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Termwise and total Hochschild theories have different grading outputs

Example

Assume the Axiom of Choice (used only in step 1.1, through the polynomial diagonal Koszul theorem). Let R=Q[x] with deg⁡x=2 and let F be the two-term complex of R-bimodules with F0=F1=R,dF0=0, concentrated in cohomological degrees 0 and 1. Since R is a polynomial ring in one variable, its diagonal Koszul complex has HH0(R,R)=R,HH1(R,R)=R{2},HHh(R,R)=0 (h≥2), where the generator of HH1 is the Koszul symbol of internal degree 2. The termwise Hochschild complex of The termwise Hochschild complex of a Rouquier complex and the groups HHH therefore has E1c,−h=HHh(R,Fc)=HHh(R,R) for (c,h)∈{0,1}2 and zero otherwise, all differentials HHh(R,dFc) vanish because dF=0, and the page E2=E∞ carries four labeled free R-generators (c,h,p)∈{(0,0,0), (1,0,0), (0,1,2), (1,1,2)}, each generating its displayed copy of R or R{2}: the termwise theory retains the full (c,h) bigrading inside the trigrading, with generator degree p=2h and further homogeneous classes in degrees p=2h+2d, d≥0, from multiplication by xd. The total Hochschild hyperhomology of Hochschild hyperhomology of a bounded bimodule complex has total degree n term Tn=⨁i−j=nCj(R,Fi); because dF=0 the total complex is the direct sum of the two shifted Hochschild complexes of the columns, and HHhyper,n≅{R{2},n=−1,R⊕R{2},n=0,R,n=1,0,otherwise. The abutment retains only the total degree n=c−h and the internal degree, together with the induced filtration: the four termwise labels regroup as (c,h)=(0,0),(1,1) in total degree 0, (0,1) in degree −1 and (1,0) in degree 1, In this zero-outer-differential example the column decomposition actually gives a canonical splitting R⊕R{2} in degree 0; the filtration is therefore split here. The declared hyperhomology grading records only (n,p), although this particular model also retains the column labels through its canonical direct sum. The spectral sequence of Termwise Hochschild spectral sequence of a bounded bimodule complex collapses at E1=E2 in this example, and no nonzero higher differential is asserted.

Facts & Assumptions

Given: the field Q, the ring R=Q[x] with deg⁡x=2, the complex F with F0=F1=R and zero differential, and AC.

[L1]

The one-variable diagonal Koszul bimodule complex for R is K(u;Re) with u=xL−xR, degree-one term Re{2} and degree-zero term Re; the symbol has internal degree 2 and the differential preserves internal degree (The polynomial diagonal Koszul bimodule complex).

[L2]

Under AC, HHh(R,M)≅Hh of the diagonal Koszul complex of The polynomial diagonal Koszul bimodule complex tensored with M, naturally in the k-central R-bimodule M and preserving internal degree (Polynomial Hochschild homology from the diagonal Koszul complex).

[L3]

F is a bounded complex of Q-central graded R-bimodules with differentials of internal degree zero; the maps induced on Hochschild homology by its (zero) differentials make (HHh(R,F∙),HHh(R,dF∙)) a cochain complex, whose cohomology is the iterated (termwise) Hochschild homology (Termwise Hochschild homology and iterated homology, The termwise Hochschild complex of a Rouquier complex and the groups HHH).

[L4]

The hyperhomology total complex has Tn=⨁i−j=n, j≥0Cj(R,Fi) with differential D=dF+(−1)ib on the (i,j) summand, and HHhyper,n=Hn(T∙); the Hochschild complex C∙(R,M)=M⊗QR⊗Q∙ has the alternating boundary b (Hochschild hyperhomology of a bounded bimodule complex, Hochschild chains and Hochschild homology with coefficients).

[L5]

The spectral sequence of the filtration FpTn=⨁i≥p, i−j=nCj(R,Fi) has E1i,−j=HHj(R,Fi), E2i,−j=Hi(HHj(R,F∙)), differentials dr ⁣:Eri,−j→Eri+r,−j−r+1, and abuts to the image filtration with E∞i,−j≅gr⁡iHHhyper,i−j (Termwise Hochschild spectral sequence of a bounded bimodule complex).

[L6]

AC is the choice-function principle (The Axiom of Choice), used only through [L2].

Verification

technique · direct
1.1L1L2L6givenalgebra

Compute HH∙(R,R). By [L1] the diagonal Koszul complex of R over R has terms R{2} in degree one and R in degree zero, and the differential is multiplication by u=xL−xR, which acts on the regular bimodule R as xm−mx=0. Hence the complex is 0→R{2}→0R→0 and [L2] gives HH0(R,R)=R with generator in internal degree 0, HH1(R,R)=R{2} with generator in internal degree 2 (the Koszul symbol contributes the degree), and HHh(R,R)=0 for h≥2, since the complex has no terms above degree one. AC is used only here, in [L2].

2.1L3step 1.1algebra

The termwise complex has four free R-generators. By [L3] the termwise complex in Hochschild degree h has terms HHh(R,Fc) for c=0,1; by step 1.1 these are nonzero exactly for h∈{0,1}, where they are copies of R{2h}, with generator degree 2h, and the induced differentials vanish because dF=0. The page E1 therefore has the four displayed free generators, and every higher differential dr ⁣:Erc,−h→Erc+r,−h−r+1 vanishes for bidegree reasons: for r≥2 the target column c+r≥2 is zero since F is concentrated in degrees 0,1. Hence E2=E∞=E1 and the termwise output is the trigraded object with the four labels (c,h,p) displayed in the Example.

2.2L3L4step 1.1algebra

The total hyperhomology. By [L4] the total complex has Tn=C−n(R,F0)⊕C1−n(R,F1) for n≤1 and Tn=0 for n≥2, with D=+b on the F0-summand and D=−b on the F1-summand because dF=0; hence T∙ is the direct sum of the two column complexes C∙(R,F0) and C∙(R,F1)[−1] in which the differential is ±b up to the index. Taking homology and using HHj(R,R)=0 for j≥2 from step 1.1 gives H−1=HH1(R,F0)=R{2}, H0=HH0(R,F0)⊕HH1(R,F1)=R⊕R{2}, H1=HH0(R,F1)=R, and Hn=HH−n(R,F0)⊕HH1−n(R,F1)=0 for n≤−2 and n≥2; this is the displayed computation of the Example.

3.1L5step 2.1step 2.2algebra∎

Compare the two outputs and identify the filtration. By [L5] the page E∞i,−j contributes to HHhyper,i−j as gr⁡i, so the four classes of step 2.1 regroup by the total degree n=c−h as follows: (c,h)=(0,1) gives gr⁡0 of HHhyper,−1, the pairs (0,0) and (1,1) give gr⁡0 and gr⁡1 of HHhyper,0, and (1,0) gives gr⁡1 of HHhyper,1. This matches step 2.2, where the two summands of HHhyper,0=R⊕R{2} are exactly the two filtered pieces HH0(R,F0) and HH1(R,F1): the declared hyperhomology grading uses (n,p), while in this particular example the vanishing outer differential supplies a canonical additional column decomposition. The termwise theory of step 2.1 keeps c and h separately inside the trigrading (c,h,p), while the total theory of step 2.2 is declared graded by n=c−h and p, with the indicated split filtration in this example; this is the asserted difference of grading outputs, and no nonzero higher differential occurs by step 2.1.

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