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Take $X$ a set and $B_X$ the quotient of $\mathbb Z[X]$ by the ideal generated by all elements $u^3-u$ with $u\in \mathbb Z[X]$, which came up in this answer to this question.

Can you describe $B_X$ explicitly for small values of $|X|$?

2 Answers 2

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Just checking exhaustively:

When |X|=0, B=Z/6Z of size 6.

When |X|=1, B=Z[x]/(6,xxx-x,3xx+3x)={a+bx+c(xx+x):a,b in Z/6Z, c in Z/3Z} of size 108.

When |X|=2, B=Z[x,y]/(6,xxx-x,yyy-y,3xx+3x,3yy+3y) = {a+bx+cy+dxy+e(xx+x)+f(xx+x)y+g(xx+x)(yy+y)+hx(yy+y)+i(yy+y) : a,b,c,d in Z/6Z, e,f,g,h,i in Z/3Z} of size 314928.

Presumably the pattern holds, so that if |X|=n, the size is 6^(2^n)*3^(3^n-2^n) with similar additive invariants, but I didn't check n=3 due to the size. At any rate, its size is a divisor.

For larger exponents (varying the "3"), the characteristic of the ring is related to the denominators of the Bernoulli numbers as in this OEIS entry.


Since 3 + 4 = 1, 3*3 = 3, 4*4 = 4, 3*4 = 0, 3*x = x*3, and 4*x = x*4 in any ring B satisfying the law uuu=u, one has that 3 and 4 are orthogonal central idempotents and B decomposes as (3B) × (4B). 3B has characteristic 2 and 4B has characteristic 3, so it suffices to consider algebras over the fields Z/2Z and Z/3Z (satisfying the law uuu=u) and take their direct product (X finite or not, B free or not).

In characteristic 2, (u+1)^3 - (u+1) = u^2-u, so the algebra 3B is always a Boolean algebra. For B free, 3B is a free Boolean algebra since the relevant homomorphisms all have 4B in their kernel.

In characteristic 3, (u+v)^3 - (u+v) = u^3-u + v^3-v, so it suffices to check the law on a basis. Since the law is clearly multiplicative, it then suffices to check it on an algebra generating set, or more importantly, to impose it there. This gives the simpler description of 4B = Z[x,y]/(3,xxx-x,yyy-y), etc.

This gives a reasonably explicit description of the free B as a direct product of a free boolean algebra and the "obvious" characteristic 3 algebra.

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    @TheSubstitute: A "free" X with gener$a$tors Y is an X so that for any other X with generators Y' there is a homomorphism of Xs taking Y to Y'. The free ring on no generators is the integers, the free ring on one generator is the ring of integer polynomials in a single varia$b$le, the free ring on two generators is the ring of polynomials in two non-commuting indeterminates with integer coefficients, etc. This answer does not discuss free rings. It discusses free "rings in which the cube of every element is itself".2014-05-19
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The arguments I gave in the other question show the following:

  • Every prime ideal is a maximal ideal, and the Jacobson radical is zero.
  • The residue field of every prime ideal is either $\mathbb{F}_2$ or $\mathbb{F}_3$.

It follows that $B_X$ has characteristic $6$. A morphism $B_X \to \mathbb{F}_2$ is freely determined by its values on $X$, so the subspace of $\text{Spec } B_X$ with residue fields $\mathbb{F}_2$ is homeomorphic to $\{ 0, 1 \}^X$. Similarly the subspace of $\text{Spec } B_X$ with residue fields $\mathbb{F}_3$ is homeomorphic to $\{ 0, 1, -1 \}^X$.

So $B_X$ embeds into $A \times B$ where $A$ is the ring of continuous functions $\{ 0, 1 \}^X \to \mathbb{F}_2$ and $B$ is the ring of continuous functions $\{ 0, 1, -1 \}^X \to \mathbb{F}_3$. It is precisely the subring of this ring generated by the images of $X$, where $x \in X$ corresponds to the product of the evaluation $e_x : \{ 0, 1 \}^X \to \mathbb{F}_2$ and the evaluation e_x': \{ 0, 1, -1 \}^X \to \mathbb{F}_2. So I guess it is a certain fiber product of $A$ and $B$.