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Positive Clifford mass differences give the actual KO classes

Statement

Assume AC. Fix q≥0 positive symmetric orthogonal Clifford generators e1,…,eq on a finite-dimensional real inner-product space. A mass is a symmetric orthogonal involution anticommuting with these generators. A mass difference consists of masses H,H0 on fixed representations W,W0, together with a specified isometric identification w:W0→W intertwining the fixed generators. Use direct sum, mass homotopy, simultaneous orthogonal change of coordinates, and addition or deletion of a common reference extension as the stable difference relations. Denote the resulting component difference group by Dq.

There is an explicit natural additive isomorphism

Ψq:Dq⟶KO−q(pt).

At q=0 it is the negative-eigenspace rank difference

Ψ0(H,H0,w)=dim⁡WH−−dim⁡(W0)H0−.

In positive degrees it is the negative of the hemisphere-clutching class constructed below. Thus a determinant reflection at q=1 gives the reduced Möbius class; an orientation-component change at q=2 gives the real complex-Hopf class; and a positive negative-rank difference at q=4 gives the chosen quaternionic Hopf generator. Under the explicit eightfold matrix stabilization T, with w stabilized along with the masses,

Ψq+8(Tδ)=Ψq(δ)⊠β,

where β is exactly the volume-graded C8 symbol supplying real Bott periodicity. In particular the negative-rank generator (−1,+1) at q=0 stabilizes to a class mapping to +β. This statement concerns the finite stable Clifford mass problem and has no disorder or interaction conclusion.

Facts & Assumptions

Given: AC, the finite gapped stable-equivalence convention, and mass differences as stated. Transport H0 using the specified w before comparing operators. Fix source-minus-target for relative triples and the volume-graded negative-square Cn=Cl0,n convention for the ABS symbol.

[F1]

Real Clifford matrix-algebra classification, including eightfold matrix periodicity and the simple-module multiplicity spaces, is supplied by Real and complex Clifford algebra classification with explicit signature. The finite stable model is Finite-dimensional gapped free Hamiltonians and stable equivalence.

[F2]

Relative triples, stable isomorphism, homotopy, relative products and their coordinate signs are supplied by Finite-CW relative KO triples and homotopy invariance and Suspension, exactness, and products in relative KO, using Reduced and relative real K-theory and compact support.

[F3]

Clutching identifies reduced sphere KO with the stable orthogonal homotopy groups; stable π0(O)=Z/2 and π1(O)=Z/2 are supplied by Real and symplectic clutching groups in low degrees. Quaternion conjugation is the double cover of SO(3) by S3 (The quaternion double cover generates the third homotopy group of SO(3)), and covering paths and homotopies lift uniquely (Existence and uniqueness of path lifts through a covering map, Existence and uniqueness of homotopy lifts through a covering map).

[F4]

The actual even-to-odd ABS symbol is multiplicative under graded tensor product, not merely abstractly isomorphic to a coefficient group (The actual ABS symbol identifies graded Clifford cokernels with KO). Its volume-graded irreducible C8 symbol is the same β used by Real Bott periodicity from the rank-eight Clifford–Thom map. The rank-four spinor symbol is the quaternionic Hopf generator by Real Spin Thom isomorphisms with real and quaternionic sources.

[F5]

Independently proved real coefficients in residues 0,…,7 are Z,Z/2,Z/2,0,Z,0,0,0, and product with β is an isomorphism (The real KO coefficient table and eightfold periodicity, Real Bott periodicity from the rank-eight Clifford–Thom map).

Proof technique: direct construction.

Proof

technique · direct construction
1.1givenalgebraconstruct

Transport H0 by w so that both masses act on the same fixed representation. Define first an auxiliary comparison Φq with the positive-rank convention, and finally set Ψq=−Φq in EVERY degree. For q=0 put Φ0=dim⁡WH+−dim⁡WH0+. Equal total dimensions give −Φ0=dim⁡WH−−dim⁡WH0−. The uniform negation is part of the comparison convention; it will also negate integer-valued Hopf classes in degrees divisible by four.

1.2givenF2F3construct

For q=1 split W=W+⊕W− by e1. Write the masses as H=(0g∗g0) and H0=(0g0∗g00), where g,g0:W+→W− are isometries. Clutch on S1 using the two values I,g0−1g on S0 and subtract the common rank. This defines Φ1. The original reference identification remains present in g0−1g; replacing it by an abstract isomorphism class of one extended module would discard this datum.

1.3F2F3algebraconstruct

For q≥2, use e2 to identify W− with W+, making e2=(0II0). The remaining fixed generators and masses have blocks ei+2=(0−JiJi0) and H=(0−AA0), with an analogous A0. Direct multiplication gives Ji∗=−Ji, Ji2=−I, pairwise anticommutation, A∗=−A, A2=−I, and AJi+JiA=0, with the same relations for A0. For x=(x0,x1,…,xq−2,t)∈Sq−1 define F(x)=x0I+∑ixiJi+tA on t≥0 and F(x)=x0I+∑ixiJi−tA0 on t≤0. The formulas coincide at t=0 and satisfy F∗F=I on each hemisphere by the anticommutation identities. They therefore define a based orthogonal clutching map. Let Φq be its reduced clutching class on Sq.

