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Section 5.1 Power Series

Subsection 5.1.1 Interval of Convergence of Power Series Summary

To determine the interval of convergence of a power series:
  1. Determine the open interval where the series converges.
    Use the ratio test (or root test).
    • Ratio test: \(L = \lim_{n \to \infty} \abs{\frac{a_{n+1}}{a_n}} \quad \rightarrow\) converges if \(L \lt 1\text{.}\)
    • Root test: \(L = \lim_{n \to \infty} \sqrt[n]{\abs{a_n}} \quad \rightarrow\) converges if \(L \lt 1\text{.}\)
    Solve the resulting inequality to find the (open) interval where the series converges, which will always be of the form \((a - R, a + R)\text{,}\) where \(R\) is the radius of convergence.
  2. Test the endpoints separately. Check the convergence of the series at \(x = a - R\) and \(x = a + R\) by substituting these values into the original series, and using a suitable test or the convergence or divergence of a known series. Some common series forms you’ll see:
    • Harmonic series
    • Alternating harmonic series
    • Divergence test (terms don’t approach 0)
    • \(p\)-series
    • Alternating series test (alternating series whose terms approach 0)
    (If \(R = \infty\) or \(R = 0\text{,}\) there are no endpoints to test)
  3. Form the complete interval of convergence, by combining the interval from step 2 and the endpoints from step 3.
Remarks:
  • Often, when testing the endpoints, the two values of \(x\) will lead to a very similar series. Contrasting where they are different can help you think about which converges and/or which diverges.
  • The ratio test is used for about 90% of all examples. However, sometimes both can be used, and the root test uses easier algebra, particularly when there are a lot of exponents of \(n\text{.}\)
  • Note that with the ratio test (or root test), the limit is with respect to \(n\text{,}\) and so \(x\) is a constant with respect to the limit.

Subsection 5.1.2 Examples

Checkpoint 5.1.1.

