We make the same assumptions as in previous section. Let i1,⋯,ik be k positive even integers. For any p∈zero(K) and 1≤j≤k, set
λij(p)=λij1+⋯+λijl.By following theorem, we reduce the computation of characteristic numbers of TM to quantities on zero(K).
Theorem 1. If i1+⋯+ik=l, then
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=(2π√−1)l∑p∈zero(K)2kλi1(p)⋯λik(p)λ(p),where RTM is the curvature of the Levi-Civita connection ∇TM. And, if i1+⋯+ik<l, then
∑p∈zero(K)2kλi1(p)⋯λik(p)λ(p)=0.
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=(2π√−1)l∑p∈zero(K)2kλi1(p)⋯λik(p)λ(p),where RTM is the curvature of the Levi-Civita connection ∇TM. And, if i1+⋯+ik<l, then
∑p∈zero(K)2kλi1(p)⋯λik(p)λ(p)=0.
Proof . We define a operator
LK=∇TMK−LK|Γ(TM).Obviously, for any Y∈Γ(TM)
LKY=∇TMKY−[K,L]=(∇TMK)(Y).i.e., LK=(∇TMK)∈Γ(End(TM))=Ω0(M,End(TM)). We need to find a ω∈Ω∗(M) satisfied dKω=0 and
∫Mω=∫Mtr[(RTM)i1]⋯tr[(RTM)ik].
Of course, it is necessary that we can calculate ∫Mω.
For any integer h, if we have dKtr[(RTM+LK)h]=0. Since
[tr[(RTM+LK)i1]⋯tr[(RTM+LK)ik]][top]=tr[(RTM)i1]⋯tr[(RTM)ik],it means
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=∫Mtr[(RTM+LK)i1]⋯tr[(RTM)ik+LK].
By the Berline-Vergne localization formula, one can easily obtain
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=(2π)l∑p∈zero(K)tr[(LK(p))i1]⋯tr[(LK(p))ik]λ(p)On the local coordinate system (Up,(x1,⋯,x2l)) from the previous section,
K=l∑i=1λi(x2i∂∂x2i−1−x2i−1∂∂x2i).And, we know LK=∇TMK is a anti-symmetry operator. So on Up,
LK(p)=(0λ1−λ10⋱0λl−λl0),then (LK(p))2=−diag{λ21,λ21,⋯,λ2l,λ2l}.
Thus, for each 1≤j≤k, tr[(LK(p))ij]=2(−1)ij/2λij(p). From this result,
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=(2π√−1)l∑p∈zero(K)2kλi1(p)⋯λik(p)λ(p).
Otherwise, we have
(d+iK)tr[(RTM+LK)h]=dtr[(RTM+LK)h]+iKtr[(RTM+LK)h]=tr[∇TM,(RTM+LK)h]+tr[iK(RTM+LK)h]=tr[∇TM,(RTM+LK)h]+tr[iK,(RTM+LK)h]=tr[∇TM+iK,(RTM+LK)h];
(∇TM+iK)2=(∇)2+(iK)2+∇TM∘iK+iK∘∇TM=RTM+0+∇TM∘iK+iK∘∇TM=RTM+0+∇TM∘iK+iK(∇TM)−∇TM∘iK=RTM+∇TMK=RTM+LK+LK;
i.e.,RTM+LK=(∇TM+iK)2−LK. By Bianchi identity, one can obtain
[∇TM+iK,RTM+LK]=[∇TM+iK,(∇TM+iK)2−LK]=−[∇TM+iK,LK].
Since both ∇TM and K are S1-invariant, one has
[∇TM,LK]=0,[iK,LK]=0.Hence [∇TM+iK,RTM+LK]=0. It shows
dKtr[(RTM+LK)h]=tr[∇TM+iK,RTM+LK]=0.
LK=∇TMK−LK|Γ(TM).Obviously, for any Y∈Γ(TM)
LKY=∇TMKY−[K,L]=(∇TMK)(Y).i.e., LK=(∇TMK)∈Γ(End(TM))=Ω0(M,End(TM)). We need to find a ω∈Ω∗(M) satisfied dKω=0 and
∫Mω=∫Mtr[(RTM)i1]⋯tr[(RTM)ik].
Of course, it is necessary that we can calculate ∫Mω.
For any integer h, if we have dKtr[(RTM+LK)h]=0. Since
[tr[(RTM+LK)i1]⋯tr[(RTM+LK)ik]][top]=tr[(RTM)i1]⋯tr[(RTM)ik],it means
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=∫Mtr[(RTM+LK)i1]⋯tr[(RTM)ik+LK].
By the Berline-Vergne localization formula, one can easily obtain
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=(2π)l∑p∈zero(K)tr[(LK(p))i1]⋯tr[(LK(p))ik]λ(p)On the local coordinate system (Up,(x1,⋯,x2l)) from the previous section,
K=l∑i=1λi(x2i∂∂x2i−1−x2i−1∂∂x2i).And, we know LK=∇TMK is a anti-symmetry operator. So on Up,
LK(p)=(0λ1−λ10⋱0λl−λl0),then (LK(p))2=−diag{λ21,λ21,⋯,λ2l,λ2l}.
Thus, for each 1≤j≤k, tr[(LK(p))ij]=2(−1)ij/2λij(p). From this result,
∫Mtr[(RTM)i1]⋯tr[(RTM)ik]=(2π√−1)l∑p∈zero(K)2kλi1(p)⋯λik(p)λ(p).
Otherwise, we have
(d+iK)tr[(RTM+LK)h]=dtr[(RTM+LK)h]+iKtr[(RTM+LK)h]=tr[∇TM,(RTM+LK)h]+tr[iK(RTM+LK)h]=tr[∇TM,(RTM+LK)h]+tr[iK,(RTM+LK)h]=tr[∇TM+iK,(RTM+LK)h];
(∇TM+iK)2=(∇)2+(iK)2+∇TM∘iK+iK∘∇TM=RTM+0+∇TM∘iK+iK∘∇TM=RTM+0+∇TM∘iK+iK(∇TM)−∇TM∘iK=RTM+∇TMK=RTM+LK+LK;
i.e.,RTM+LK=(∇TM+iK)2−LK. By Bianchi identity, one can obtain
[∇TM+iK,RTM+LK]=[∇TM+iK,(∇TM+iK)2−LK]=−[∇TM+iK,LK].
Since both ∇TM and K are S1-invariant, one has
[∇TM,LK]=0,[iK,LK]=0.Hence [∇TM+iK,RTM+LK]=0. It shows
dKtr[(RTM+LK)h]=tr[∇TM+iK,RTM+LK]=0.
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