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Any Clifford algebra $\operatorname{Cl}(k, p)$ carries an induced inner product, which is the "trace" on its 0-blade: $\langle AB\rangle_0$ for given elements $A, B$ of the algebra.

This inner product, when restricted to the generating vector space $V$, gives back the inner product on $V$.

Now, my question (maybe entirely trivial, but I could not find it in the standard literature, for instance in Lounesto's great book Clifford algebras and spinors):

What is the interplay of the Clifford product and its induced inner product? Are there any formal laws?

PS. I have done some little googling, and I came up with some references on associative algebras with inner product, but unfortunately there seems to be a certain latitude as far as their definition.

PPS In light of the comments below, I realize that the phrasing of my post is not as clear as it should. Please refer to the comments below and my additional replies for the proper context.

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    Would have said the symmetrised "Clifford product" is this "induced inner product" up to a factor of 2 but perhaps I'm misunderstanding one or both of the definitions – AlexArvanitakis Jun 14 '20 at 02:41
  • @AlexArvanitakis they are not the same, though they are related: you refer to (ab+ ba) /2 which is the symmetric side of the clifford product. I was referring to _0, which is taking the o-dimensional component of the product. On vectors these two are the same, but not in general – Mirco A. Mannucci Jun 14 '20 at 12:53
  • I am afraid your question is not clear (at least the way it is phrased). Do you mean that you are looking for some relation between the $0$-component of the product $A\cdot B$ (for arbitrary elements $A$, $B$) and the bilinear form asssociated to the quadratic form involved in the definition of the Clifford algebra ? – Konstantinos Kanakoglou Jun 14 '20 at 16:12
  • @KonstantinosKanakoglou I agree: my question is not as crystal clear as it should, and I apologize. Let me see if I can make it a bit clearer: you start from a vector space with an inner product (V, Q), you build the clifford algebra. Now, the clifford agebra is an associative algebra PLUS the inner product given by the 0-blade (which of course restrict to the starting Q for V). – Mirco A. Mannucci Jun 14 '20 at 16:50
  • Now, forget completely about V, and just look at what you have: you have a unital associative algebra AND a inner product on it. Now, comes the question: forget entirely about V, Q and just look at what you have: you have an associative unital algebra, with an inner product. Now, can I describe how these two products are related in the algebra from an entirely algebraic standpoint? – Mirco A. Mannucci Jun 14 '20 at 16:52
  • Why I am asking: I want to clarify the structure of the clifford algebra per se, regardless of the underlying vector space. Does it make a bit more sense? – Mirco A. Mannucci Jun 14 '20 at 16:54
  • If this is the case (and if i understand correctly), isn't $xy+yx=B(x,y)1$ the relation you are looking for ? – Konstantinos Kanakoglou Jun 14 '20 at 17:15
  • @KonstantinosKanakoglou this is certainly true if x and y are vectors. But, as I have said, I wish to forget about V. Not sure it holds for arbitrary x and y in the algebra (see above what I said to Alex) – Mirco A. Mannucci Jun 14 '20 at 17:38
  • Recalling the construction of the Clifford algebra $C$, if we start from a vector space $V$ then the Clifford algebra is a suitable quotient of the tensor algebra $T(V)$. The canonical projection $\pi : T(V)\to C$, enables one to view $\pi (V)$ as a subspace of $C$, and this subspace generates $C$ (as an algebra). So, in this sense (if by vectors you mean the elements of $\pi(V)$), yes you are right. But their multiplication is in $C$. – Konstantinos Kanakoglou Jun 14 '20 at 18:15
  • So, you have a relation between the multiplication of generators of $C$ and the bilinear form associated to the quadratic form (via which $C$ is defined). What else do you need here? – Konstantinos Kanakoglou Jun 14 '20 at 18:17
  • I see that I am still unable to make myself clear (my fault). What you say is true, but play one second the following game: take your favorite Clifford (mine is Cl(3, 0) and FORGET completely HOW it has been generated. There is no grading, no V, nothing, only the algebra, with its structure as unital associative – Mirco A. Mannucci Jun 14 '20 at 18:22
  • algebra. Now, it so happens that the 0-blade trace of the product endows the algebra with a inner product (so the entire algebra is a vector space with an inner product). Now I ask: ok, what is the interplay of the two products? Are they completely separated (remember, no V)? I suspect not. I suspect there are some structural relations between them..... – Mirco A. Mannucci Jun 14 '20 at 18:24
  • PS Notice that I have not contradicted myself: the 0 blade exists independently, simply because it is a unital algebra (1 is there as a subspace of the algebra). – Mirco A. Mannucci Jun 14 '20 at 18:25
  • Mirco, maybe it is due to my insufficient knowledge, but i do not really understand the expression: "the 0-blade trace of the product endows the algebra with a inner product". What is the 0-blade on the first place ? – Konstantinos Kanakoglou Jun 14 '20 at 18:28
  • take two elements of the algebra, say V and W. Multiply them: V W. Now expand the result in any basis you like: VW = a * 1 + ....................... a is the 0-blade. Fact: a is a inner product (you can amuse yourself with some calculation, for instance try it on Cl(0, 1), aka the complex numbers seen as a clifford . The result will perhaps surprise you....) – Mirco A. Mannucci Jun 14 '20 at 18:40
  • Have you tried to figure out what is the signature of the bilinear from on $Cl(p,q)$ ? – InfiniteLooper Jun 14 '20 at 18:45
  • oh yes. That is easy: write down the standard basis 1 e1...... e1e2......... etc. Each squared is either + 1 or -1 . That determines uniquely the signature of the form on Cl(p, q). For instance, going back to Cl(0, 1). in that case the signature is + -, which when restricted to the generating one-dim space is simply -. It is kind of funny: C from this angle is 2-dimensional space-time + -....... – Mirco A. Mannucci Jun 14 '20 at 19:14
  • if you do not like that basis approach there is some combinatorial formula which tells you the signature of Cl(p, q). But that is perhaps beside the point. – Mirco A. Mannucci Jun 14 '20 at 19:16
  • This trace on the 0-blade is presumably the honest matrix trace in the usual faithful matrix representation? In that case you can find very explicit formulae for all values of the trace, in the physics/quantum field theory literature. – AlexArvanitakis Jun 16 '20 at 00:39
  • When I used the word "trace" I put a quote around it. But now, on a second thought, you may have a point. 1 is certainly represented as the diagonal of ones in any matrix representation, so it should be easy to verify what happens when two elements A and B get multiplied: what is the coefficient of 1? Indeed their inner product ... – Mirco A. Mannucci Jun 16 '20 at 19:40
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    @MircoA.Mannucci: Right at the bottom of the Wikipedia entry on the Clifford scalar product: is this what you wanted? The fact that Clifford transposition is the adjoint under the scalar product? – Alex M. Jun 22 '22 at 05:26

