Introduction to Vector Spaces

A vector space (over $\mathbb {R}$) consists of a set Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V} along with two operations "$+$" and "$\cdot$" subject to these conditions.

1. For any ${\vec {v}},{\vec {w}}\in V:{\vec {v}}+{\vec {w}}\in V$.
2. For any ${\vec {v}},{\vec {w}}\in V:{\vec {v}}+{\vec {w}}={\vec {w}}+{\vec {v}}$.
3. For any ${\vec {u}},{\vec {v}},{\vec {w}}\in V:({\vec {v}}+{\vec {w}})+{\vec {u}}={\vec {v}}+({\vec {w}}+{\vec {u}})$.
4. There is a zero vector ${\vec {0}}\in V$ such that ${\vec {v}}+{\vec {0}}={\vec {v}}$ for all ${\vec {v}}\in V$.
5. Each ${\vec {v}}\in V$ has an additive inverse ${\vec {w}}\in V$ such that ${\vec {w}}+{\vec {v}}={\vec {0}}$.
6. If $r$ is a scalar, that is, a member of $\mathbb {R}$ and ${\vec {v}}\in V$ then the scalar multiple $r\cdot {\vec {v}}$ is in $V$.
7. If $r,s\in \mathbb {R}$ and ${\vec {v}}\in V$ then $(r+s)\cdot {\vec {v}}=r\cdot {\vec {v}}+s\cdot {\vec {v}}$.
8. If $r\in \mathbb {R}$ and ${\vec {v}},{\vec {w}}\in V$ , then $r\cdot ({\vec {v}}+{\vec {w}})=r\cdot {\vec {v}}+r\cdot {\vec {w}}$.
9. If $r,s\in \mathbb {R}$ and ${\vec {v}}\in V$ , then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle (rs)\cdot\vec v=r\cdot(s\cdot\vec v)} .
10. For any Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \vec v\in V} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1\cdot\vec v=\vec v} .

Remark: Because it involves two kinds of addition and two kinds of multiplication, that definition may seem confused. For instance, in condition 7 "$(r+s)\cdot {\vec {v}}=r\cdot {\vec {v}}+s\cdot {\vec {v}}$", the first "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle +} " is the real number addition operator while the "$+$" to the right of the equals sign represents vector addition in the structure Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle V} . These expressions aren't ambiguous because, e.g., $r$ and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle s} are real numbers so "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle r+s} " can only mean real number addition.

Example: The set Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \R^2} is a vector space if the operations "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle + } " and "$\cdot$" have their usual meaning.

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \binom{x_1}{x_2}+\binom{y_1}{y_2}=\binom{x_1+y_1}{x_2+y_2}\qquad r\cdot\binom{x_1}{x_2}=\binom{rx_1}{rx_2}}

We shall check all of the conditions.

There are five conditions in item 1. For 1, closure of addition, note that for any Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle v_1,v_2,w_1,w_2\in\R} the result of the sum

${\binom {v_{1}}{v_{2}}}+{\binom {w_{1}}{w_{2}}}={\binom {v_{1}+w_{1}}{v_{2}+w_{2}}}$

is a column array with two real entries, and so is in Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \R^2} . For 2, that addition of vectors commutes, take all entries to be real numbers and compute

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \binom{v_1}{v_2}+\binom{w_1}{w_2}=\binom{v_1+w_1}{v_2+w_2}=\binom{w_1+v_1}{w_2+v_2}=\binom{w_1}{w_2}+\binom{v_1}{v_2}}

(the second equality follows from the fact that the components of the vectors are real numbers, and the addition of real numbers is commutative). Condition 3, associativity of vector addition, is similar.

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{align} \Bigg(\binom{v_1}{v_2}+\binom{w_1}{w_2}\Bigg)+\binom{u_1}{u_2} &=\binom{(v_1+w_1)+u_1}{(v_2+w_2)+u_2}\\ &=\binom{v_1+(w_1+u_1)}{v_2+(w_2+u_2)}\\ &=\binom{v_1}{v_2}+\Bigg(\binom{w_1}{w_2}+\binom{u_1}{u_2}\Bigg) \end{align}}

For the fourth condition we must produce a zero element — the vector of zeroes is it.

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \binom{v_1}{v_2}+\binom00=\binom{v_1}{v_2}}

For 5, to produce an additive inverse, note that for any Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle v_1,v_2\in\R} we have

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \binom{-v_1}{-v_2}+\binom{v_1}{v_2}=\binom00}

so the first vector is the desired additive inverse of the second.

The checks for the five conditions having to do with scalar multiplication are just as routine. For 6, closure under scalar multiplication, where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle r,v_1,v_2\in\R} ,

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle r\cdot\binom{v_1}{v_2}=\binom{rv_1}{rv_2}}

is a column array with two real entries, and so is in $\mathbb {R} ^{2}$ . Next, this checks 7.

$(r+s)\cdot {\binom {v_{1}}{v_{2}}}={\binom {(r+s)v_{1}}{(r+s)v_{2}}}={\binom {rv_{1}+sv_{1}}{rv_{2}+sv_{2}}}=r\cdot {\binom {v_{1}}{v_{2}}}+s\cdot {\binom {v_{1}}{v_{2}}}$

For 8, that scalar multiplication distributes from the left over vector addition, we have this.

$r\cdot {\Bigg (}{\binom {v_{1}}{v_{2}}}+{\binom {w_{1}}{w_{2}}}{\Bigg )}={\binom {r(v_{1}+w_{1})}{r(v_{2}+w_{2})}}={\binom {rv_{1}+rw_{1}}{rv_{2}+rw_{2}}}=r\cdot {\binom {v_{1}}{v_{2}}}+r\cdot {\binom {w_{1}}{w_{2}}}$

The ninth

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle (rs)\cdot\binom{v_1}{v_2}=\binom{(rs)v_1}{(rs)v_2}=\binom{r(sv_1)}{r(sv_2)}=r\cdot\left(s\cdot\binom{v_1}{v_2}\right)}

and tenth conditions are also straightforward.

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 1\cdot\binom{v_1}{v_2}=\binom{1v_1}{1v_2}=\binom{v_1}{v_2}}

In a similar way, each $\mathbb {R} ^{n}$ is a vector space with the usual operations of vector addition and scalar multiplication. (In Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbb{R}^1 } , we usually do not write the members as column vectors, i.e., we usually do not write "Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle (\pi) } ". Instead we just write "$\pi$".)