2015 년 봄학기 강원대학교 컴퓨터과학전공 문양세 이산수학 (Discrete Mathematics) 수열과 합 (Sequences and Summations)

Discrete Mathematics by Yang-Sae Moon Page 2 Introduction Sequences and Summations A sequence or series is just like an ordered n-tuple (a 1, a 2, …, a n ), except: Each element in the sequences has an associated index number. ( 각 element 는 색인 (index) 번호와 결합되는 특성을 가진다.) A sequence or series may be infinite. ( 무한할 수 있다.) Example: 1, 1/2, 1/3, 1/4, … A summation is a compact notation for the sum of all terms in a (possibly infinite) series. (  )

Discrete Mathematics by Yang-Sae Moon Page 3 Sequences Formally: A sequence {a n } is identified with a generating function f:S  A for some subset S  N (S=N or S=N  {0}) and for some set A. ( 수열 {a n } 은 자연수 집합으로부터 A 로의 함수 …) If f is a generating function for a sequence {a n }, then for n  S, the symbol a n denotes f(n). The index of a n is n. (Or, often i is used.) SA 12341234 a 1 = f(1) a 2 = f(2) a 3 = f(3) a 4 = f(4)  f Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 4 Sequence Examples Example of an infinite series ( 무한 수열 ) Consider the series {a n } = a 1, a 2, …, where (  n  1) a n = f(n) = 1/n. Then, {a n } = 1, 1/2, 1/3, 1/4, … Example with repetitions ( 반복 수열 ) Consider the sequence {b n } = b 0, b 1, … (note 0 is an index) where b n = (  1) n. {b n } = 1,  1, 1,  1, … Note repetitions! {b n } denotes an infinite sequence of 1’s and  1’s, not the 2-element set {1,  1}. Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 5 Recognizing Sequences (1/2) Sometimes, you’re given the first few terms of a sequence, and you are asked to find the sequence’s generating function, or a procedure to enumerate the sequence. ( 순열의 몇몇 값들에 기반하여 f(n) 을 발견하는 문제에 자주 직면하게 된다.) Examples: What’s the next number and f(n)? 1, 2, 3, 4, … (the next number is 5. f(n) = n 1, 3, 5, 7, … (the next number is 9. f(n) = 2n − 1 Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 6 Recognizing Sequences (2/2) Trouble with recognition (of generating functions) The problem of finding “the” generating function given just an initial subsequence is not well defined. ( 잘 정의된 방법이 없음 ) This is because there are infinitely many computable functions that will generate any given initial subsequence. ( 세상에는 시퀀스를 생성하는 셀 수 없이 많은 함수가 존재한다.) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 7 Summation Notation Given a sequence {a n }, an integer lower bound j  0, and an integer upper bound k  j, then the summation of {a n } from j to k is written and defined as follows: ({a n } 의 j 번째에서 k 번째까지의 합, 즉, a j 로부터 a k 까지의 합 ) Here, i is called the index of summation. Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 8 Generalized Summations For an infinite series, we may write: To sum a function over all members of a set X={x 1, x 2, …}: ( 집합 X 의 모든 원소 x 에 대해서 ) Or, if X={x|P(x)}, we may just write: (P(x) 를 true 로 하는 모든 x 에 대해서 ) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 9 Summation Examples A simple example An infinite sequence with a finite sum: Using a predicate to define a set of elements to sum over: Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 10 Summation Manipulations (1/2) Some useful identities for summations: (Distributive law) (Application of commutativity) (Index shifting) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 11 Summation Manipulations (2/2) Some more useful identities for summations: (Grouping) (Order reversal) (Series splitting) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 12 An Interesting Example “I’m so smart; give me any 2-digit number n, and I’ll add all the numbers from 1 to n in my head in just a few seconds.” (1 에서 n 까지의 합을 수초 내에 계산하겠다 !) I.e., Evaluate the summation: There is a simple formula for the result, discovered by Euler at age 12! Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 13 Euler’s Trick, Illustrated Consider the sum: … + (n/2) + ((n/2)+1) + … + (n-1) + n n/2 pairs of elements, each pair summing to n+1, for a total of (n/2)(n+1). n/2 pairs of elements, each pair summing to n+1, for a total of (n/2)(n+1). ( 합이 n+1 인 두 쌍의 element 가 n/2 개 있다.) … n+1 Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 14 Geometric Progression ( 등비수열 ) A geometric progression is a series of the form a, ar, ar 2, ar 3, …, ar k, where a,r  R. The sum of such a sequence is given by: We can reduce this to closed form via clever manipulation of summations... Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 15 Nested Summations These have the meaning you’d expect. Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 16 Some Shortcut Expressions SumClosed Form Infinite series ( 무한급수 ) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 17 Using the Shortcuts Example: Evaluate. Use series splitting. Solve for desired summation. Apply quadratic series rule. Evaluate. Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 18 Cardinality: Formal Definition For any two (possibly infinite) sets A and B, we say that A and B have the same cardinality (written |A|=|B|) iff there exists a bijection (bijective function) from A to B. ( 집합 A 에서 집합 B 로의 전단사함수가 존재하면, A 와 B 의 크기는 동일하다.) When A and B are finite, it is easy to see that such a function exists iff A and B have the same number of elements n  N. ( 집합 A, B 가 유한집합이고 동일한 개수의 원소를 가지면, A 와 B 가 동일한 크기 임을 보이는 것은 간단하다.) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 19 Countable versus Uncountable For any set S, if S is finite or if |S|=|N|, we say S is countable. Else, S is uncountable. ( 유한집합이거나, 자연수 집합과 크기가 동일하면 countable 하며, 그렇지 않으 면 uncountable 하다.) Intuition behind “countable:” we can enumerate (sequentially list) elements of S. Examples: N, Z. ( 집합 S 의 원소에 번호를 매길 수 ( 순차적으로 나열할 수 ) 있다.) Uncountable means: No series of elements of S (even an infinite series) can include all of S’s elements. Examples: R, R 2 ( 어떠한 나열 방법도 집합 S 의 모든 원소를 포함할 수 없다. 즉, 집합 S 의 원소에 번호를 매길 수 있는 방법이 없다.) Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 20 Countable Sets: Examples Theorem: The set Z is countable. Proof: Consider f:Z  N where f(i)=2i for i  0 and f(i) =  2i  1 for i<0. Note f is bijective. (…, f(  2)=3, f(  1)=1, f(0)=0, f(1)=2, f(2)=4, …) Theorem: The set of all ordered pairs of natural numbers (n,m) is countable. (1,1) (1,2) (1,3) (1,4) (1,5) (2,1) (2,2) (2,3) (2,4) (2,5) (3,1) (3,2) (3,3) (3,4) (3,5) (4,1) (4,2) (4,3) (4,4) (4,5) (5,1) (5,2) (5,3) (5,4) (5,5) …………… … … … … … … consider sum is 2, then consider sum is 3, then consider sum is 4, then consider sum is 5, then consider sum is 6, then consider … Note a set of rational numbers is countable! Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 21 Uncountable Sets: Example (1/2) - skip Theorem: The open interval [0,1) :  {r  R| 0  r < 1} is uncountable. ([0,1) 의 실수는 uncountable) Proof by Cantor Assume there is a series {r i } = r 1, r 2,... containing all elements r  [0,1). Consider listing the elements of {r i } in decimal notation in order of increasing index: r 1 = 0.d 1,1 d 1,2 d 1,3 d 1,4 d 1,5 d 1,6 d 1,7 d 1,8 … r 2 = 0.d 2,1 d 2,2 d 2,3 d 2,4 d 2,5 d 2,6 d 2,7 d 2,8 … r 3 = 0.d 3,1 d 3,2 d 3,3 d 3,4 d 3,5 d 3,6 d 3,7 d 3,8 … r 4 = 0.d 4,1 d 4,2 d 4,3 d 4,4 d 4,5 d 4,6 d 4,7 d 4,8 … … Now, consider r’ = 0.d 1 d 2 d 3 d 4 … where d i = 4 if d ii  4 and d i = 5 if d ii = 4. Sequences and Summations

Discrete Mathematics by Yang-Sae Moon Page 22 Uncountable Sets: Example (2/2) - skip E.g., a postulated enumeration of the reals: r 1 = … r 2 = … r 3 = … r 4 = … … OK, now let’s make r’ by replacing d ii by the rule. (Rule: r’ = 0.d 1 d 2 d 3 d 4 … where d i = 4 if d ii  4 and d i = 5 if d ii = 4) r’ = … can’t be on the list anywhere! ( 왜냐면, 4 가 아니면 4 로, 4 이면 5 로 바꾸었기 때문에 ) This means that the assumption({r i } is countable) is wrong, and thus, [0,1), {r i }, is uncountable. Sequences and Summations