+ [mathcal{D}_2]_inftyleft(
+mathbb{E}left[mathrm{Tr}left(mathcal{tilde{D}}_2right)right],endaligned$$ where $mathbb Eleft[Xright]$ is the expectation of $X$ over the random variables $X$. The first term in the right hand side of Eq. ([eq:delta2]) is the leading term in Eq.([eq1]), and the second term is the next-to-leading term. The second term in this equation is the sum of the second and third terms. The third term is a constant that does not depend on $X$, and the fourth term is an additional constant that depends on $N$. This constant is the second-order correction to the leading order. The constant in the fourth line of Eqs. (1) and (2) is the same as the one in Eqs.(ref{eq:D2}), (ref {eq:tilde D2}). The constant is equal to $1/2$ if the number of sites in the system is even, and to $0$ if it is odd.

The leading term of the density of states of the system can be calculated by using the fact that the density operator $mathcal D_2$ is diagonal in the basis of the eigenstates of the Hamiltonian. The density of the states of a system with $N$ sites is equal $$label{eq1}
n(omega)=sum_{i=1}^{N}delta(omeg_i-omega).$$ The density operator is diagonalized by the transformation $$mathbf{S}_i=left{
&frac{omega_i}{sqrt{N}},&quad i=1,dots,N,\
&0,&qquad ineq 1,
label {eq2}
\ &1,& qquad i=N+1,N+2,ldots,2N.
right.$$ The transformed density operator can be written as $$begin {split}
&mathsf{D_2}(omegas)=mathop{sum_{omegas_1, dots omegas_{2N}}}_{mathclap{substack{omegas’_1delta_{{1,2}},dagger_{{omeg 1,omeg 2}}}}}\
&{timesmathfrak{S}}_{left(omegs_1+omegs’_2, omega_{rm max}+domegaright)}
mathscr{D_{2}}(omegap)=frac 1{2sq{N}}sum_idgd_i(omegp_i)^2,
end{split}$$ where the subscripts of the transformed density operators are the indices of the sites. The transformed densities are calculated by the following equations: $$dgp_1=dgs_2-frac 12dgb_1^2dga_1;quad dgp_{2i}=frac dgb_{2,i}-left|fracdgc_{2,i}right|^2-2frac {dgd_{2}^2}{dgas_{1}^*}dgb^*_1.$$
The density matrix of the reduced density operator of the whole system is equal $
rho_N=mathsfs{D^dag_2}mathsf D_1
$ and the density matrix is diagonal.
The density of a single site is equal: $$n_1(0)=dtg_2^2+frac 14dgg_2+2left.frac {dmathds{1}}{domegp}rightvert_{dgap=0}dgp.$$ This is the density in the case of the even number of site.