exact-closed form solutions of recurrences












0














How can I find the exact-closed form solution of this recurrence?



$$f(n) = (1/2)f(n-2) + n/2,$$
where n is even, $f(0)=1$



My answer:



I try to guess a solution by unwinding the formula for f(n)



$f(n) = (1/2)f(n-2) + n/2$$



$= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$



$=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$



$=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$



$=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$



Continuing in this manner suggest the sum:



$f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$



$f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$



How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?










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    0














    How can I find the exact-closed form solution of this recurrence?



    $$f(n) = (1/2)f(n-2) + n/2,$$
    where n is even, $f(0)=1$



    My answer:



    I try to guess a solution by unwinding the formula for f(n)



    $f(n) = (1/2)f(n-2) + n/2$$



    $= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$



    $=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$



    $=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$



    $=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$



    Continuing in this manner suggest the sum:



    $f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$



    $f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$



    How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?










    share|cite|improve this question

























      0












      0








      0







      How can I find the exact-closed form solution of this recurrence?



      $$f(n) = (1/2)f(n-2) + n/2,$$
      where n is even, $f(0)=1$



      My answer:



      I try to guess a solution by unwinding the formula for f(n)



      $f(n) = (1/2)f(n-2) + n/2$$



      $= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$



      $=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$



      $=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$



      $=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$



      Continuing in this manner suggest the sum:



      $f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$



      $f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$



      How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?










      share|cite|improve this question













      How can I find the exact-closed form solution of this recurrence?



      $$f(n) = (1/2)f(n-2) + n/2,$$
      where n is even, $f(0)=1$



      My answer:



      I try to guess a solution by unwinding the formula for f(n)



      $f(n) = (1/2)f(n-2) + n/2$$



      $= (1/2) ((1/2)f(n-4) + ((n-2)/2) )+n/2$



      $=(1/2)^2f(n-4)+n/2^2+n/2 -2/2^2$



      $=(1/2)^3f(n-6)+n/2^3+n/2^2+n/2 -2/2^2-4/2^3$



      $=(1/2)^4f(n-8)+n/2^4+n/2^3+n/2^2+n/2 -2/2^2-4/2^3-6/2^4$



      Continuing in this manner suggest the sum:



      $f(n)=1/2^{(n/2)}f(0)+n(1/2^{n/2}+1/2^{(n/2)-1}+...+1/2)-(2/2^2+4/2^3+6/2^4+...)$



      $f(n)=1/2^{(n/2)}+n(1-(1/2)^{n/2})-(2/2^2+4/2^3+6/2^4+...)$



      How could I solve the expression $(2/2^2+4/2^3+6/2^4+...)$ and find the exact-closed form solution of this recurrence?







      discrete-mathematics recurrence-relations






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      asked Nov 26 '18 at 14:28









      user614642

      278




      278






















          3 Answers
          3






          active

          oldest

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          2














          Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$






          share|cite|improve this answer





















          • Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
            – user614642
            Nov 26 '18 at 15:26



















          0














          Hint



          Let $g(m)=f(m)+a_0+a_1m+cdots$



          $n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$



          Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$



          so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$



          Set $2r=n$






          share|cite|improve this answer





















          • I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
            – user614642
            Nov 27 '18 at 3:36





















          0














          Your direct question is about the sum
          $$
          eqalign{
          & 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
          & = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
          & = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
          $$



          Since
          $$
          eqalign{
          & sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
          = z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
          & = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
          $$

          then the sum equals
          $$
          sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
          $$






          share|cite|improve this answer





















          • Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
            – user614642
            Nov 27 '18 at 3:26











          Your Answer





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          3 Answers
          3






          active

          oldest

          votes








          3 Answers
          3






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          2














          Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$






          share|cite|improve this answer





















          • Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
            – user614642
            Nov 26 '18 at 15:26
















          2














          Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$






          share|cite|improve this answer





















          • Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
            – user614642
            Nov 26 '18 at 15:26














          2












          2








          2






          Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$






          share|cite|improve this answer












          Hint: Look for a solution in the form $$f(n) = A r^n + B n + C$$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Nov 26 '18 at 14:37









          Robert Israel

          318k23208457




          318k23208457












          • Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
            – user614642
            Nov 26 '18 at 15:26


















          • Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
            – user614642
            Nov 26 '18 at 15:26
















          Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
          – user614642
          Nov 26 '18 at 15:26




