Vector Spaces
Example used in the Exercise below: Look at the abelian group `langle RR_n \ , + rangle = RR times RR times ... times RR` for n factors. These are ordered n-tuples under addition component-wise. We define scalar multiplication for scalars in `RR` by
`r alpha = (ra_1 \ , ra_2 \ , ... \ , ra_n)` for `r in RR` and `alpha = (a_1 \ , a_2 \ , ... \ , a_n) in RR^n`. These operations make `RR^n` a vector space over `RR`. For example `RR^2 = RR times RR` is a vector space over `RR` and can be visualized as all vectors starting at the origin of the Euclidean plane.
The Mechanics in this Example:
-The Set Construction: We want to generalize to `bbb"F"^n` for any field `bbb"F"`. Just as `RR^n` is a product of n copies of `RR` , `bbb"F"^n` is the product of n copies of `bbb"F"`.
-Component-wise Logic: The example shows that addition and multiplication happen "slot by slot". This is the "engine" you carry over to the general field `bbb"F"`.
-The Geometric Analogy: Mentioning the "origin of the Euclidean plane" is useful for intuition , even if `bbb"F"` is a field where "distance" or "planes" look very different.
The following exercise is the "Big Bang" of linear algebra. By moving from `RR^n` (which we can visualize) to `bbb"F"^n` (which is abstract) , you are learning how to apply geometri to things that aren't necessarily geometric - like codes , logic , or even finite fields in cryptography.
Exercise: The goal is to generalize the specific case of the real vector space `RR^n` over the field `RR` to an arbitrary field `bbb"F"`. We must:
1. Define the set `bbb"F"^n`.
2. Define the operations of addition and scalar multiplication.
3. Identify a standard basis for this space.
General Strategy: We will use the properties of a field `bbb"F"` (which has its own addition and multiplication) to define "component-wise" operatons on n-tuples. Then we will identify the standard basis - the set of vectors that act as the building blocks for every other vector in the space.
Specific Theory Used:
- Field (`bbb"F"`): A set equipped with two operations `(+ "and" *)` satisfying the field axioms
(commutativity , associativity , identity , inverses).
- Cartsian Product: `bbb"F"^n = bbb"F" times bbb"F" times ... times bbb"F"` (n times).
- Basis: A set of vectors `{v_1 \ , v_2 \ , ... \ , v_n}` that is linearly independent and spans the entire space.
Solution:
Model Case: We begin by looking at the provided example of `RR^n`. In `RR^n` , the vector space properties rely on the fact that `RR` is a field.
Generalization: To obtain `bbb"F"^n` we replace the field `RR` with an arbitrary field `bbb"F"`. Since the definition of a vector space only requires that the scalars belong to some field , the operations of component-wise addition and scalar multiplication used in `RR^n` remain valid for `bbb"F"^n`.
Part A. Generalizing the Space: We define `bbb"F"^n` as the set of all ordered n-tuples:
`bbb"F"^n = {(a_1 \ , a_2 \ , ... \ , a_n) : a_i in bbb"F" \ "for" \ i = 1 \ , ... \ , n}`.
We define operations exactly as in `RR^n` , but using the arithmetic of field `bbb"F"`.
1. Addition: For `alpha =(a_1 \ , a_2 \ , ... \ , a_n) \ "and" \ beta =(b_1 \ , b_2 \ , ... \ , b_n)` ,
`alpha + beta = (a_1 + beta_1 \ , a_2 + beta_2 \ , ... \ , a_n + beta_n)`.
2. Scalar Multiplication: For any scalar `c in bbb"F"` and vector `alpha in bbb"F"^n \ , c alpha = (ca_1 \ , ca_2 \ , ... \ , ca_n)`.
Since `bbb"F"` is a field , the vector space axioms (like the existence of a zero vector `(0 \ , 0 \ , ... \ , 0)` and additive inverses) are automatically satisfied by the properties of `bbb"F"`.
Part B. Generalizing the Basis: To find a basis , we look for the simplest set of vectors that can "build" any tuple. Consider the vectors `e_i` where the i-th position is the multiplicative identity `1 in bbb"F"` and all other positions are 0:
`e_1 = (1 \ , 0 \ , 0 \ , ... \ , 0)`
`e_2 = (0 \ , 1 \ , 0 \ , ... \ , 0)`
      .
      .
      .
`e_n = (0 \ , 0 \ , 0 \ , ... \ , 1)`
Proof that this is a Basis:
1. Spanning: Any vector `alpha =(a_1 \ , a_2 \ , ... \ , a_n)` can be written as:
`alpha = a_1 e_1 + a_2 e_2 + ... + a_n e_n`.
2. Linear Independence: If `c_1 e_1 + c_2 e_2 + ... + c_n e_n = (0 \ , 0 \ , ... \ , 0)` , then `(c_1 \ , c_2 \ , ... \ , c_n) = (0 \ , 0 \ , ... \ , 0)` , which forces every `c_i = 0`.
QED
Conclusion: The set `{e_1 \ , e_2 \ , ... \ , e_n}` is called the Standard Basis for the general vector space `bbb"F"^n`.
END
Question
If our field `bbb"F"` is the finite field `ZZ_2` (containing only `{0 \ , 1}`) , how many distinct vectors exists in the vector space `bbb"F"^2` (the set of all 2-tuples)
?
A) `2^2 = 4` vectors.
B) Infinitely many vectors , because it is a vector space.