1.4F1F4algebraconstruct

Choose positive orthogonal generators a1,…,a8 on the sixteen-dimensional irreducible Cl8,0 module S, and put Γ8=a1⋯a8. Then Γ82=I, Γ8∗=Γ8 and Γ8aj=−ajΓ8. The graded negative C8 module B with grading Γ8 and generators bj=Γ8aj has ordered volume b1⋯b8=Γ8. It is therefore the volume-graded irreducible symbol module of [F4], with s8(B)=β. Define T by old fixed generators Ei=ei⊗Γ8, eight new fixed generators Fj=1⊗aj, masses H′=H⊗Γ8, H0′=H0⊗Γ8, and reference w′=w⊗1. All Clifford and mass identities hold by direct multiplication.

2.1F2F3step 1.1step 1.2step 1.3construct

If A=A0, the clutching map factors through the closed upper hemisphere by t↦∣t∣. That disk contracts to the chosen equatorial basepoint while fixing the point, so its clutching map is based nullhomotopic. Common reference blocks contribute zero. Mass homotopies give homotopies of clutching maps; simultaneous coordinate changes conjugate the clutching map and give isomorphic bundles; direct sums give block sums and hence sums of reduced classes. These conclusions hold also in step 1.1 and step 1.2. Reversing the two masses reflects the clutching sphere and negates its class by the reflection/block-rotation proof in [F2]. Thus Φq, and consequently Ψq, respect stable difference relations and inverses.

2.2F1step 1.1step 1.2step 1.3algebraconstruct

We compute the actual component generators rather than infer them from the KO table. At q=0, real involutions are classified up to orthogonal conjugacy by their eigenspace dimensions; their fixed-rank orbits are real Grassmannians. At q=1 the mass parameter is g∈O(n) relative to g0. At q=2 it is an orthogonal complex structure A, with orbit O(2n)/U(n): an adapted basis consists of pairs (v,Av), and its stabilizer is precisely the complex-linear isometry group. The latter is connected by complex plane rotations, whereas O(2n) has its two determinant components. At q=4, the fixed J1,J2 give a quaternionic multiplicity space. Put K=J1J2; then A=KS with S=−KA a quaternionic-linear symmetric involution. Its eigenspace orbit is Sp(k+m)/(Sp(k)×Sp(m)). Hence the nonzero stable difference groups in these rows are the real rank difference, the determinant parity, the complex-orientation parity, and the quaternionic rank difference. Their elementary differences are respectively one opposite real line, one reflected real isometry, one reversed complex line, and one opposite quaternionic line.

2.3F4step 1.1step 1.2step 1.3algebraconstruct

To identify the exact symbol, first take an OPPOSITE mass pair (H,−H) on a summand. For q≥2, A0=−A, so its transition is the globally linear map h(x)=x0I+∑ixiJi+tA. On W put grading e1 and negative-square Clifford generators c1=e1e2,…,cq−1=e1eq,cq=e1H. Each squares to −I, is skew-adjoint and odd for this grading, and the generators anticommute. Identify the odd space with the even space by −e2. The even-to-odd blocks then become I,J1,…,Jq−2,A, respectively. Thus the actual ABS symbol of this graded Cq module M is EXACTLY h, and Φq(H,−H)=sq(M) whereas Ψq(H,−H)=−sq(M). At q=1 use grading e1 and generator e1H; the two opposite boundary isometries normalize to I,−I and give the same statement for an elementary reflected line. At q=0 the auxiliary positive-rank convention corresponds to the elementary graded real line. This use of a module is confined to an actual opposite DIFFERENCE pair whose reference is retained.

3.1F1step 1.4algebraconstructstep 2.2step 2.1

This stabilization is an equivalence of fixed-representation mass problems. The Fj generate 1⊗M16(R). An operator anticommuting with every Fj, multiplied by 1⊗Γ8, commutes with that matrix algebra and hence is K⊗1; consequently the original operator is K⊗Γ8. Matrix units extract its multiplicity space, and multiplication by those units reconstructs it. This recovers the old fixed generators and masses, with their homotopies, sums and reference identifications. The classification of [F1] supplies the same decomposition for every admissible representation of the stabilized fixed algebra. Hence T:Dq→Dq+8 is an isomorphism. In particular it transports the elementary component generators from step 2.2 and common folded-null blocks from step 2.1 to all nonnegative degrees.

