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# 2: Sequences

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A sequence is simply an ordered list of numbers. For example, here is a sequence: 0, 1, 2, 3, 4, 5, …. This is different from the set N because, while the sequence is a complete list of every element in the set of natural numbers, in the sequence we very much care what order the numbers come in.

• 2.1: Definitions
While we often just think of sequences as an ordered list of numbers, they really are a type of function. Later we will manipulate sequences in much the same way you have manipulated functions in algebra or calculus. We can shift a sequence up or down, add two sequences, or ask for the rate of change of a sequence. These are done exactly as you would for functions.
• 2.2: Arithmetic and Geometric Sequences
If the terms of a sequence differ by a constant, we say the sequence is arithmetic. A sequence is called geometric if the ratio between successive terms is constant.
• 2.3: Polynomial Fitting
So far we have seen methods for finding the closed formulas for arithmetic and geometric sequences. Since we know how to compute the sum of the first n terms of arithmetic and geometric sequences, we can compute the closed formulas for sequences which have an arithmetic (or geometric) sequence of differences between terms. But what if we consider a sequence which is the sum of the first n terms of a sequence which is itself the sum of an arithmetic sequence?
• 2.4: Solving Recurrence Relations
We have seen that it is often easier to find recursive definitions than closed formulas. Lucky for us, there are a few techniques for converting recursive definitions to closed formulas. Doing so is called solving a recurrence relation. Recall that the recurrence relation is a recursive definition without the initial conditions.
• 2.5: Induction
Induction is a style of argument we use to convince ourselves and others that a mathematical statement is always true. Many mathematical statements can be proved by simply explaining what they mean. Others are very difficult to prove—in fact, there are relatively simple mathematical statements which nobody yet knows how to prove. To facilitate the discovery of proofs, it is important to be familiar with some standard styles of arguments
• 2.E: Sequences (Exercises)
• 2.S: Chapter Summary