          Sorry but I do not know how is this hint related to this question, could you please explain more about it?(There is only one way to solve this kind of recurrences in my lecture notes: unwind the formula and guess for a solution, so I do not know if there are other ways to solve it)
          – user614642
          Nov 26 '18 at 15:26











          0














          Hint



          Let $g(m)=f(m)+a_0+a_1m+cdots$



          $n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$



          Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$



          so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$



          Set $2r=n$






          share|cite|improve this answer





















          • I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
            – user614642
            Nov 27 '18 at 3:36


















          0














          Hint



          Let $g(m)=f(m)+a_0+a_1m+cdots$



          $n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$



          Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$



          so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$



          Set $2r=n$






          share|cite|improve this answer





















          • I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
            – user614642
            Nov 27 '18 at 3:36
















          0












          0








          0






          Hint



          Let $g(m)=f(m)+a_0+a_1m+cdots$



          $n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$



          Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$



          so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$



          Set $2r=n$






          share|cite|improve this answer












          Hint



          Let $g(m)=f(m)+a_0+a_1m+cdots$



          $n=2f(n)-f(n-2)=2g(n)-g(n-2)+a_0(2-1)+a_1{2n-(n-2)}$



          Set $a_i=0$ for $ige2$ and $a_1=1$ and $a_0+2a_1=0$



          so that $g(n)=dfrac{g(n-2)}2=dfrac{g(n-2r)}{2^r}$



          Set $2r=n$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Nov 26 '18 at 14:50









          lab bhattacharjee

          223k15156274




          223k15156274












          • I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
            – user614642
            Nov 27 '18 at 3:36




















          • I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
            – user614642
            Nov 27 '18 at 3:36


















          I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
          – user614642
          Nov 27 '18 at 3:36






          I don’t understand why 2f(n)-2f(n-2)=2g(n)-g(n-2)+a(2-1)+a1(2n-(n-2)),where does the term a(2-1)+a1(2n-(n-2)) come from?How do you know that?
          – user614642
          Nov 27 '18 at 3:36













          0














          Your direct question is about the sum
          $$
          eqalign{
          & 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
          & = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
          & = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
          $$



          Since
          $$
          eqalign{
          & sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
          = z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
          & = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
          $$

          then the sum equals
          $$
          sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
          $$






          share|cite|improve this answer





















          • Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
            – user614642
            Nov 27 '18 at 3:26
















          0














          Your direct question is about the sum
          $$
          eqalign{
          & 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
          & = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
          & = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
          $$



          Since
          $$
          eqalign{
          & sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
          = z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
          & = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
          $$

          then the sum equals
          $$
          sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
          $$






          share|cite|improve this answer





















          • Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
            – user614642
            Nov 27 '18 at 3:26














          0












          0








          0






          Your direct question is about the sum
          $$
          eqalign{
          & 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
          & = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
          & = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
          $$



          Since
          $$
          eqalign{
          & sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
          = z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
          & = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
          $$

          then the sum equals
          $$
          sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
          $$






          share|cite|improve this answer












          Your direct question is about the sum
          $$
          eqalign{
          & 2/2^{,2} + 4/2^{,3} + 6/2^{,4} + cdots = cr
          & = 1 cdot left( {{1 over 2}} right)^{,1} + 2 cdot left( {{1 over 2}} right)^{,2} + 3 cdot left( {{1 over 2}} right)^{,3} + cdots = cr
          & = sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} cr}
          $$



          Since
          $$
          eqalign{
          & sumlimits_{0, le ,left( {,1, le } right),k} {k,z^{,k} } = zsumlimits_{0, le ,k} {k,z^{,k - 1} }
          = z{d over {dz}}sumlimits_{0, le ,k} {z^{,k} } = cr
          & = z{d over {dz}}{1 over {left( {1 - z} right)}} = {z over {left( {1 - z} right)^{,2} }} cr}
          $$

          then the sum equals
          $$
          sumlimits_{1, le ,k} k left( {{1 over 2}} right)^{,k} = 2
          $$







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Nov 26 '18 at 16:57









          G Cab

          18k31237




          18k31237












          • Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
            – user614642
            Nov 27 '18 at 3:26


















          • Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
            – user614642
            Nov 27 '18 at 3:26
















          Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
          – user614642
          Nov 27 '18 at 3:26




          Sorry.Maybe I did not explain clearly about this question.The sequence 2/2^2 +4/2^2+... is a summation to n, not to infinity,and its first term 2/2^2 first appears when n=4, as you can see in my try to unwind the formula
          – user614642
          Nov 27 '18 at 3:26


















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