σµ+,µ−(A) = where in the last step we made use of estimate (B.3.9) andclis defined in the statement of the proposition. It follows from this relation and formula (B.3.15) thatR−lσµ+,µ−R−lis a bounded functional. Making use of relation (B.3.15) and exploiting the following formula, valid for any n∈N, we prove estimate (B.3.4) from the statement of the proposition. In order to verify rela-tion (B.3.5), suppose thatb+i =b−i fori∈Nand{b+i }∞1 form an orthogonal system. Then Substituting this bound to formula (B.3.15) and making use again of identity (B.3.17), we conclude the proof of the proposition.
B.4 Expansions in Single-Particle Space
In this section we derive two expansions of the functions F± ∈D(Or)R which enter into definition (B.2.6) of the local algebra. The first expansion relies on the fact that ˜F± are analytic functions and thus can be expanded into convergent Taylor series. The coefficients of these expansions are given in terms of vectors {˜b±κj,r}∞1 defined by (B.4.1) below. We show in Lemma B.4.3, stated below, that these vectors have finite Sobolev norms. Thus the corresponding functionalsσµ+,µ−, constructed in Proposition B.3.1, satisfy the energy
bounds (B.3.4). Due to the rapid growth with |µ+|+|µ−|, this estimate does not suffice to establish the convergence of the sum (B.0.2) in the massless case. In order to exploit the more tame estimate (B.3.5), in the second part of this section we construct a suitable orthonormal basis expansion.
χEF˜± and note that the functions ˜F± are analytic. From their Taylor expansions we obtain [Bos00] where in the second step we numbered the s-indices κ with some index j ∈N in such a way that κ1 = 0. There holds the following proposition.
Proposition B.4.1. [Bos00] Expansion (B.4.3) converges in L2(Rs, dsp).
This statement follows from estimate (B.4.17) below which relies on the bounds on the norms of functions (B.4.1) and (B.4.2), established in Lemmas B.4.3 and B.4.4, respec-tively. Actually, we derive here more general estimates on the Sobolev norms of these functions, (see definition (B.3.2)), which appear in Proposition B.3.1 above. First, we note the following auxiliary fact.
Lemma B.4.2. Let χ ∈ C0∞(Rs). Then, for any s-index κ, i ∈ {1, . . . , s} and n ∈ N,
there holds Z
dsx|∂xni(xκχ(~x))|2 ≤(cn)|κ|+1 (B.4.4) for some constant cn, independent of κ.
Proof. We note the following identity which follows from the Leibniz rule
∂xni(xκχ)(~x) =xˆκ where thes-index ˆκis obtained fromκby settingκ(i) = 0. There easily follows the bound
Z for some constant cn, independent ofκ.
Now we are ready to prove the required bounds on the Sobolev norms of b±κ.
B.4. Expansions in Single-Particle Space 85
Lemma B.4.3. For any l≥0 the functions b±κ,r, defined by (B.4.1), satisfy the bound kb±κ,rk2,l ≤ (cl,r)|κ|+1
κ! , (B.4.7)
where the constant cl,r, is independent of κ.
