summation operator
DESCRIPTION
Properties of the summation operator for my introductory econometrics students.TRANSCRIPT
OutlineOperators
SummationDouble summation
Applied Statistics for EconomicsSummation Operator
SFC - [email protected]
Spring 2012
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Operators
Summation
Double summation
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
On math as a language
Math is, among other things, a language. We use language tothink ideas and share them with others.
In principle, the same ideas we express with math symbols we canexpress with words (which are also symbols). Math symbols arejust abbreviations for words.
However, when we abbreviate and express our ideas in mathlanguage, we economize resources. It is easier, for example, tomake the shared or communicable meaning of words clearer andmore precise when we use math symbols.
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Operators
Operators are mathematical symbols that compress or abbreviatefurther our math language. That is why they can be extremelypowerful tools in econometrics.
These are some familiar examples of operators:
I Addition: +
I Subtraction: −I Multiplication: ×I Division: ÷
In the context of a statement in math language, these operatorstell 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 - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summation 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 bevariables. The following are some properties of the summationoperator.
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summation (∑
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 thenatural numbers (1, 2, 3, . . .) to index these ages. Thus, xi meansthe age of student i , where i = 1, 2, . . . , 6). Then:
x1 + x2 + x3 + x4 + x5 + x6 = x1 + x2 + . . . + x6 =6∑
i=1
xi
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∑i=1
xi = 20 + 19 + 22 + 19 + 21 + 18 = 119.
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summation (∑
xi)
Note the following:
n∑i=1
xi =m∑i=1
xi +n∑
i=m+1
xi
Example:
6∑i=1
xi =3∑
i=1
xi+6∑
i=4
xi = (20+19+22)+(19+21+18) = 61+58 = 119.
We can always split the sum into various sub-sums.
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summing n times the constant number (k)
This property also holds for the summation operator:
n∑i=1
k = nk
Example:4∑
i=1
3 = 3 + 3 + 3 + 3 = 4 × 3 = 12.
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summing n times the product of a constant k and avariable x
n∑i=1
kxi = kn∑
i=1
xi
Example:
3∑i=1
5xi = 5x1 + 5x2 + 5x3 = 5(x1 + x2 + x3) = 53∑
i=1
xi .
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summing the sum of two variables (x and y)
n∑i=1
(xi + yi ) =n∑
i=1
xi +n∑
i=1
yi
Example:
2∑i=1
(xi + yi ) = (x1 + y1) + (x2 + y2) = x1 + y1 + x2 + y2
= x1 + x2 + y1 + y2 = (x1 + x2) + (y1 + y2) =2∑
i=1
xi +2∑
i=1
yi .
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Summing 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), thenwe can express the sum of this linear rule of x over its n values asfollows:
n∑i=1
(a + bxi ) = na + bn∑
i=1
xi
Example:
3∑i=1
(4 + 5xi ) =3∑
i=1
4 +3∑
i=1
5xi = (3 × 4) + 53∑
i=1
xi = 12 + 53∑
i=1
xi .
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Double summation
The double summation operator is used to sum up twice for thesame variable:
n∑i=1
m∑j=1
xij =n∑
i=1
(xi1 + xi2 + . . . + xim)
= (x11+x21+. . .+xn1)+(x12+x22+. . .+xn2)+. . .+(x1m+x2m+. . .+xnm)
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
Double summation
A property of the double summation operator is that thesummations are interchangeable:
n∑i=1
m∑j=1
xij =m∑i=1
n∑j=1
xij .
SFC - [email protected] Applied Statistics for Economics Summation Operator
OutlineOperators
SummationDouble summation
The product operator
The product operator (∏
) is defined as:
n∏i=1
xi = x1 · x2 · · · xn.
Example: Let x be a list of numbers: 20, 19, 22. Then,
3∏i=1
xi = 20 × 19 × 22 = 8, 360.
Note that∏n
i=1 k = kn. The n-product of a constant is theconstant raised to the n-th power.
SFC - [email protected] Applied Statistics for Economics Summation Operator