Find the interval of convergence and radius of convergence of each series.
  1. \(\displaystyle \sum_{n=1}^{\infty} \frac{3(x+1)^n}{n \cdot 4^n}\)
  2. \(\displaystyle \sum_{n=1}^{\infty} \frac{(x-3)^n}{n^2}\)
  3. \(\displaystyle \sum_{n=1}^{\infty} n! (2x + 1)^n\)
  4. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^n}{n}\)
  5. \(\displaystyle \sum_{n=1}^{\infty} (-1)^n n x^n\)
  6. \(\displaystyle \sum_{n=1}^{\infty} \sqrt{n} \, x^n\)
  7. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n x^n}{\sqrt[3]{n}}\)
  8. \(\displaystyle \sum_{n=1}^{\infty} \frac{n}{5^n} x^n\)
  9. \(\displaystyle \sum_{n=2}^{\infty} \frac{5^n}{n} x^n\)
  10. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^n}{n 3^n}\)
  11. \(\displaystyle \sum_{n=1}^{\infty} \frac{n}{n+1} x^n\)
  12. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^n}{2n-1}\)
  13. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n x^n}{n^2}\)
  14. \(\displaystyle \sum_{n=1}^{\infty} n^n x^n\)
  15. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^n}{n 4^n}\)
  16. \(\displaystyle \sum_{n=1}^{\infty} 2^n n^2 x^n\)
  17. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n 4^n}{\sqrt{n}} x^n\)
  18. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n-1}}{n 5^n} x^n\)
  19. \(\displaystyle \sum_{n=1}^{\infty} \frac{n}{2^n (n^2+1)} x^n\)
  20. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^{2n}}{n!}\)
  21. \(\displaystyle \sum_{n=0}^{\infty} \frac{(x-2)^n}{n^2+1}\)
  22. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n}{(2n-1) 2^n} (x-1)^n\)
  23. \(\displaystyle \sum_{n=1}^{\infty} \frac{1 + n}{n^3 2^n} x^n\)
  24. \(\displaystyle \sum_{n=0}^{\infty} \frac{(x + 1)^n}{n 2^n}\)
  25. \(\displaystyle \sum_{n=0}^{\infty} n! x^n\)
  26. \(\displaystyle \sum_{n=1}^{\infty} \frac{(x-1)^n}{n^3 \cdot 3^n}\)
  27. \(\displaystyle \sum_{n=1}^{\infty} \frac{\sqrt{n} \cdot (x+6)^n}{8^n}\)
  28. \(\displaystyle \sum_{n=1}^{\infty} \frac{2}{3^n (x-1)^{n+1}}\)
  29. \(\displaystyle \sum_{n=1}^{\infty} \frac{(4x+5)^n}{n^2}\)
  30. \(\displaystyle \sum_{n=2}^{\infty} \frac{(x+2)^n}{2^n \ln n}\)
  31. \(\displaystyle \sum_{n=1}^{\infty} \frac{(x-2)^n}{n^n}\)
  32. \(\displaystyle \sum_{n=4}^{\infty} \frac{\ln n}{n} x^n\)
  33. \(\displaystyle \sum_{n=2}^{\infty} \frac{(-1)^n}{n \ln n} x^n\)
  34. \(\displaystyle \sum_{n=1}^{\infty} n!(2x-1)^n\)
  35. \(\displaystyle \sum_{n=1}^{\infty} \frac{(5x-4)^n}{n^3}\)
  36. \(\displaystyle \sum_{n=2}^{\infty} \frac{x^{2n}}{n (\ln n)^2}\)
  37. \(\displaystyle \sum_{n=0}^{\infty} (x+5)^n\)
  38. \(\displaystyle \sum_{n=0}^{\infty} (-1)^n(4x+1)^n\)
  39. \(\displaystyle \sum_{n=1}^{\infty} \frac{(3x-2)^n}{n}\)
  40. \(\displaystyle \sum_{n=0}^{\infty} \frac{(x-2)^n}{10^n}\)
  41. \(\displaystyle \sum_{n=0}^{\infty} (2x)^n\)
  42. \(\displaystyle \sum_{n=0}^{\infty} \frac{n x^n}{n+2}\)
  43. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n(x+2)^n}{n}\)
  44. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^n}{n\sqrt{n}\,3^n}\)
  45. \(\displaystyle \sum_{n=1}^{\infty} \frac{(x-1)^n}{\sqrt{n}}\)
  46. \(\displaystyle \sum_{n=0}^{\infty} \frac{(-1)^n x^n}{n!}\)
  47. \(\displaystyle \sum_{n=0}^{\infty} \frac{3^n x^n}{n!}\)
  48. \(\displaystyle \sum_{n=1}^{\infty} \frac{4^n x^{2n}}{n}\)
  49. \(\displaystyle \sum_{n=0}^{\infty} \frac{x^n}{\sqrt{n^2+3}}\)
  50. \(\displaystyle \sum_{n=0}^{\infty} \frac{(-1)^n x^{n+1}}{\sqrt{n}+3}\)
  51. \(\displaystyle \sum_{n=0}^{\infty} \frac{n(x+3)^n}{5^n}\)
  52. \(\displaystyle \sum_{n=0}^{\infty} \frac{n x^n}{4^n (n^2+1)}\)
  53. \(\displaystyle \sum_{n=0}^{\infty} \frac{\sqrt{n}\,x^n}{3^n}\)
  54. \(\displaystyle \sum_{n=1}^{\infty} \sqrt[n]{n}\,(2x+5)^n\)
  55. \(\displaystyle \sum_{n=1}^{\infty} \bigl(2+(-1)^n\bigr)\,(x+1)^{n-1}\)
  56. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n 3^{2n}(x-2)^n}{3n}\)
  57. \(\displaystyle \sum_{n=1}^{\infty} \left(1+\frac{1}{n}\right)^n x^n\)
  58. \(\displaystyle \sum_{n=1}^{\infty} (\ln n)\,x^n\)
  59. \(\displaystyle \sum_{n=0}^{\infty} n! (x-4)^n\)
  60. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1}(x+2)^n}{n\,2^n}\)
  61. \(\displaystyle \sum_{n=0}^{\infty} (-2)^n (n+1) (x-1)^n\)
  62. \(\displaystyle \sum_{n=0}^{\infty} \left(\frac{x}{4}\right)^n\)
  63. \(\displaystyle \sum_{n=0}^{\infty} \frac{(-1)^n x^n}{n}\)
  64. \(\displaystyle \sum_{n=0}^{\infty} (-1)^{n+1} (n+1)x^n\)
  65. \(\displaystyle \sum_{n=0}^{\infty} \frac{5^n}{n!}\)
  66. \(\displaystyle \sum_{n=0}^{\infty} \frac{(3x)^n}{(2n)!}\)
  67. \(\displaystyle \sum_{n=0}^{\infty} (2n)! \left(\frac{x}{3}\right)^n\)
  68. \(\displaystyle \sum_{n=0}^{\infty} \frac{(-1)^n x^n}{(n+1)(n+2)}\)
  69. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1} x^n}{6^n}\)
  70. \(\displaystyle \sum_{n=0}^{\infty} \frac{(-1)^n n! (x-5)^n}{3^n}\)
  71. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1}(x-4)^n}{n 9^n}\)
  72. \(\displaystyle \sum_{n=0}^{\infty} \frac{(x-3)^{n+1}}{(n+1)4^{n+1}}\)
  73. \(\displaystyle \sum_{n=0}^{\infty} \frac{(-1)^{n+1}(x-1)^{n+1}}{n+1}\)
  74. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1}(x-2)^n}{n 2^n}\)
  75. \(\displaystyle \sum_{n=1}^{\infty} \frac{(x-3)^{n-1}}{3^{n-1}}\)
  76. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^n x^{2n+1}}{2n+1}\)
  77. \(\displaystyle \sum_{n=1}^{\infty} \frac{n}{n+1} (-2x)^{n-1}\)
  78. \(\displaystyle \sum_{n=0}^{\infty} \frac{x^{3n+1}}{(3n+1)!}\)
  79. \(\displaystyle \sum_{n=1}^{\infty} \frac{n! x^n}{(2n)!}\)

Subsection 5.1.3 Advanced Examples

Checkpoint 5.1.2.