Proof. First, we note that kb+κ,rk2,l ≤(1 +m)1/4kb−κ,rk2,l+1
2, so it suffices to consider the (−) case. Clearly l → kb−κ,rk2,l is a monotonically increasing function, so it is enough to establish the bound forl=n+12,n∈N. We consider the expression
kω−12xκ^χ(Or)k22,n+1
2
= xκ^χ(Or)
ω(~p)−1(1 +|~p|2)n+12xκ^χ(Or)
≤cn
kω−12xκ^χ(Or)k22+ xκ^χ(Or)( Xs
i=1
|pi|2n)xκ^χ(Or)
, (B.4.8)
where on the r.h.s. above we mean the scalar product in L2(Rs, dsp) and the constantcn depends only on n and s. In order to study the first term on the r.h.s. above, we write ω−12 = (ω−12)++ (ω−12)−, where (ω−12)±(~p) =ω(~p)−12θ(±(|~p| −1)). There clearly holds
k(ω−12)+xκ^χ(Or)k2 ≤ kxκχ(Or)k2 ≤ 1
2(cr)|κ|+1, (B.4.9) k(ω−12)−xκ^χ(Or)k2 ≤ kxκ^χ(Or)k∞k(ω−12)−k2 ≤ 1
2(cr)|κ|+1, (B.4.10) for some constant cr ≥0, independent of κ, and therefore
kω−12xκ^χ(Or)k2 ≤(cr)|κ|+1. (B.4.11) Next, we study the second term on the r.h.s. of (B.4.8). Making use of Lemma B.4.2, we obtain
xκ^χ(Or)
|pi|2nxκ^χ(Or)
= Z
dsx|∂xni(xκχ(Or))(~x)|2 ≤(cn,r)|κ|+1, (B.4.12) where the constantcn,r does not depend onκ. This concludes the proof of the lemma.
While the above result holds for anym≥0, in the next statement, concerning the Sobolev norms of the functions ˜h±κ,E, we have to make a distinction between the massive and the massless case.
Lemma B.4.4. In massive scalar free field theory for s≥ 1 the functions ˜h±κ,E, defined by relation (B.4.2), satisfy, for any λ≥0, β ∈R,
kω−β˜h±κ,Ek2,λ ≤(cλ,β,E)|κ|+1, (B.4.13) where the constants cλ,β,E are independent of κ. In massless scalar free field theory for s≥3 the bound (B.4.13) holds (in particular) in the following two cases:
(a) For any β <1 and λ= 0.
(b) For β = ∓12 and any λ ≥ 0. (The ± signs are correlated with these appearing in formula (B.4.13)).
Proof. We defineχ±E(~p) :=ω(~p)−β∓12χE(~p). Then
(ω−β˜h±κ,E)(~p) = (−1)|κ|(ip)κχ±E(~p). (B.4.14) We first consider the case m= 0. Then the functions χ±E are square-integrable forβ <1 and there holds for some constant cE, independent ofκ,
kω−β˜h±κ,Ek2 ≤c|Eκ|kχ±Ek2, (B.4.15) what proves part (a) and part (b) for λ= 0. For m >0 inequality (B.4.15) holds for any β ∈R.
In the massive case and the case considered in part (b), ~p → χ±E(~p) are smooth, compactly supported functions. It suffices to take into account λ∈N, since the functions λ → kω−βh˜±κ,Ek2,λ are monotonically increasing. By H¨older’s inequality applied to the term (1 +|~x|2)λ, identity (B.4.14), Lemma B.4.2 and relation (B.4.15) there holds the following bound
kω−β˜h±κ,Ek22,λ ≤cλ
kω−β˜h±κ,Ek22+ Xs
i=1
Z
dsp|∂pλi pκχ±E(~p)
|2
≤(cλ,β,E)|κ|+1,(B.4.16) where the constant cλ,β,E is independent of κ.
From Lemmas B.4.3 and B.4.4 we obtain the bound for any combination of ±-signs, any 0≤β ≤1 and 0< p≤1
X
κ∈Ns
0
kb±κ,rkp2kω−β˜h±κ,Ekp2≤ X
κ∈Ns
0
(c0,rc0,β,E)p(|κ|+1)
(κ!)p <∞. (B.4.17) Thus we have proven Proposition B.4.1.
B.4.2 Orthonormal Basis Expansion
Let QE be the projection on the single-particle space onto states of energy lower than E. Let hr ∈ D(Or)R be s.t. ˜hr > 0. We introduce the closed, linear subspaces L±r = [ω±12D(e Or)] in L2(Rs, dsp) and denote the respective projections by the same symbols.
