Differentiation of Infinite series

Differentiation of Infinite Series:

  • Infinite series refers to the sum of an infinite sequence of terms. Differentiating such series involves differentiating term by term.
  • nfinite series represents an infinite sum of terms involving a variable x, often written in the form of f(x)=n=0anxn, where an are coefficients and x is the variable.

Conditions for Differentiation of Infinite Series:

  1. Uniform Convergence:

    • To differentiate an infinite series term by term, the series must converge uniformly within a given interval.
    • Uniform convergence ensures that the derivative of the sum is equal to the sum of the derivatives of each term.
  2. Continuity and Differentiability:

    • Each term of the series should be continuous and differentiable within the interval of interest for the differentiation to hold.

Differentiation of Power Series:

  • Power series is a type of infinite series that can be differentiated and integrated term by term within its interval of convergence.

Steps for Differentiating Infinite Series:

  1. Write the function as a power series:

    • Express the function as an infinite sum involving powers of x.
  2. Differentiate each term individually:

    • Differentiate each term of the series with respect to x using standard differentiation rules.
    • Derivative of xn is nxn1.
  3. Find the resulting series:

    • Sum up the individual derivatives to get the derivative of the original series.


Consider the series f(x)=n=1xnn within the interval (1,1).

  1. Express the series as a function:

    • f(x)=n=1xnn=ln(1x), 1<x1
  2. Differentiate term by term:

    • Differentiate f(x) term by term to find its derivative: f(x)=n=1xn1=11x, 1<x<1
  3. Verify the convergence and interval:

    • The derivative f(x) converges within the interval (1,1) and is equal to the sum of the derivatives of each term in the original series.


Given the function f(x)=n=0xnn! which represents ex:

  1. Express the function as a power series:

    • f(x)=ex is a well-known power series.
  2. Differentiate each term individually:

    • Differentiate each term of the power series ex term by term.
    • ddx(xnn!)=nxn1n!=xn1(n1)!
  3. Find the resulting series:

    • The derivative of f(x) is ddxex=n=0xn1(n1)!.

Tips for Differentiating Infinite Series:

  • Term-by-Term Differentiation:

    • Differentiate each term of the series individually as if it were a separate function.
  • Convergence Analysis:

    • Ensure that the resulting series after differentiation converges within the same interval as the original series.
  • Uniform Convergence:

    • Confirm uniform convergence to ensure that differentiation can be performed term by term.

Special Cases:

  • Power Series:

    • Often used in differentiating functions represented as power series expansions, such as Taylor or Maclaurin series.
  • Functions with Special Forms:

    • Differentiating trigonometric, exponential, or logarithmic series often involves specific differentiation rules tailored to these functions.

Advantages of Differentiating Infinite Series:

  • Extension of Differentiation Rules:

    • Allows differentiation of functions represented as infinite series using standard differentiation rules.
  • Representation of Functions:

    • Infinite series often represent functions and their derivatives, aiding in function manipulation and analysis.