# Modern research into Grassman's "theory of forms"?

I quote from Petsche's Hermann Graßmann: Biography (emphasis mine):

The mathematical part of the book begins with the conception of the “General Theory of Forms”. Starting with a perspective on mathematics as a theory of forms, Graßmann analyses in the most abstract way possible the general structures of concrete “conjunctions of forms”. Here, he places special emphasis on “elementary conjunctions”, demanding they have module properties, that is, associativity, commutativity and an inverse and neutral element. The so-defined conjunction of the first order, or “formal addition”, is then followed up by an investigation of a conjunction of the second order (“formal multiplication”), for which he only requires distributivity with respect to formal addition. Graßmann directly posits the validity of the module properties for formal addition and distributivity for formal multiplication as the principles for constructing these conjunctions: “This generally is the way”, he wrote, “that initially, that is when no species of conjunction is yet given, such a conjunction of next higher order is defined.”

Since Graßmann does not require the forms generated by conjunctions of the second order to be embedded in the fundamental domain, he can use this form of conjunction for the formal generation of new mathematical objects in the further course of the text.

...

After Graßmann has laid down the foundation for all mathematical disciplines by presenting these uniquely generalized group-theoretical and structural abstractions he starts with the actual presentation of his new mathematical discipline.

What is the modern terminology for Grassmann's "General Theory of Forms"? What research work has been done in order to continue this line of thinking? Which resources could I acquaint myself with in order to answer these questions?

I think the answer is simply (and very generally, thus unhelpfully) "universal algebra": http://en.wikipedia.org/wiki/Algebraic_structure

Am I on the right track? I am not sure. See also this question of mine, which is looking for something similar in spirit.

Can someone with familiarity weave together a proper answer from these three resources and others as appropriate?

• Group actions? en.wikipedia.org/wiki/Group_action Feb 7, 2014 at 21:21
• @Martín-BlasPérezPinilla I think it's more general than that, see: "by presenting these uniquely generalized group-theoretical and structural abstractions he..." Feb 7, 2014 at 22:32
• I'm not clear what you're looking for, here. Grassman is one of the founding figures of abstract algebra, but after over a hundred years of development his influence is diffuse. Feb 12, 2014 at 13:49
• Exterior forms, differential forms and geometric algebra (when composed with Clifford algebra) all seem relevant. Jun 5, 2014 at 22:45
• This MO thread may be of interest too. Jun 5, 2014 at 23:00

Universal Algebra: Broadly speaking, Whitehead in his Treatise on Universal Algebra was motivated by the work of Boole, Hamilton and Grassmann, and eventually Maltsev and Polish mathematicians developed the general theory. However, Grassmann himself was mostly focused on binary associative operations.

Hypercomplex numbers: From abstract algebra point of view Grassmann studied some finitely generated associative algebras over the field of real numbers. In 19th century this area came to be known as linear associative algebra, and was further developed by Peirce and Wedderburn among others. The modern name is hypercomplex numbers and associativity is no longer required, examples are dual and split-complex numbers, quaternions, octonions, etc. Although purely algebraic approach is more in the spirit of Hamilton than Grassmann some hypercomplex numbers are closely related to non-Euclidean geometries and appear in mechanics, relativity and quantum theory. New interesting hypercomplex numbers were introduced by Musès in 1980s, although most of his work is more in the realm of numerology than mathematics.

Multilinear Algebra: Grassmann's own work was more specific than universal algebra and more intuitive than abstract algebra, in modern terms he was interested in algebras generated by a vector space, like exterior or Clifford algebras, with emphasis on geometric interpretations. And it's not just the scope but also the viewpoint, he was interested in geometric content of the operations (wedge product, Clifford product, etc.) more than their algebraic properties. Formally, the closest modern analog would be multilinear algebra if not for the prevailing tensor approach that reduces everything to index manipulations and assigns geometric interpretations after the fact if at all.

Geometric Algebra: Gian-Carlo Rota is the most prominent recent proponent of Grassmann's original point of view, he and his students introduced new geometric operations in his spirit, and developed their theory. Rota's motivation came from the classical invariant theory and he injected some combinatorial flavor into Grassmann's constructions. "Geometric algebra" is a term occasionally used to describe this circle of ideas, but most authors treat it as a synonym of Clifford algebra, which is just one example albeit universal in some sense (not to be confused with geometric algebra of ancient greeks, which is something else). And Rota's approach is an exception rather than a rule in multilinear algebra.