Find the interval of convergence and radius of convergence of each series.
  1. \(\displaystyle \sum_{n=1}^{\infty} \frac{(x-2)^n}{n^{4/5} (5^n - 4)}\)
  2. \(\displaystyle \sum_{n=1}^{\infty} \frac{n}{b^n} (x - a)^n \quad (b\gt 0)\)
  3. \(\displaystyle \sum_{n=1}^{\infty} \frac{(2x-1)^n}{5^n \sqrt{n}}\)
  4. \(\sum_{n=1}^{\infty} \frac{1}{b^n} (x-a)^n\) (where \(b \gt 0\))
  5. \(\sum_{n=2}^{\infty} \frac{b^n}{\ln n} (x-a)^n\) (where \(b \gt 0\))
  6. \(\displaystyle \sum_{n=1}^{\infty} \frac{n^2 x^n}{2 \cdot 4 \cdot 6 \cdots (2n)}\)
  7. \(\displaystyle \sum_{n=1}^{\infty} \frac{x^n}{1 \cdot 3 \cdot 5 \cdots (2n-1)}\)
  8. \(\displaystyle \sum_{n=1}^{\infty} \frac{n! x^n}{1 \cdot 3 \cdot 5 \cdots (2n-1)}\)
  9. \(\displaystyle \sum_{n=2}^{\infty} \frac{x^n}{n(\ln n)^2}\)
  10. \(\displaystyle \sum_{n=2}^{\infty} \frac{x^n}{n \ln n}\)
  11. \(\displaystyle \sum_{n=1}^{\infty} \frac{(4x-5)^{2n+1}}{n^{3/2}}\)
  12. \(\displaystyle \sum_{n=1}^{\infty} \frac{(3x+1)^{n+1}}{2n+2}\)
  13. \(\displaystyle \sum_{n=1}^{\infty} \frac{1}{2\cdot 4 \cdot 6 \cdots (2n)}\,x^n\)
  14. \(\displaystyle \sum_{n=1}^{\infty} \frac{3\cdot 5 \cdot 7 \cdots (2n+1)}{n^2\,2^n}\,x^{n+1}\)
  15. \(\displaystyle \sum_{n=1}^{\infty} \frac{2 \cdot 3 \cdot 4 \cdots (n+1)}{n!} x^n\)
  16. \(\displaystyle \sum_{n=1}^{\infty} \left(\frac{2 \cdot 4 \cdot 6 \cdots (2n)}{3 \cdot 5 \cdot 7 \cdots (2n+1)}\right) x^{2n+1}\)
  17. \(\displaystyle \sum_{n=1}^{\infty} \frac{(-1)^{n+1} 3 \cdot 7 \cdot 11 \cdots (4n-1)}{4^n} (x-3)^n\)
  18. \(\displaystyle \sum_{n=1}^{\infty} \frac{n! (x+1)^n}{1 \cdot 3 \cdot 5 \cdots (2n-1)}\)
  19. \(\displaystyle \sum_{n=1}^{\infty} \frac{1+2+3+\cdots+n}{1^2+2^2+3^2+\cdots+n^2}\,x^n\)
  20. \(\displaystyle \sum_{n=1}^{\infty} \bigl(\sqrt{n+1}-\sqrt{n}\bigr)\,(x-3)^n\)
  21. \(\sum_{n=0}^{\infty} \frac{(n!)^k}{(kn)!} x^n\) (where \(k\) is a positive integer)

Checkpoint 5.1.3.

Find the radius of convergence of each power series.
  1. \(\displaystyle \sum_{n=1}^{\infty} \brac{\frac{2 \cdot 4 \cdot 6 \cdots (2n)}{2 \cdot 5 \cdot 8 \cdots (3n-1)}}^2 x^n\)
  2. \(\displaystyle \sum_{n=1}^{\infty} \frac{n!}{3 \cdot 6 \cdot 9 \cdots 3n} \, x^n\)
  3. \(\displaystyle \sum_{n=1}^{\infty} \frac{(n!)^2}{2^n (2n)!} \, x^n\)
  4. \(\displaystyle \sum_{n=1}^{\infty} \brac{\frac{n}{n+1}}^{n^2} x^n\)

Checkpoint 5.1.4.

Find the interval of convergence and radius of convergence of each series. Then, find the sum of the series for \(x\) in that interval.
  1. \(\sum_{n=0}^{\infty} \frac{(x+1)^{2n}}{9^n}\text{.}\)
Hint.
The series is geometric.
Answer.
\((-4,2)\text{,}\) converges to \(\frac{9}{8-x^2-2x}\text{.}\)