Prove that a function is indefinitely differentiableFinding the general term of two related recurrence relationsPiecewise interpolation with derivatives that is also twice differentiableShow that there is a series in R^infinity has some term greater than or equal to 1/n but that also is arbitrary close to the zero sequence.Uniform convergence to a differentiable functionFinding the general formula for $a_n+1=2^n a_n +4$, where $a_1=1$.When does the limit of derivatives coincide with the derivative of the limit function?Solving a recurrence with an alternating pattern e.g. do $mathrm X$, then do $mathrm Y$; repeatProve that if $f$ is derivable in an interval $I$ and $f'(a)=0$ and $f''(a) ne 0$, then $a$ must be a local minimum or maximum of the function.How to prove that a function has directional derivative?Formula for an m derivative of a function from $mathbbR^ntomathbbR$

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Prove that a function is indefinitely differentiable


Finding the general term of two related recurrence relationsPiecewise interpolation with derivatives that is also twice differentiableShow that there is a series in R^infinity has some term greater than or equal to 1/n but that also is arbitrary close to the zero sequence.Uniform convergence to a differentiable functionFinding the general formula for $a_n+1=2^n a_n +4$, where $a_1=1$.When does the limit of derivatives coincide with the derivative of the limit function?Solving a recurrence with an alternating pattern e.g. do $mathrm X$, then do $mathrm Y$; repeatProve that if $f$ is derivable in an interval $I$ and $f'(a)=0$ and $f''(a) ne 0$, then $a$ must be a local minimum or maximum of the function.How to prove that a function has directional derivative?Formula for an m derivative of a function from $mathbbR^ntomathbbR$













1












$begingroup$



Show that if
$$f: mathbbR to mathbbR$$ is differentiable in second order which satisfies the equation $$f'' =f'+f$$ then $f$ is indefinitely differentiable.




I was thinking to write the $n$the derivative as
$$f^(n)=a_nf+b_nf', a_n=b_n-1, b_n=a_n-1+b_n-1 $$
I calculated a few derivatives so I think this must be the form of the sequence, but I don't know how to solve it and I cannot see any other pattern to write the general term.










share|cite|improve this question











$endgroup$
















    1












    $begingroup$



    Show that if
    $$f: mathbbR to mathbbR$$ is differentiable in second order which satisfies the equation $$f'' =f'+f$$ then $f$ is indefinitely differentiable.




    I was thinking to write the $n$the derivative as
    $$f^(n)=a_nf+b_nf', a_n=b_n-1, b_n=a_n-1+b_n-1 $$
    I calculated a few derivatives so I think this must be the form of the sequence, but I don't know how to solve it and I cannot see any other pattern to write the general term.










    share|cite|improve this question











    $endgroup$














      1












      1








      1


      1



      $begingroup$



      Show that if
      $$f: mathbbR to mathbbR$$ is differentiable in second order which satisfies the equation $$f'' =f'+f$$ then $f$ is indefinitely differentiable.




      I was thinking to write the $n$the derivative as
      $$f^(n)=a_nf+b_nf', a_n=b_n-1, b_n=a_n-1+b_n-1 $$
      I calculated a few derivatives so I think this must be the form of the sequence, but I don't know how to solve it and I cannot see any other pattern to write the general term.










      share|cite|improve this question











      $endgroup$





      Show that if
      $$f: mathbbR to mathbbR$$ is differentiable in second order which satisfies the equation $$f'' =f'+f$$ then $f$ is indefinitely differentiable.




      I was thinking to write the $n$the derivative as
      $$f^(n)=a_nf+b_nf', a_n=b_n-1, b_n=a_n-1+b_n-1 $$
      I calculated a few derivatives so I think this must be the form of the sequence, but I don't know how to solve it and I cannot see any other pattern to write the general term.







      calculus sequences-and-series functions derivatives recurrence-relations






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited 8 hours ago









      Chase Ryan Taylor

      4,53621531




      4,53621531










      asked 9 hours ago









      AndreiAndrei

      134




      134




















          3 Answers
          3






          active

          oldest

          votes


















          12












          $begingroup$

          Since $f$ is twice differentiable, $f''=f'+f$ is the sum of two differentiable functions, so $f'''$ exists and is equal to $f'''=f''+f'$. By induction you can now show that it's infinitely differentiable, as $f^(n)=f^(n-1)+f^(n-2)$, implying that $f^(n+1)$ exists.






          share|cite|improve this answer









          $endgroup$












          • $begingroup$
            Can we simply neglect the coefficients of f and f' ?
            $endgroup$
            – Andrei
            8 hours ago










