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# 11: Sequences and Series

• • Contributed by David Guichard
• Professor (Mathematics) at Whitman College
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A power series (in one variable) is an infinite series. Any polynomial can be easily expressed as a power series around any center c, although most of the coefficients will be zero since a power series has infinitely many terms by definition. One can view power series as being like "polynomials of infinite degree," although power series are not polynomials. The content in this Textmap's chapter is complemented by OpenStax's Calculus Textmap.

• 11.1: Prelude to Sequences and Series
Our first task is to investigate infinite sums, called series, is to investigate limits of sequences of numbers.
• 11.2: Sequences
While the idea of a sequence of numbers is straightforward, it is useful to think of a sequence as a function. We have up until now dealt with functions whose domains are the real numbers, or a subset of the real numbers, like f(x)=sinx. A sequence is a function with domain the natural numbers N={1,2,3,…} or the non-negative integers, Z≥0={0,1,2,3,…}. The range of the function is still allowed to be the real numbers; in symbols, we say that a sequence is a function f:N→R.
• 11.3: Series
Recall that a series, roughly speaking, is the sum of a sequence. Associated with a series is a second sequence, called the sequence of partial sums.  A series converges if the sequence of partial sums converges, and otherwise the series diverges.
• 11.4: The Integral Test
It is generally quite difficult, often impossible, to determine the value of a series exactly. In many cases it is possible at least to determine whether or not the series converges, and so we will spend most of our time on this problem. If all of the terms anan in a series are non-negative, then clearly the sequence of partial sums snsn is non-decreasing. This means that if we can show that the sequence of partial sums is bounded, the series must converge.
• 11.5: Alternating Series
Next we consider series with both positive and negative terms, but in a regular pattern: they alternate.
• 11.6: Comparison Test
As we begin to compile a list of convergent and divergent series, new ones can sometimes be analyzed by comparing them to ones that we already understand.
• 11.7: Absolute Convergence
Roughly speaking there are two ways for a series to converge: (1) the individual terms get small very quickly, so that the sum of all of them stays finite, or (2) the terms do not get small fast enough, but a mixture of positive and negative terms provides enough cancellation to keep the sum finite. You might guess from what we've seen that if the terms get small fast enough to do the job, then whether or not some terms are negative and some positive the series converges.
• 11.8: The Ratio and Root Tests
Sometimes it is possible, but a bit unpleasant, to evaluate if a series converges with the integral test or the comparison test, but there are easier ways. The ratio and root tests are two such approaches.
• 11.9: Power Series
The geometric series has a special feature that makes it unlike a typical polynomial---the coefficients of the powers of xx are the same, namely kk . We will need to allow more general coefficients if we are to get anything other than the geometric series.
• 11.10: Calculus with Power Series
Now we know that some functions can be expressed as power series, which look like infinite polynomials. Since calculus, that is, computation of derivatives and antiderivatives, is easy for polynomials, the obvious question is whether the same is true for infinite series. The answer i
• 11.11: Taylor Series
We have seen that some functions can be represented as series, which may give valuable information about the function. So far, we have seen only those examples that result from manipulation of our one fundamental example, the geometric series. We would like to start with a given function and produce a series to represent it, if possible.
• 11.12: Taylor's Theorem
ne of the most important uses of infinite series is the potential for using an initial portion of the series for f to approximate ff . We have seen, for example, that when we add up the first n terms of an alternating series with decreasing terms that the difference between this and the true value is at most the size of the next term. A similar result is true of many Taylor series.
Thumbnail: The graph shows the function $$\displaystyle y=sinx$$ and the Maclaurin polynomials $$\displaystyle p_1,p_3$$ and $$\displaystyle p_5$$. Image used with permission (CC BY-SA 3.0; OpenStax).