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$ \newcommand\lcontr{\,\lrcorner\,} \newcommand\rcontr{\,\llcorner\,} \newcommand\lcontrr{{\rfloor}} \newcommand\rcontrr{{\lfloor}} \newcommand\form[1]{\langle#1\rangle} \newcommand\Ext\bigwedge \newcommand\rev\widetilde $

I hope that this is in the vein of what you were looking for.

In a review (Marcel Riesz’s Work on Clifford Algebras, 1993) of Marcel Riesz's lecture notes (Clifford Numbers and Spinors, 1958) on Clifford algebras, Pertti Lounesto demonstrates a natural linear isomorphism between $\Ext(V)$ and $Cl(V)$.

Given a symmetric bilinear form $\form{\cdot,\cdot}$ on a vector space $V$ over a field $K$ with characteristic $\not=2$, this extends to a bilinear form on $\Ext V$. On $k$-blades (i.e. simple $k$-vectors), we define $$ \form{x_1\wedge\cdots\wedge x_k, y_1\wedge\cdots\wedge y_k} = \det\form{x_i, y_j}, $$ which is to say that we take the determinant over the matrix with entries $a_{ij} = \form{x_i, y_j}$. For blades $A, B \in \Ext V$ with different grades we define $\form{A, B} = 0$, and then extend by linearity to the entirety of $\Ext V$. Though maybe not obvious when described like this, this extension of the form on $V$ is natural, see here and here.

With this form on $\Ext V$, the adjoints of the exterior product $$ \form{X \wedge Y, Z} = \form{Y, X \lcontr Z},\quad \form{X \wedge Y, Z} = \form{X, Z \rcontr Y} $$ (where $X, Y, Z \in \Ext V$ are arbitrary multivectors) are found to be the left ($\lcontr$) and right ($\rcontr$) contractions of the Clifford algebra defined by $\form{\cdot,\cdot}$ on $V$. They are indispensable when studying Clifford algebras for applications. There are various different conventions that can be used when defining them; see the appendix of this preprint (Compendium on Multivector Contractions, 2022) by André Mandolesi. It's worth noting that the alternative contractions $$ X\lcontrr Y = \rev X\lcontr Y,\quad X\rcontrr Y = X\rcontr\rev Y, $$ where $\rev X$ is the reverse of $X$, are particularly popular; see Lounesto or this article (The Inner Products of Geometric Algebra, 2002) by Leo Dorst for an exposition on their basic properties.

We may note that $a \lcontr b = a \rcontr b = \form{a, b}$ for $a, b \in V$. Now define the products $$ aX = a\lcontr X + a\wedge X,\quad Xa = X\rcontr a + X\wedge a, $$ which can be seen to be exactly the Clifford algebra products of $a$ and $X$. By this definition, this product is self-adjoint: $\form{aX, Y} = \form{X, aY}$ with an analogous formula for $a$ on the right. But any blade $A$ can be written as $A = a_1a_2\cdots a_k$ for some $k$ and $a_1,\dotsc, a_k \in V$ using Clifford products (which are associative); hence $$ \form{AX, Y} = \form{a_1a_2\cdots a_kX, Y} = \form{a_2a_3\cdots a_kX, a_1Y} = \form{a_3a_4\cdots a_kX, a_2a_1Y} = \form{X, \rev AY}. $$ By linearity, the same holds for when $A$ is an arbitrary multivector. Thus, $$ \form{X, Y} = \form{X\cdot 1, Y} = \form{1, \rev XY} = \form{\rev XY}_0. $$


It's worth noting that the contractions have a very direct connection with the Clifford product. For any $X, Y \in \Ext V$, $$ X\lcontr Y = \sum_{j=0}^n\sum_{k=0}^n\form{\form{\rev X}_j\form{Y}_k}_{k-j},\quad X\rcontr Y = \sum_{j=0}^n\sum_{k=0}^n\form{\form{X}_j\form{\rev Y}_k}_{j-k}, $$ where $\form{\cdot}_l = 0$ if $l < 0$. For $a \in V$ and $X \in \Ext V$ in particular, $$ a\lcontr X = \frac12(aX - \hat Xa),\quad X\rcontr a = \frac12(Xa - a\hat X), $$ where $\hat X$ is the grade involution (i.e. main involution) applied to $X$.

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