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PhyslibAlpha.ProbabilisticTheory.Algebra.Alternative

Alternative algebras

i. Overview

An algebra is alternative when the associator `(a, b, c) = (a b) c - a (b c)` is alternating. The octonions are alternative, and they coordinatize the exceptional Albert Jordan algebra. Alternative algebras satisfy the flexible law and the left Moufang identity.

ii. Key results

  • `IsAlternative` : alternative algebras.
  • `associator` : the associator.
  • `IsAlternative.mul_flexible` : the flexible law.
  • `IsAlternative.moufang_left` : the left Moufang identity.

15 declarations

instance

Semigroups are Alternative Algebras

Every semigroup AA is an alternative algebra. That is, if the multiplication in AA is associative, then the associator (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz) satisfies the alternating property.

definition

Associator (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz)

For elements x,y,zx, y, z in an algebra AA, the associator is defined as the ternary operation (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz), which measures the failure of the associative law.

theorem

(x+y,z,w)=(x,z,w)+(y,z,w)(x + y, z, w) = (x, z, w) + (y, z, w)

In an alternative algebra AA, for any elements x,y,z,w∈Ax, y, z, w \in A, the associator satisfies the left-additive identity (x+y,z,w)=(x,z,w)+(y,z,w)(x + y, z, w) = (x, z, w) + (y, z, w), where the associator is defined as (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz).

theorem

Additivity of the Associator in the Middle Argument

In an algebra AA, the associator of three elements is defined as (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz). For any elements x,y,z,w∈Ax, y, z, w \in A, the associator is additive in its middle argument: (x,y+z,w)=(x,y,w)+(x,z,w) (x, y + z, w) = (x, y, w) + (x, z, w)

theorem

The associator is additive in its third argument

For any elements x,y,zx, y, z, and ww in an algebra AA, the associator (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz) is additive in its third argument, satisfying the identity (x,y,z+w)=(x,y,z)+(x,y,w)(x, y, z + w) = (x, y, z) + (x, y, w).

theorem

Teichmüller's Identity for the Associator

For any elements x,y,z,wx, y, z, w in an algebra AA, let the associator be defined as (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz). Then the following identity holds: (xy,z,w)−(x,yz,w)+(x,y,zw)=(x,y,z)w+x(y,z,w)(xy, z, w) - (x, yz, w) + (x, y, zw) = (x, y, z)w + x(y, z, w) This is known as Teichmüller's identity and holds in any non-associative algebra.

theorem

The left alternative identity (x,x,y)=0(x, x, y) = 0

In an alternative algebra AA, for any elements x,y∈Ax, y \in A, the associator (x,x,y)(x, x, y) is equal to 00.

theorem

The associator identity (y,x,x)=0(y, x, x) = 0 in alternative algebras

In an alternative algebra AA, for any elements x,y∈Ax, y \in A, the associator of y,x,xy, x, x is zero, which is expressed as (y,x,x)=0(y, x, x) = 0. Using the definition of the associator, this implies the right alternative law (yx)x=y(xx)(yx)x = y(xx).

theorem

[x,y,z]=−[y,x,z][x, y, z] = -[y, x, z] in Alternative Algebras

In an alternative algebra AA, for any elements x,y,z∈Ax, y, z \in A, the associator [x,y,z]=(xy)z−x(yz)[x, y, z] = (xy)z - x(yz) is skew-symmetric with respect to the first two arguments, satisfying [x,y,z]=−[y,x,z][x, y, z] = -[y, x, z].

theorem

(x,y,z)=−(x,z,y)(x, y, z) = -(x, z, y)

In an alternative algebra AA, for any elements x,y,z∈Ax, y, z \in A, the associator (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz) satisfies the identity (x,y,z)=−(x,z,y)(x, y, z) = -(x, z, y).

theorem

(x,y,z)=(y,z,x)(x, y, z) = (y, z, x) for the Associator

In an alternative algebra AA, the associator (x,y,z)=(xy)z−x(yz)(x, y, z) = (xy)z - x(yz) is cyclically invariant. That is, for any elements x,y,z∈Ax, y, z \in A, it holds that (x,y,z)=(y,z,x)(x, y, z) = (y, z, x).

theorem

The associator (x,y,x)=0(x, y, x) = 0

In an alternative algebra AA, for any elements x,y∈Ax, y \in A, the associator (x,y,x)(x, y, x), defined as (xy)x−x(yx)(xy)x - x(yx), is equal to 00.

theorem

Flexible Law: x(yx)=(xy)xx(yx) = (xy)x

In an alternative algebra AA, for any two elements x,y∈Ax, y \in A, the flexible law x(yx)=(xy)xx(yx) = (xy)x holds.

theorem

Left Moufang identity (x(zx))y=x(z(xy))(x(zx))y = x(z(xy))

In an alternative algebra AA, for any elements x,y,z∈Ax, y, z \in A, the left Moufang identity holds: (x(zx))y=x(z(xy))(x(zx))y = x(z(xy)) where the associator (a,b,c)=(ab)c−a(bc)(a, b, c) = (ab)c - a(bc) is alternating.

theorem

McCrimmon's Left Bumping Formula: (x,y,zx)=x(y,z,x)(x, y, zx) = x(y, z, x)

In an alternative algebra AA, for any x,y,z∈Ax, y, z \in A, the following identity holds: (x,y,zx)=x(y,z,x)(x, y, zx) = x(y, z, x) where (a,b,c)=(ab)c−a(bc)(a, b, c) = (ab)c - a(bc) denotes the associator. This result is known as McCrimmon's left bumping formula, a key alternative-algebra identity used in the study of Albert matrices.