The Euclidean inner product $⟨x,y⟩={\sum }_{k<n}{x}_k{y}_k$ on ${\mathbb{R}}^n$

Let $n\in \mathbb{N}$. A natural number is a von Neumann natural, that is a set, and $n=\{0,1,\dots ,n-1\}$ (The natural numbers $\mathbb{N}$ (von Neumann), On $\mathbb{N}$ the order is membership: $m<n⟺m\in n$), so

${\mathbb{R}}^n=\{x:x\text{ is a function }n\to \mathbb{R}\}$

is the function space of The vector space $F^X$ of all functions $X\to F$ with pointwise operations, and $F^n$ as the case $X=n=\{0,1,\dots ,n-1\}$ at $F=\mathbb{R}$ and $X=n$, a vector space over $\mathbb{R}$ under the pointwise operations (Vector space over a field). We write $x_k:=x(k)$ for $k<n$, and two elements of ${\mathbb{R}}^n$ are equal exactly when they agree at every $k<n$. This is the same set that ${\mathbb{R}}^n$ as the set of functions $n\to \mathbb{R}$, and $d_1$, $d_2$, $d_{\mathrm{\infty }}$ are metrics on it calls ${\mathbb{R}}^n$.

The Euclidean inner product of $x,y\in {\mathbb{R}}^n$ is the real number

$⟨x,y⟩:={\sum }_{k<n}{x}_k{y}_k,$

the finite sum of Finite sums and finite products, by recursion applied to the list $k\mapsto x_k{y}_k$ (extended by $0$ beyond $n$, as every finite list in this library is). The Euclidean norm of $x$ is

$\parallel x{\parallel }_2:=\sqrt{⟨x,x⟩},$

which is defined because $⟨x,x⟩={\sum }_{k<n}{x}_k^2\ge 0$ (a sum of nonnegative terms, Laws of finite sums and finite products clause 4 and Squares of nonzero elements are positive, the case $x_k=0$ giving $x_k^2=0$ by Integer powers $a^m$) and every nonnegative real has a unique nonnegative square root (Square roots exist: a unique $\sqrt{a}\ge 0$ with $(\sqrt{a}{)}^2=a$; the positives are $\{x^2:x\mathrm{\ne }0\}$).

Both are defined for every $n$, including $n=0$

At $n=0$ the set ${\mathbb{R}}^0$ has exactly one element, the empty function, and it is the zero vector space (The standard list $e:n\to F^n$ with $e_i(i)=1_F$ and $e_i(j)=0_F$ for $j\mathrm{\ne }i$ is an ordered basis of $F^n$; hence ${\dim }_F{F}^n=n$, and $F^0$ is the zero space with basis $\mathrm{\varnothing }$ and dimension $0$ clause 5); the sum above is the empty sum, so $⟨x,y⟩=0$ and $\parallel x{\parallel }_2=0$. This is the first place on this page where the two index regimes diverge, and the divergence is deliberate. The published metrics $d_1$, $d_2$, $d_{\mathrm{\infty }}$ of ${\mathbb{R}}^n$ as the set of functions $n\to \mathbb{R}$, and $d_1$, $d_2$, $d_{\mathrm{\infty }}$ are metrics on it are defined only for $n\ge 1$, because $d_{\mathrm{\infty }}$ would otherwise be a maximum over the empty index set; the algebra above needs no such restriction. The boundary in this page runs between the algebra and the metric, not where a reader would guess, and Conventions of this page, the standing $n\ge 1$ hypothesis, and what is taken up elsewhere in the reading order lists exactly which items inherit $n\ge 1$.

The algebra of the inner product

For all $x,y,z\in {\mathbb{R}}^n$ and $\lambda \in \mathbb{R}$:

1. Symmetry. $⟨x,y⟩=⟨y,x⟩$, since $x_k{y}_k=y_k{x}_k$ termwise.
2. Additivity in the first argument. $⟨x+y,z⟩=⟨x,z⟩+⟨y,z⟩$: the list $k\mapsto (x_k+y_k)z_k$ is the termwise sum of $k\mapsto x_k{z}_k$ and $k\mapsto y_k{z}_k$, so Laws of finite sums and finite products clause 1 applies.
3. Homogeneity in the first argument. $⟨\lambda x,y⟩=\lambda ⟨x,y⟩$, by Laws of finite sums and finite products clause 2.
4. Bilinearity. Clauses 2 and 3 together with symmetry give the same two laws in the second argument.
5. Positive definiteness. $⟨x,x⟩\ge 0$, and $⟨x,x⟩=0$ if and only if $x=0$. Indeed a vanishing sum of nonnegative terms has every term $0$ (Laws of finite sums and finite products clause 4), so $x_k^2=0$ for every $k<n$, and a nonzero real has a positive square (Squares of nonzero elements are positive), whence $x_k=0$ for every $k<n$ and $x=0$.
6. Agreement with the published Euclidean metric. For $n\ge 1$ and $x,y\in {\mathbb{R}}^n$, $\parallel x-y{\parallel }_2=\sqrt{{\sum }_{k<n}(x_k-y_k{)}^2}=d_2(x,y)$, the two sides being the same expression (${\mathbb{R}}^n$ as the set of functions $n\to \mathbb{R}$, and $d_1$, $d_2$, $d_{\mathrm{\infty }}$ are metrics on it). In particular $\parallel x{\parallel }_2=d_2(x,0)$.

That $\parallel \cdot {\parallel }_2$ is a norm in the sense of A norm on a real vector space, the induced metric, and the dictionary with the metric axioms is proved in Cauchy-Schwarz $\mid ⟨x,y⟩\mid \le \parallel x{\parallel }_2\parallel y{\parallel }_2$ with its equality case, the triangle inequality for $\parallel \cdot {\parallel }_2$, the parallelogram law and polarisation, where the triangle inequality is obtained from the Cauchy-Schwarz inequality; it is not assumed here.

• Scope: the concrete form only. What is defined above is the Euclidean inner product on ${\mathbb{R}}^n$ and nothing more. The general theory of inner product spaces — abstract inner products, orthonormal bases, Gram-Schmidt, orthogonal projection and orthogonal complements of arbitrary subspaces — is planned for a page of this library that comes earlier in the plan order and is not yet built. No item on this page claims anything about abstract inner product spaces, and no item on this page introduces the general notion.

• The standard basis and coordinates. For $i<n$ the standard unit vector $e_i\in {\mathbb{R}}^n$ has $e_i(i)=1$ and $e_i(j)=0$ for $j\mathrm{\ne }i$ (The standard list $e:n\to F^n$ with $e_i(i)=1_F$ and $e_i(j)=0_F$ for $j\mathrm{\ne }i$ is an ordered basis of $F^n$; hence ${\dim }_F{F}^n=n$, and $F^0$ is the zero space with basis $\mathrm{\varnothing }$ and dimension $0$). Then $⟨x,e_i⟩=x_i$: the list $k\mapsto x_k(e_i{)}_k$ vanishes except at $k=i$, where its value is $x_i$, and a list vanishing off one index sums to its value there (Laws of finite sums and finite products clause 3, splitting the range at $i$). So the coordinates of $x$ are recovered by testing against the standard basis, which is the form used repeatedly below.

• Powers here are integer powers. $x_k^2$ means the integer power of Integer powers $a^m$, and $\parallel x{\parallel }_2^2=⟨x,x⟩$ by Square roots exist: a unique $\sqrt{a}\ge 0$ with $(\sqrt{a}{)}^2=a$; the positives are $\{x^2:x\mathrm{\ne }0\}$ and Laws of integer exponents.