0^infty(1-cos (1/x^2))dx= int_0^1(1-cos (1/x^2))dx+int_1^infty(1-cos (1/x^2))dxleq 2+int_1^infty2sin^2 left(frac{1}{2x^2}right)dxleq 2+int_1^infty2sin left(frac{1}{2x^2}right)dxleq2+int_1^infty2 frac{1}{2x^2}dx text{ (as } sin xleq x text{ forall } x>0) =2+int_1^infty frac{1}{x^2}dx .$






                                  share|cite|improve this answer


























                                    1














                                    $int_0^infty(1-cos (1/x^2))dx= int_0^1(1-cos (1/x^2))dx+int_1^infty(1-cos (1/x^2))dxleq 2+int_1^infty2sin^2 left(frac{1}{2x^2}right)dxleq 2+int_1^infty2sin left(frac{1}{2x^2}right)dxleq2+int_1^infty2 frac{1}{2x^2}dx text{ (as } sin xleq x text{ forall } x>0) =2+int_1^infty frac{1}{x^2}dx .$






                                    share|cite|improve this answer
























                                      1












                                      1








                                      1






                                      $int_0^infty(1-cos (1/x^2))dx= int_0^1(1-cos (1/x^2))dx+int_1^infty(1-cos (1/x^2))dxleq 2+int_1^infty2sin^2 left(frac{1}{2x^2}right)dxleq 2+int_1^infty2sin left(frac{1}{2x^2}right)dxleq2+int_1^infty2 frac{1}{2x^2}dx text{ (as } sin xleq x text{ forall } x>0) =2+int_1^infty frac{1}{x^2}dx .$






                                      share|cite|improve this answer












                                      $int_0^infty(1-cos (1/x^2))dx= int_0^1(1-cos (1/x^2))dx+int_1^infty(1-cos (1/x^2))dxleq 2+int_1^infty2sin^2 left(frac{1}{2x^2}right)dxleq 2+int_1^infty2sin left(frac{1}{2x^2}right)dxleq2+int_1^infty2 frac{1}{2x^2}dx text{ (as } sin xleq x text{ forall } x>0) =2+int_1^infty frac{1}{x^2}dx .$







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                                      answered Dec 2 '18 at 14:22









                                      John_Wick

                                      1,381111




                                      1,381111























                                          1














                                          One way is to use $ 1 - costheta = 2sin^2theta$ and $u = x^{-2}$ to get



                                          begin{multline}
                                          int_0^inftyleft[1-cos(x^{-2})right]dx = int_0^1 2sin^2(x^{-2})dx + int_1^infty 2sin^2(u)frac{du}{2u^{3/2}}
                                          \ = 2int_0^1sin^2(x^{-2})dx +int_0^1 u^{1/2}left[frac{sin(u)}{u}right]^2 du
                                          end{multline}



                                          Since both integrals are of bounded functions over a finite range, they must be finite.






                                          share|cite|improve this answer


























                                            1














                                            One way is to use $ 1 - costheta = 2sin^2theta$ and $u = x^{-2}$ to get



                                            begin{multline}
                                            int_0^inftyleft[1-cos(x^{-2})right]dx = int_0^1 2sin^2(x^{-2})dx + int_1^infty 2sin^2(u)frac{du}{2u^{3/2}}
                                            \ = 2int_0^1sin^2(x^{-2})dx +int_0^1 u^{1/2}left[frac{sin(u)}{u}right]^2 du
                                            end{multline}



                                            Since both integrals are of bounded functions over a finite range, they must be finite.






                                            share|cite|improve this answer
























                                              1












                                              1








                                              1






                                              One way is to use $ 1 - costheta = 2sin^2theta$ and $u = x^{-2}$ to get



                                              begin{multline}
                                              int_0^inftyleft[1-cos(x^{-2})right]dx = int_0^1 2sin^2(x^{-2})dx + int_1^infty 2sin^2(u)frac{du}{2u^{3/2}}
                                              \ = 2int_0^1sin^2(x^{-2})dx +int_0^1 u^{1/2}left[frac{sin(u)}{u}right]^2 du
                                              end{multline}



                                              Since both integrals are of bounded functions over a finite range, they must be finite.






                                              share|cite|improve this answer












                                              One way is to use $ 1 - costheta = 2sin^2theta$ and $u = x^{-2}$ to get



                                              begin{multline}
                                              int_0^inftyleft[1-cos(x^{-2})right]dx = int_0^1 2sin^2(x^{-2})dx + int_1^infty 2sin^2(u)frac{du}{2u^{3/2}}
                                              \ = 2int_0^1sin^2(x^{-2})dx +int_0^1 u^{1/2}left[frac{sin(u)}{u}right]^2 du
                                              end{multline}



                                              Since both integrals are of bounded functions over a finite range, they must be finite.







                                              share|cite|improve this answer












                                              share|cite|improve this answer



                                              share|cite|improve this answer










                                              answered Dec 2 '18 at 14:49









                                              eyeballfrog

                                              6,048629




                                              6,048629






























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