Sequences - ECE 3250 MORE ON REAL AND COMPLEX NUMBERS Fall...

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Unformatted text preview: ECE 3250 MORE ON REAL AND COMPLEX NUMBERS Fall 2008 My purpose in this handout is to summarize fairly briskly the main results on sequences and series of real and complex numbers. Ill be lifting some of the definitions (e.g. con- vergent sequences, Cauchy sequences, etc.) from the NUMBERS handout, but I think its useful to collect the principal facts in one place in a kind of bulleted-list format. The lack of a lot of intervening text makes for some high-density mathematics, but the layout, I hope, will facilitate easy reference. To start with, Ill assume you have some basic familiarity with the real numbers R and the complex numbers C . Ill assume you understand the algebra of complex numbers at the level of the ALGEBRA OF COMPLEX NUMBERS handout. For a real number a , | a | denotes the absolute value of a ; for a complex number c , | c | denotes the magnitude of c . So if c = a + jb , with a and b in R , then | c | = p a 2 + b 2 . The distance between two real numbers a and b is the absolute value of a- b and the distance between two complex numbers c 1 and c 2 is the magnitude of c 1- c 2 . To avoid having to type real or complex numbers a zillion times, Ill use the notation F to denote the phrase R or C . The F is supposed to mean field. Notational Convention: In what follows, F = R or C . Definition 1: A sequence in F is an ordered list of elements of F indexed by N . We use notation such as { a n } or { c n } to denote such a sequence. So, for example, { a n } = a , a 1 , a 2 , a 3 , . . . . Definition 2: We say that a sequence { a n } in F converges to a F if and only if the distance between a n and a approaches zero as n . In this case, we write lim n a n = a . A precise mathematical definition of convergence: { a n } converges to a if and only if for every > 0 there exists an integer N > 0 such that | a n- a | < for every n > N . Fact 1: A sequence { c n = a n + jb n } in C converges to c = a + j b C if and only if { a n } converges to a and { b n } converges to b . Proof: First of all, for every n N , | c n- c | = q | a n- a | 2 + | b n- b | 2 , If { c n } converges to c , then for every > 0 we can find N > 0 so that | c n- c | < when n > N . Hence for n > N , we have q | a n- a | 2 + | b n- b | 2 < , which implies that both | a n- a | < and | b n- b | < for every n > N . Accordingly, { a n } converges to a and { b n } converges to b . Conversely, if { a n } converges to a and { b n } converges to b , then for every > 0 we can find N > 0 so that both | a n- a | < 2 / 2 and | b n- b | < 2 / 2 when n > N . Hence for n > N , we have | c n- c | < p 2 / 2 + 2 / 2 = ....
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This note was uploaded on 08/10/2010 for the course ECE 4370 at Cornell University (Engineering School).

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Sequences - ECE 3250 MORE ON REAL AND COMPLEX NUMBERS Fall...

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