In the paper GM18, they say that the sampling algorithm, SampleG, is shown in Figure 2. It takes as input a modulus $q$, an integer variance $s$, a coset $u$ of $\Lambda^{\perp}(g^T )$, and outputs a sample statistically close to $D_{\Lambda^{\perp}_u(g^T),s}$. SampleG relies on subroutines Perturb and SampleD where Perturb($\sigma$) returns a perturbation, $p$, statistically close to $D_{L(\Sigma_2),\sigma \cdot \sqrt{\Sigma_2}}$ , and SampleD($σ, c$) returns a sample $z$ such that $Dz$ is statistically close to $D_{L(D),−c,σ}$.
I can't really understand why SampleG outputs a sample statistically close to $D_{\Lambda^{\perp}_u(g^T),s}$, where the distribution is spherical(I think it's covariance should be $\sqrt{\Sigma_G}$, not $I$).
Also, I can't understand the meaning of $\beta$ in subroutin Perturb($\sigma$).
In addition, the perturbation $p$ statistically close to $D_{L(\Sigma_2),\sigma \cdot \sqrt{\Sigma_2}}$, why the lattice is $L(\Sigma_2)$, in other words, why the lattice basis is $\Sigma_2$.
I understand the framework of the SampleG algorithm in high level, but I am confused in pseudocode of the algorithm, anyone can help or discuss with me? Thanks a lot.