3.2F1step 2.2algebraconstruct

For completeness, the other four multiplicity models have connected mass spaces. At q=3, a further complex structure anticommuting the fixed one is a quaternionic structure on a complex space, with orbit U(2n)/Sp(n). At q=5, the two quaternionic isotypic halves must have equal dimension and a mass is an isometry between them, giving Sp(n). At q=6, an orthogonal quaternionic-linear complex structure has orbit Sp(n)/U(n): its complex +i eigenspace and its right-j image provide an adapted quaternionic frame. At q=7, a further anticommuting structure determines an antiunitary involution on that eigenspace, hence a real form, with orbit U(n)/O(n). These descriptions follow by diagonalizing the fixed Clifford algebra of [F1] and taking adapted frames; their stabilizers are exactly the transformations commuting with the indicated structures. Each quotient is connected because U and Sp are connected, the latter by quaternionic-frame induction over the connected sphere fibrations. Thus D3,D5,D6,D7 vanish. The parity and equal-rank conditions are imposed on admissible representations before stabilization.

3.3F3step 1.2step 1.3step 2.2construct

A reflected real line in step 1.2 gives transition values +1,−1 on S0, so transport around the clutched line returns a vector to its negative. This is the Möbius line and is nonzero stably because its determinant is negative. At q=2 take A=J=(0−110) and A0=−J on one complex line. The two hemispheres in step 1.3 combine to F(x0,t)=x0I+tJ, the one-turn rotation on S1 and the underlying real transition of the complex Hopf line. Its lift to S3→SO(3) is cos⁡(θ/2)+ksin⁡(θ/2), which runs from 1 to −1, so it is not nullhomotopic. The stable inclusion preserves its nonzero class by [F3]. Replacing the direction of the difference negates this class, which does not change a group of order two. Thus Ψ1 and Ψ2 send the actual parity generators to the stated actual bundle generators.

3.4F2F4step 2.3step 1.4algebra

For q≥1, apply step 2.3 to the stabilized opposite pair, using grading E1=e1⊗Γ8. Its negative generators, in order, are the old c1⊗1,…,cq−1⊗1, then e1⊗b1,…,e1⊗b8, then cq⊗1. Move the last coordinate across the eight new coordinates. This is an even permutation, so its orthogonal action is homotopic to identity and introduces no suspension sign. The resulting ordered module has generators ci⊗1 and ΓM⊗bj, and grading ΓM⊗Γ8, where ΓM=e1. It is literally M⊗^B. The actual symbol product in [F4] gives Φq+8(Tδ)=sq+8(M⊗^B)=Φq(δ)⊠β for every elementary opposite pair. Uniform negation gives the same identity for Ψ, since −(u⊠β)=(−u)⊠β.

3.5F1F4step 1.1step 2.1step 2.3step 1.4algebra

Check the integer base case and its sign separately. Stabilizing the POSITIVE-rank elementary difference (+1,−1) produces opposite masses Γ8,−Γ8 on S. Its module from step 2.3 has grading a1 and negative generators a1a2,…,a1a8,a1Γ8. Their ordered product is a1: multiplying eight pairs contributes (−1)28=1, and (a2⋯a8)2=−1 cancels the sign from moving a1 past seven factors. Thus it is the volume-graded irreducible C8 module. Classification identifies it with B, and the grading intertwines because it is the same volume element; polar normalization makes an algebraic intertwiner orthogonal without changing its Clifford action. Consequently its RAW Φ8 class is +β. Reversing the difference to the NEGATIVE-rank generator (−1,+1) gives raw Φ8=−β, hence final Ψ8=+β. Since Ψ0(−1,+1)=+1, this proves Ψ8(Tδ)=Ψ0(δ)⊠β with the stated native negative-rank convention. There is no volume-sign obstruction and no silent reversal of β.

4.1F2F4F5step 1.1step 2.1step 2.2step 3.2step 3.3algebra

At q=4 use J1=Li,J2=Lj,K=Lk on one quaternionic line. For S=+1,S0=−1, the transition is F(x0,x1,x2,t)=Lx0+x1i+x2j+tk, exactly the quaternionic Hopf transition. Thus Φ4 gives the positive Hopf class for a POSITIVE-rank difference. The negative-rank generator is instead S=−1,S0=+1: its Φ4 value is the negative Hopf class, and its Ψ4=−Φ4 value is the positive Hopf class. The source-minus-target/volume convention of [F4] identifies the latter with the chosen generator. Arbitrary involutions are connected to diagonal ones with the same real or quaternionic ranks; common sign blocks give the folded-null transition in step 2.1. Therefore these elementary computations prove Ψq is an isomorphism in rows 0,1,2,4. Steps 3.2 and [F5] prove it for the four zero rows.

5.1F4F5step 2.1step 4.1step 3.1step 3.4step 3.5algebrastep 2.2step 3.2∎

In each of the first eight rows, every stable component difference is a sum of the elementary opposite differences and common null blocks identified in step 2.1, step 2.2, step 3.2 and step 4.1. Step 3.1 transports this generating description through every eighth row. Additivity, homotopy invariance and step 3.4 and step 3.5 therefore prove Ψq+8T=(−Φq+8)T=β⊠(−Φq)=β⊠Ψq on every class. Since T and product by β are isomorphisms, induction from the low rows in step 4.1 proves that every Ψq is an isomorphism. All constructions retained the fixed reference identification w; abstract isomorphism of the modules attached to two separate masses was never used as a substitute for the difference. This proves the stated finite-model comparison with the exact negative-eigenspace and volume conventions.

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