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# Summation Operator

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Properties of the summation operator for my introductory econometrics students.

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### Summation Operator

1. 1. Outline Operators Summation Double summationApplied Statistics for Economics Summation Operator SFC - juliohuato@gmail.com Spring 2012 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
2. 2. Outline Operators Summation Double summationOperatorsSummationDouble summation SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
3. 3. Outline Operators Summation Double summationOn math as a language Math is, among other things, a language. We use language to think ideas and share them with others. In principle, the same ideas we express with math symbols we can express with words (which are also symbols). Math symbols are just abbreviations for words. However, when we abbreviate and express our ideas in math language, we economize resources. It is easier, for example, to make the shared or communicable meaning of words clearer and more precise when we use math symbols. SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
4. 4. Outline Operators Summation Double summationOperators Operators are mathematical symbols that compress or abbreviate further our math language. That is why they can be extremely powerful tools in econometrics. These are some familiar examples of operators: Addition: + Subtraction: − Multiplication: × Division: ÷ In the context of a statement in math language, these operators tell us to execute speciﬁc operations: (a + b) add b to a; (a − b) subtract b from a; (a × b) multiply b times the number a; (a ÷ b) divide a by b (or b into a). SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
5. 5. Outline Operators Summation Double summationSummation Operator ( ) The summation operator is heavily used in econometrics. We now let a, b, k, and n be constant numbers, and x, y , and i be variables. The following are some properties of the summation operator. SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
6. 6. Outline Operators Summation Double summationSummation ( xi ) Suppose we have a list of numbers (the ages of 6 students): 20, 19, 22, 19, 21, 18. Let x be the age of a student and use the natural numbers (1, 2, 3, . . .) to index these ages. Thus, xi means the age of student i, where i = 1, 2, . . . , 6). Then: 6 x1 + x2 + x3 + x4 + x5 + x6 = x1 + x2 + . . . + x6 = xi i=1 The last expression is the most compact. It reads: “The sum of xi , where i goes from 1 to 6.” The summation operator tells us to add up the values of the variable x from the ﬁrst to the sixth value: 6 xi = 20 + 19 + 22 + 19 + 21 + 18 = 119. i=1 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
7. 7. Outline Operators Summation Double summationSummation ( xi ) Note the following: n m n xi = xi + xi i=1 i=1 i=m+1 Example: 6 3 6 xi = xi + xi = (20+19+22)+(19+21+18) = 61+58 = 119. i=1 i=1 i=4 We can always split the sum into various sub-sums. SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
8. 8. Outline Operators Summation Double summationSumming n times the constant number (k) This property also holds for the summation operator: n k = nk i=1 Example: 4 3 = 3 + 3 + 3 + 3 = 4 × 3 = 12. i=1 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
9. 9. Outline Operators Summation Double summationSumming n times the product of a constant k and avariable x n n kxi = k xi i=1 i=1 Example: 3 3 5xi = 5x1 + 5x2 + 5x3 = 5(x1 + x2 + x3 ) = 5 xi . i=1 i=1 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
10. 10. Outline Operators Summation Double summationSumming the sum of two variables (x and y ) n n n (xi + yi ) = xi + yi i=1 i=1 i=1 Example: 2 (xi + yi ) = (x1 + y1 ) + (x2 + y2 ) = x1 + y1 + x2 + y2 i=1 2 2 = x1 + x2 + y1 + y2 = (x1 + x2 ) + (y1 + y2 ) = xi + yi . i=1 i=1 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
11. 11. Outline Operators Summation Double summationSumming the linear rule of a variable (x) The linear rule of a variable x is: a + bx. E.g.: 4 + 5x. If the n values of the variables are indexed (i = 1, 2, . . . , n), then we can express the sum of this linear rule of x over its n values as follows: n n (a + bxi ) = na + b xi i=1 i=1 Example: 3 3 3 3 3 (4 + 5xi ) = 4+ 5xi = (3 × 4) + 5 xi = 12 + 5 xi . i=1 i=1 i=1 i=1 i=1 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
12. 12. Outline Operators Summation Double summationDouble summation The double summation operator is used to sum up twice for the same variable: n m n xij = (xi1 + xi2 + . . . + xim ) i=1 j=1 i=1 = (x11 +x21 +. . .+xn1 )+(x12 +x22 +. . .+xn2 )+. . .+(x1m +x2m +. . .+xnm ) SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
13. 13. Outline Operators Summation Double summationDouble summation A property of the double summation operator is that the summations are interchangeable: n m m n xij = xij . i=1 j=1 i=1 j=1 SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator
14. 14. Outline Operators Summation Double summationThe product operator The product operator ( ) is deﬁned as: n xi = x1 · x2 · · · xn . i=1 Example: Let x be a list of numbers: 20, 19, 22. Then, 3 xi = 20 × 19 × 22 = 8, 360. i=1 n Note that i=1 k = k n . The n-product of a constant is the constant raised to the n-th power. SFC - juliohuato@gmail.com Applied Statistics for Economics Summation Operator