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Associativity and coefficient calculus for the finite superspace Grassmann algebra

Statement

For the basis-indexed vector space and product eSeT=0 when S∩T≠∅ and eSeT=(−1)ν(S,T)eS∪T otherwise, with ν(S,T)=∣{(s,t)∈S×T:s>t}∣, multiplication is associative, unital and supercommutative. The graded tensor product with a supercommutative coefficient algebra is also associative and supercommutative. The basis formulas for left and right odd differentiation extend to derivations with the stated Leibniz rules. The ordered four-variable Berezin integral is a well-defined linear functional extracting the coefficient of θ1θ2θˉ1˙θˉ2˙, and the stated conjugation is an involutive anti-automorphism.

Statement references

Martin, §4.1.

Gates et al., §3.7.

Four-dimensional N=1 superspace Grassmann algebra and Berezin integration: Four-dimensional N=1 superspace Grassmann algebra and Berezin integration.

Facts & Assumptions

Given: The finite basis eS, multiplication sign ν(S,T), coefficient superalgebra, derivative actions and ordered measures in the Statement.

[F1]

Four-dimensional N=1 superspace Grassmann algebra and Berezin integration: the basis is indexed by all subsets of {1,2,3,4}, eSeT=0 on overlap and otherwise equals (−1)ν(S,T)eS∪T; coefficient multiplication is the graded tensor product, and the left/right derivative actions on homogeneous coefficient-left tensors are (−1)∣a∣a⊗∂LeS and a⊗∂ReS, respectively.

[F2]

Four-dimensional N=1 superspace Grassmann algebra and Berezin integration: for i∈S, ∂ξiLeS=(−1)∣{s∈S:s<i}∣eS∖{i} and ∂ξiReS=(−1)∣{s∈S:s>i}∣eS∖{i}, and both vanish when i∉S.

[F3]

Four-dimensional N=1 superspace Grassmann algebra and Berezin integration: the one-variable integral extracts the coefficient with that variable ordered first; the four-variable measure is ∫dθˉ2˙dθˉ1˙dθ2dθ1.

[F4]

Four-dimensional N=1 superspace Grassmann algebra and Berezin integration: conjugation reverses product order and exchanges θα with θˉα˙.

Proof

Given: The finite basis and signed product in [F1], and the specified derivative, integral and conjugation operations in [F2]–[F4].

1.1F1algebra

The element e∅ is a two-sided identity because ν(∅,S)=ν(S,∅)=0. For S,T,U⊆{1,2,3,4}, if any two sets overlap, both parenthesizations (eSeT)eU and eS(eTeU) vanish: if the first pair overlaps one side vanishes immediately and the other retains that common generator, and the same argument applies to the other pairs. If S,T,U are pairwise disjoint, the left sign exponent is ν(S,T)+ν(S∪T,U)=ν(S,T)+ν(S,U)+ν(T,U), while the right exponent is ν(T,U)+ν(S,T∪U)=ν(T,U)+ν(S,T)+ν(S,U). The signs agree, proving associativity on basis elements and hence by trilinearity on all elements.

2.1F1step 1.1algebra

If S∩T≠∅, both products vanish. If the sets are disjoint, every ordered pair in S×T has exactly one of s>t or t>s, so ν(S,T)+ν(T,S)=∣S∣∣T∣. Thus eSeT=(−1)∣S∣∣T∣eTeS. This proves supercommutativity.

2.2F1F2step 1.1algebra

Put a=∣{s∈S:s<i}∣, c=∣{s∈S:s>i}∣, b=∣{t∈T:t<i}∣, and d=∣{t∈T:t>i}∣. If S and T are disjoint and i∈S, then ν(S,T)=ν(S∖{i},T)+b; the left-hand derivative exponent is a+b+ν(S,T), equal modulo two to the right-hand side exponent a+ν(S∖{i},T). If i∈T, then ν(S,T)=ν(S,T∖{i})+c; the left-hand exponent a+b+ν(S,T) equals the right-hand exponent ∣S∣+b+ν(S,T∖{i}), since i∉S and ∣S∣=a+c. For the right derivative, if i∈S the identity ν(S,T)=ν(S∖{i},T)+b makes the left exponent c+d+ν(S,T) equal to the right exponent ∣T∣+c+ν(S∖{i},T); if i∈T, the identity ν(S,T)=ν(S,T∖{i})+c makes both exponents d+ν(S,T∖{i}) modulo two. If S∩T contains a common element other than i, all Leibniz terms vanish because an overlap remains. If S∩T={i}, both terms can be nonzero, but their exponents differ by one: for the left rule they are a+ν(S∖{i},T) and ∣S∣+b+ν(S,T∖{i}); for the right rule they are ∣T∣+c+ν(S∖{i},T) and d+ν(S,T∖{i}). Thus they cancel in both rules, matching the derivative of eSeT=0. For homogeneous coefficient-left tensors a⊗eS and b⊗eT, the left derivative of their product has the common sign (−1)∣S∣∣b∣+∣a∣+∣b∣ multiplying the exterior left Leibniz expansion; the first tensor Leibniz term has sign (−1)∣a∣+(∣S∣−1)∣b∣, which is equal, and the second has the additional (−1)∣S∣ required by the exterior rule. For the right derivative, the first tensor Leibniz term has sign (−1)∣b∣+∣T∣+(∣S∣−1)∣b∣=(−1)∣S∣∣b∣+∣T∣, while the second carries (−1)∣S∣∣b∣; these are exactly the two exterior right-rule terms. Thus the stated coefficient-left derivative extensions satisfy the graded-tensor left and right Leibniz identities.

3.1F1step 1.1step 2.1algebra

For homogeneous tensors a⊗eS, b⊗eT and c⊗eU, the sign in ((a⊗eS)(b⊗eT))(c⊗eU) is ∣S∣∣b∣+(∣S∣+∣T∣)∣c∣, while the sign in (a⊗eS)((b⊗eT)(c⊗eU)) is ∣T∣∣c∣+∣S∣(∣b∣+∣c∣). These exponents agree modulo two; associativity of A and step 1.1 then gives associativity of the graded tensor product. Combining its sign ∣S∣∣b∣ with supercommutativity of both factors gives the total sign (−1)(∣a∣+∣S∣)(∣b∣+∣T∣) when the tensors are exchanged, so the tensor product is supercommutative.

3.2F1F3step 1.1step 2.1

The vectors eS are a specified basis, so each coefficient polynomial has a unique finite expansion. For a fixed i, grouping terms with and without i and moving ξi to the left by the signed product rule gives a unique expression f=f0+ξif1 with f0,f1 independent of ξi; hence the linear coefficient extraction in [F3] is well-defined. Applying the four ordered coefficient maps successively to eS returns 1 exactly when S={1,2,3,4} and 0 otherwise. By linearity, the full measure extracts precisely the stated top coefficient.

4.1F1F4step 1.1step 2.1∎

Reversing a word in the four generators and exchanging each undotted generator with its dotted conjugate preserves the square-zero and anticommutation relations. The coefficient operation preserves parity and reverses products by [F4]. For homogeneous a∈A and eS, moving eS∗ back to the right of a∗ contributes (−1)∣a∣∣S∣, so the induced tensor formula is (a⊗eS)∗=(−1)∣a∣∣S∣a∗⊗eS∗, as stated in [F1]. This formula respects the graded-tensor relation: applying it to (1⊗eS)(a⊗1)=(−1)∣a∣∣S∣(a⊗eS) gives the same identity after reversing the factors. Applying the operation twice restores every coefficient and every generator, so it descends to an involutive anti-automorphism.

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