          • $begingroup$
            @Andrei What coefficients are you talking about?
            $endgroup$
            – DonAntonio
            8 hours ago






          • 3




            $begingroup$
            @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
            $endgroup$
            – InterstellarProbe
            8 hours ago



















          3












          $begingroup$

          You wanted to know the general form for $f^(n)$ in terms of $f$ and $f'$. The solution Alex R. provided is the correct solution. But, if you just want to see what form the $n$-th derivative takes (in terms of $f$ and $f'$), you have:



          $$f^(n) = a_nf' + b_nf$$



          Take the derivative derivatives:



          $$f^(n+1) = a_nf''+b_nf' = a_n(f'+f)+b_nf' = (a_n+b_n)f' + a_nf$$



          From this, we get:



          $$b_n+1 = a_n$$



          $$a_n+1 = a_n+b_n$$



          So, we have:



          $$a_n+2 = a_n+1+b_n+1 = a_n+1+a_n$$



          This is the Fibonacci recurrence relation. From this, we obtain:



          $$f^(n) = F_n+1f'+F_nf, nge 0$$



          where $$F_0=1, F_1=0$$ and $$F_n+2 = F_n+1+F_n, nge 0$$ is the Fibonacci sequence.






          share|cite|improve this answer











          $endgroup$












          • $begingroup$
            a very interesting approach to acquire some extra information
            $endgroup$
            – Chase Ryan Taylor
            8 hours ago


















          1












          $begingroup$

          Another way (different way) to look at this is to think of it as an ODE:
          $$ fracd^2 fdx^2 - fracdfdx - f = 0 $$
          This is a pretty simple ODE which has the solution of:
          $$ f(x) = a_1 e^fracsqrt5 + 1 2x + a_2e^-fracsqrt5 + 1 2x $$
          Where $a_1$ and $a_2$ are some constant. Since we know that exponential functions are infinite differentiable, this imply that $f(x)$ is infinite differentiable since it is just linear combination of exponential functions.






          share|cite|improve this answer









          $endgroup$













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            3 Answers
            3






            active

            oldest

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            3 Answers
            3






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            12












            $begingroup$

            Since $f$ is twice differentiable, $f''=f'+f$ is the sum of two differentiable functions, so $f'''$ exists and is equal to $f'''=f''+f'$. By induction you can now show that it's infinitely differentiable, as $f^(n)=f^(n-1)+f^(n-2)$, implying that $f^(n+1)$ exists.






            share|cite|improve this answer









            $endgroup$












            • $begingroup$
              Can we simply neglect the coefficients of f and f' ?
              $endgroup$
              – Andrei
              8 hours ago










            • $begingroup$
              @Andrei What coefficients are you talking about?
              $endgroup$
              – DonAntonio
              8 hours ago






            • 3




              $begingroup$
              @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
              $endgroup$
              – InterstellarProbe
              8 hours ago
















            12












            $begingroup$

            Since $f$ is twice differentiable, $f''=f'+f$ is the sum of two differentiable functions, so $f'''$ exists and is equal to $f'''=f''+f'$. By induction you can now show that it's infinitely differentiable, as $f^(n)=f^(n-1)+f^(n-2)$, implying that $f^(n+1)$ exists.






            share|cite|improve this answer









            $endgroup$












            • $begingroup$
              Can we simply neglect the coefficients of f and f' ?
              $endgroup$
              – Andrei
              8 hours ago










            • $begingroup$
              @Andrei What coefficients are you talking about?
              $endgroup$
              – DonAntonio
              8 hours ago






            • 3




              $begingroup$
              @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
              $endgroup$
              – InterstellarProbe
              8 hours ago














            12












            12








            12





            $begingroup$

            Since $f$ is twice differentiable, $f''=f'+f$ is the sum of two differentiable functions, so $f'''$ exists and is equal to $f'''=f''+f'$. By induction you can now show that it's infinitely differentiable, as $f^(n)=f^(n-1)+f^(n-2)$, implying that $f^(n+1)$ exists.






            share|cite|improve this answer









            $endgroup$



            Since $f$ is twice differentiable, $f''=f'+f$ is the sum of two differentiable functions, so $f'''$ exists and is equal to $f'''=f''+f'$. By induction you can now show that it's infinitely differentiable, as $f^(n)=f^(n-1)+f^(n-2)$, implying that $f^(n+1)$ exists.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered 8 hours ago









            Alex R.Alex R.