Differential geometry: Where the spirit of Grassman is preserved more it's differential geometry. After Elie Cartan's introduction of differential forms there was a tendency to abolish tensor algebra's "debauchery of indices" in favor of coordinate invariant notation because there is no global choice of coordinates on manifolds. Differential forms are Grassman's exterior forms that vary over a manifold, i.e. one generates exterior algebras over tangent spaces and takes smooth cross sections across the manifold. Of course, other Grassman type algebras can and are used instead. The advantage is that Grassman's definitions of operations are intrinsic, and automatically extend to bundles once some underlying structure is fixed (e.g. Riemannian metric for Clifford algebras). They can then be used to define differential operators that are explicitly invariant under transformations that preserve the underlying structure (e.g. defining codifferential using Hodge star). Physicists working with gauge theories on manifolds sometimes favor this approach, but even in differential geometry tensors dominate because they can be manipulated with much less geometric insight.

Sources: Yaglom’s book has an excellent historical exposition of Grassman’s ideas and their relation to Hamilton’s quaternions and hypercomplex numbers in general. This is a more recent historical survey of the subsequent work of Clifford, Lipschitz, Study and Cartan with emphasis on applications to physics. Wikipidea article on hypercomplex numbers gives a good overview with many historical and modern references, for Musèan hypernumbers see here. Rota’s programmatic paper reviving Grassman’s ideas and applying them to the classical invariant theory is quite readable, its short and elementary review is here. Applications to spinors and representation theory can be found in this book. Warner’s text systematically applies invariant approach in differential geometry.

• David Hestenes is also another major proponent of the Geometric Algebra angle. What you wrote in your answer is what I am familiar with so far, but I think this passage actually refers to Grassman's general viewpoints on mathematics and how to create structure in mathematics. This is what I would someone to address, not the geometric algebra angle, which is already well known to me. Jun 6, 2014 at 3:21
• "After Graßmann has laid down the foundation for all mathematical disciplines by presenting these uniquely generalized group-theoretical and structural abstractions he starts with the actual presentation of his new mathematical discipline." Jun 6, 2014 at 3:23
• My interpretation of the passage you quoted is that conjunction of the first order creates a vector space or module, and conjunction of the second order is a formal product, i.e. tensor product. He can then generate the tensor algebra of new mathematical objects, and carve out more specialized structures by imposing additional relations. They admit new conjunctions like progressive and regressive products. Higher order conjunctions may refer to iterating second order ones. Insisting on commutativity, associativity and distributivity significantly narrows the scope though. Jun 6, 2014 at 6:10
• Not sure if it's of interest to you, but you get additional structure by having a group representation on the underlying module, it induces representations on the tensor algebra and its descendants, and you can ask for invariants and covariants that transform nicely, classify tensors, etc. This is the context of the classical invariant theory, but it goes beyond Grassman jones.math.unibas.ch/~kraft/Papers/KP-Primer.pdf. Jun 6, 2014 at 6:43
• Grassmann tries to be general, but imposes algebraic conditions that lead him to modules without any geometry. If you are looking for constructions for "all of mathematics", as your other question suggests, you will not find them in Grassmann. Whitehead later suggested studying sets with general operations (using Boolean and Grassmann algebras as motivation), which eventually led to universal algebra of Maltsev and the Polish school. Even that is not truly "universal" though since structures on sets do not have to be operations. But is this more what you are looking for? Jun 6, 2014 at 20:30

I think this passage is referring to the exterior algebra of a vector space. It's designed to have properties suitable for Grassmann's approach, and indeed can be viewed as a universal arrow in a certain category, but it does not really have anything to do with universal algebra, per se.

Grassmann's work later was applied in multilinear algebra and seeded the study of differential forms. Other than this, Grassmann's ideas were also built upon by Clifford. This eventually lead to the study of Clifford algebras and so-called "geometric algebras," which are special cases of Clifford algebras. This direction has a slightly different flavor than that of differential forms, but that's not to say they don't overlap and work together.

A great thing to read to get a better feel for the role of differential forms and Grassmann's and Clifford's legacies is Roger Penrose's Road to reality chapters 6 and 11.

• I would have to disagree with you. The passage that I quote specifically refers to a more general insight of Grassman's, which he uses to then build his geometric algebra upon. Jun 6, 2014 at 3:22
• "After Graßmann has laid down the foundation for all mathematical disciplines by presenting these uniquely generalized group-theoretical and structural abstractions he starts with the actual presentation of his new mathematical discipline." Jun 6, 2014 at 3:24
• Dear @user89 : honestly, this quote is a bit weird. I've never heard of Grassmann's work providing a foundation for "all mathematical disciplines," and I doubt that is literally true. Considering it sounds like a sensationalized passage in a biography, might I recommend you don't take it as gospel? Regards Jun 6, 2014 at 10:11
• @user89 the entire paragraph after your first highlight appears to be talking about exterior algebra. Jun 6, 2014 at 10:13
• @user89 OK, but that is my reading of After Graßmann has laid down the foundation for all mathematical disciplines Jun 6, 2014 at 16:41

Another resource is John Browne's book Grassmann Algebra Volume 1: Foundations.

There is also a fairly extensive free Mathematica Application available to reasonably serious individuals described at Grassmann Calculus.