This post continues the previous one 3D crossings in graphic lambda calculus . Other relevant posts are:
In the previous post we saw that we can “construct” crossings by using both the abstraction operation and the composition operation from lambda calculus. These operations appear as elementary gates in graphic lambda calculus, along with other two gates, namely the FAN-OUT gate denoted by and the gate (with an element in a commutative group ). This last gate models the family of operations of an emergent algebra.
The FAN-OUT gate is used in two different contexts. The first one is as a properly defined FAN-OUT, with behaviour described by the global fan-out move, needed in the construction which attaches to any term in untyped lambda calculus a graph in the lambda calculus sector of the graphic lambda calculus as explained in “Local and global moves on locally planar trivalent graphs … ” step 3 in section 3. The second one is in relation with the decorated knot formalism of emergent algebras, “Computing with space, …” section 3.1″.
There is an astounding similarity between the and composition gates from lambda calculus, on one side, and FAN-OUT and gates from emergent algebras, in the context of defining crossings. I shall explain this further.
In the decorated knots formalism, crossings of oriented wires are decorated with elements of a commutative group . The relation between these crossings and their representations in terms of trivalent graphs is as following:
Comparing with the notations from the previous post, we see that in both cases the gate corresponds to a FAN-OUT, but, depending on the type of crossing, the composition operation gate corresponds to one of the gates decorated by OR .
There is a second remark to be made, namely that the crossings constructed from FAN-OUT and gates satisfy Reidemeister I and II moves but not Reidemeister III move. This is not a bad feature of the construction, in fact is the most profound feature, because it leads to the “chora” construction and introduction of composite gates which “in the infinitesimal limit”, satisfy also Reidemeister III, see section 6 from “Computing with space”.
In contradistinction, the crossings constructed from the abstraction and composition operation do satisfy the three Reidemeister moves.