            25.8k12455




            25.8k12455











            • $begingroup$
              Can we simply neglect the coefficients of f and f' ?
              $endgroup$
              – Andrei
              8 hours ago










            • $begingroup$
              @Andrei What coefficients are you talking about?
              $endgroup$
              – DonAntonio
              8 hours ago






            • 3




              $begingroup$
              @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
              $endgroup$
              – InterstellarProbe
              8 hours ago

















            • $begingroup$
              Can we simply neglect the coefficients of f and f' ?
              $endgroup$
              – Andrei
              8 hours ago










            • $begingroup$
              @Andrei What coefficients are you talking about?
              $endgroup$
              – DonAntonio
              8 hours ago






            • 3




              $begingroup$
              @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
              $endgroup$
              – InterstellarProbe
              8 hours ago
















            $begingroup$
            Can we simply neglect the coefficients of f and f' ?
            $endgroup$
            – Andrei
            8 hours ago




            $begingroup$
            Can we simply neglect the coefficients of f and f' ?
            $endgroup$
            – Andrei
            8 hours ago












            $begingroup$
            @Andrei What coefficients are you talking about?
            $endgroup$
            – DonAntonio
            8 hours ago




            $begingroup$
            @Andrei What coefficients are you talking about?
            $endgroup$
            – DonAntonio
            8 hours ago




            3




            3




            $begingroup$
            @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
            $endgroup$
            – InterstellarProbe
            8 hours ago





            $begingroup$
            @Andrei I see what you are trying to do, but you are overthinking the problem. $$dfracddxf'' = dfracddx(f'+f)$$ Then $$f''' = f''+f'$$ You don't need to then convert $f''$ into $f'+f$. You can immediately apply induction. This is the base case.
            $endgroup$
            – InterstellarProbe
            8 hours ago












            3












            $begingroup$

            You wanted to know the general form for $f^(n)$ in terms of $f$ and $f'$. The solution Alex R. provided is the correct solution. But, if you just want to see what form the $n$-th derivative takes (in terms of $f$ and $f'$), you have:



            $$f^(n) = a_nf' + b_nf$$



            Take the derivative derivatives:



            $$f^(n+1) = a_nf''+b_nf' = a_n(f'+f)+b_nf' = (a_n+b_n)f' + a_nf$$



            From this, we get:



            $$b_n+1 = a_n$$



            $$a_n+1 = a_n+b_n$$



            So, we have:



            $$a_n+2 = a_n+1+b_n+1 = a_n+1+a_n$$



            This is the Fibonacci recurrence relation. From this, we obtain:



            $$f^(n) = F_n+1f'+F_nf, nge 0$$



            where $$F_0=1, F_1=0$$ and $$F_n+2 = F_n+1+F_n, nge 0$$ is the Fibonacci sequence.






            share|cite|improve this answer











            $endgroup$












            • $begingroup$
              a very interesting approach to acquire some extra information
              $endgroup$
              – Chase Ryan Taylor
              8 hours ago















            3












            $begingroup$

            You wanted to know the general form for $f^(n)$ in terms of $f$ and $f'$. The solution Alex R. provided is the correct solution. But, if you just want to see what form the $n$-th derivative takes (in terms of $f$ and $f'$), you have:



            $$f^(n) = a_nf' + b_nf$$



            Take the derivative derivatives:



            $$f^(n+1) = a_nf''+b_nf' = a_n(f'+f)+b_nf' = (a_n+b_n)f' + a_nf$$



            From this, we get:



            $$b_n+1 = a_n$$



            $$a_n+1 = a_n+b_n$$



            So, we have:



            $$a_n+2 = a_n+1+b_n+1 = a_n+1+a_n$$



            This is the Fibonacci recurrence relation. From this, we obtain:



            $$f^(n) = F_n+1f'+F_nf, nge 0$$



            where $$F_0=1, F_1=0$$ and $$F_n+2 = F_n+1+F_n, nge 0$$ is the Fibonacci sequence.






            share|cite|improve this answer











            $endgroup$












            • $begingroup$
              a very interesting approach to acquire some extra information
              $endgroup$
              – Chase Ryan Taylor
              8 hours ago













            3












            3








            3





            $begingroup$

            You wanted to know the general form for $f^(n)$ in terms of $f$ and $f'$. The solution Alex R. provided is the correct solution. But, if you just want to see what form the $n$-th derivative takes (in terms of $f$ and $f'$), you have:



            $$f^(n) = a_nf' + b_nf$$



            Take the derivative derivatives:



            $$f^(n+1) = a_nf''+b_nf' = a_n(f'+f)+b_nf' = (a_n+b_n)f' + a_nf$$



            From this, we get:



            $$b_n+1 = a_n$$



            $$a_n+1 = a_n+b_n$$



            So, we have:



            $$a_n+2 = a_n+1+b_n+1 = a_n+1+a_n$$



            This is the Fibonacci recurrence relation. From this, we obtain:



            $$f^(n) = F_n+1f'+F_nf, nge 0$$



            where $$F_0=1, F_1=0$$ and $$F_n+2 = F_n+1+F_n, nge 0$$ is the Fibonacci sequence.