We choose 12 ≤ γ < s−12 and define operators TE,± = ω−12QEL±r, Th,± = ω−γh˜1/2r L±r, where ˜hr is the corresponding multiplication operator in momentum space. We recall that the p-norm of an operator A is given, for any p > 0, by kAkp = k|A|pk1/p1 , where k · k1
denotes the trace norm. By a slight modification of Lemma 3.5 from [BP90], one obtains the following result:
Lemma B.4.5. For anyp >0 the operators TE,± and Th,± arep-nuclear i.e. there holds
kTE,±kp < ∞, (B.4.18)
kTh,±kp < ∞. (B.4.19)
B.4. Expansions in Single-Particle Space 87 Proof. In order to show that the operators Th,± are p-nuclear for any p >0, it suffices to demonstrate that their adjoints Th,∗± are products of an arbitrary number of Hilbert-Schmidt operators. (The Hilbert-Hilbert-Schmidt property is preserved under the adjoint opera-tion due to the cyclicity of the trace). We set, as in [BP90], for i∈N
hi = ω(1 +ω2)(i−1)sχ(Or)(1 +ω2)−isω−1 (B.4.20) ki = (1 +ω2)(i−1)sχ(Or)(1 +ω2)−is. (B.4.21) These operators, and also ω−12h1, ω−21k1, are in the Hilbert-Schmidt class [BP90]. For any n∈Nthere hold the identities
Th,+∗ = L+rω−12h1. . . hnω−γ+12(1 +ω2)ns˜h1/2, (B.4.22) Th,∗− = L−rω−12k1. . . knω−γ+12(1 +ω2)ns˜h1/2, (B.4.23) so it suffices to check thathnω−γ+12 andknω−γ+12 are Schmidt. Since the Hilbert-Schmidt norm k · kHS of an operator is equal to the L2-norm of its integral kernel, we obtain
khnω−γ+12k2HS= (2π)−s Z
dsp dsq ω(~p)2(1 +ω(~p)2)2(n−1)s|χ(^Or)(~p−~q)|2
· 1
(1 +ω(~q)2)2ns 1
ω(~q)2γ+1. (B.4.24) Making use of the following two identities (see e.g. formula (7.2.109) of [Bos00] for the proof of the first statement)
1 +ω(~p)2
1 +ω(~q)2 ≤(|~p−~q|+ 1)2, (B.4.25) ω(~p)2 ≤2(ω(~p−~q)2+ω(~q)2), (B.4.26) we arrive at the bound
khnω−γ+12k2HS
≤ 2(2π)−s Z
dsp dsq 1 (1 +ω(~q)2)2s
1
ω(~q)2γ−1(|~p|+ 1)4(n−1)s|χ(^Or)(~p)|2 + 2(2π)−s
Z
dsp dsq 1 (1 +ω(~q)2)2s
1
ω(~q)2γ+1ω(~p)2(|~p|+ 1)4(n−1)s|χ(^Or)(~p)|2.(B.4.27) These integrals are clearly convergent for 0≤γ < s−12 . The (simpler) case ofknω−γ+12 is treated analogously. Finally, the p-nuclearity of the operators TE,± follows from the fact that QE˜h−r1 is a bounded operator and there holds TE,±= (QE˜h−r1)Th,± forγ = 12. We define the operator T as follows
T = (|TE,+|2+|TE,−|2+|Th,+|2+|Th,−|2)12. (B.4.28) Making use of the fact [Ko84] that for any 0 < p≤ 1 and any pair of positive operators A, B s.t. Ap,Bp are trace-class, there holds k(A+B)pk1 ≤ kApk1+kBpk1, we get
kTkpp≤ kTE,+kpp+kTE,−kpp+kTh,+kpp+kTh,−kpp. (B.4.29)
Since T commutes with the operator J of complex conjugation in configuration space, it has aJ-invariant orthonormal basis of eigenvectors{ej}∞1 and we denote the corresponding eigenvalues by {tj}∞1 . As we will see in the next section, the expansion
QEf±= X∞ j=1
hej|f±iQEL±rej, (B.4.30)
valid for any f±∈L˚±r, has the required convergence properties.