            share|cite|improve this answer











            $endgroup$



            You wanted to know the general form for $f^(n)$ in terms of $f$ and $f'$. The solution Alex R. provided is the correct solution. But, if you just want to see what form the $n$-th derivative takes (in terms of $f$ and $f'$), you have:



            $$f^(n) = a_nf' + b_nf$$



            Take the derivative derivatives:



            $$f^(n+1) = a_nf''+b_nf' = a_n(f'+f)+b_nf' = (a_n+b_n)f' + a_nf$$



            From this, we get:



            $$b_n+1 = a_n$$



            $$a_n+1 = a_n+b_n$$



            So, we have:



            $$a_n+2 = a_n+1+b_n+1 = a_n+1+a_n$$



            This is the Fibonacci recurrence relation. From this, we obtain:



            $$f^(n) = F_n+1f'+F_nf, nge 0$$



            where $$F_0=1, F_1=0$$ and $$F_n+2 = F_n+1+F_n, nge 0$$ is the Fibonacci sequence.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited 8 hours ago

























            answered 8 hours ago









            InterstellarProbeInterstellarProbe

            4,311931




            4,311931











            • $begingroup$
              a very interesting approach to acquire some extra information
              $endgroup$
              – Chase Ryan Taylor
              8 hours ago
















            • $begingroup$
              a very interesting approach to acquire some extra information
              $endgroup$
              – Chase Ryan Taylor
              8 hours ago















            $begingroup$
            a very interesting approach to acquire some extra information
            $endgroup$
            – Chase Ryan Taylor
            8 hours ago




            $begingroup$
            a very interesting approach to acquire some extra information
            $endgroup$
            – Chase Ryan Taylor
            8 hours ago











            1












            $begingroup$

            Another way (different way) to look at this is to think of it as an ODE:
            $$ fracd^2 fdx^2 - fracdfdx - f = 0 $$
            This is a pretty simple ODE which has the solution of:
            $$ f(x) = a_1 e^fracsqrt5 + 1 2x + a_2e^-fracsqrt5 + 1 2x $$
            Where $a_1$ and $a_2$ are some constant. Since we know that exponential functions are infinite differentiable, this imply that $f(x)$ is infinite differentiable since it is just linear combination of exponential functions.






            share|cite|improve this answer









            $endgroup$

















              1












              $begingroup$

              Another way (different way) to look at this is to think of it as an ODE:
              $$ fracd^2 fdx^2 - fracdfdx - f = 0 $$
              This is a pretty simple ODE which has the solution of:
              $$ f(x) = a_1 e^fracsqrt5 + 1 2x + a_2e^-fracsqrt5 + 1 2x $$
              Where $a_1$ and $a_2$ are some constant. Since we know that exponential functions are infinite differentiable, this imply that $f(x)$ is infinite differentiable since it is just linear combination of exponential functions.






              share|cite|improve this answer









              $endgroup$















                1












                1








                1





                $begingroup$

                Another way (different way) to look at this is to think of it as an ODE:
                $$ fracd^2 fdx^2 - fracdfdx - f = 0 $$
                This is a pretty simple ODE which has the solution of:
                $$ f(x) = a_1 e^fracsqrt5 + 1 2x + a_2e^-fracsqrt5 + 1 2x $$
                Where $a_1$ and $a_2$ are some constant. Since we know that exponential functions are infinite differentiable, this imply that $f(x)$ is infinite differentiable since it is just linear combination of exponential functions.






                share|cite|improve this answer









                $endgroup$



                Another way (different way) to look at this is to think of it as an ODE:
                $$ fracd^2 fdx^2 - fracdfdx - f = 0 $$
                This is a pretty simple ODE which has the solution of:
                $$ f(x) = a_1 e^fracsqrt5 + 1 2x + a_2e^-fracsqrt5 + 1 2x $$
                Where $a_1$ and $a_2$ are some constant. Since we know that exponential functions are infinite differentiable, this imply that $f(x)$ is infinite differentiable since it is just linear combination of exponential functions.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered 6 hours ago









                user209663user209663

                40129




                40129



























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