Couldn't connect the link between the RLS sampling formula and code implemented of `compute_tau`

[emonhossainraihan]

I give a try to read the arXiv paper of Calandriello et al. 2017 but failed to understand the link between the actual paper formula in section 3, Sequential RLS Sampling 👇

and the code implementation of compute_tau:

def compute_tau(centers_dict: CentersDictionary,
                X: np.ndarray,
                similarity_func: callable,
                lam_new: float,
                force_cpu=False):
                .
                .
                .
    diag_norm = np.asarray(similarity_func.diag(X))
    # (m x n) kernel matrix between samples in dictionary and dataset X
    K_DU = xp.asarray(similarity_func(centers_dict.X, X))
    # The estimator proposed in Calandriello et al. 2017 is
    # diag(XX' - XX'S(SX'XS + lam*I)^(-1)SXX')/lam
    # Here for efficiency we collect an S inside the inverse and compute
    # diag(XX' - XX'(X'X + lam*S^(-2))^(-1)XX')/lam
    # note that in the second term, we take care of dropping the rows/columns of X associated
    # with 0 entries in S
    U_DD, S_DD, _ = np.linalg.svd(xp.asnumpy(similarity_func(centers_dict.X, centers_dict.X)
                                             + lam_new * np.diag(centers_dict.probs)))
    U_DD, S_root_inv_DD = __stable_invert_root(U_DD, S_DD)
    E = xp.asarray(S_root_inv_DD * U_DD.T)
    # compute (X'X + lam*S^(-2))^(-1/2)XX'
    X_precond = E.dot(K_DU)
    # the diagonal entries of XX'(X'X + lam*S^(-2))^(-1)XX' are just the squared
    # ell-2 norm of the columns of (X'X + lam*S^(-2))^(-1/2)XX'
    tau = (diag_norm - xp.asnumpy(xp.square(X_precond, out=X_precond).sum(axis=0))) / lam_new 👈

Like,

• Is X'X reflect the kernel matrix $\mathbf{K}_t=\boldsymbol{\Phi}_t^{\top}\boldsymbol{\Phi}_t$? And what about $XX'$?
• I couldn't understand what is X_precond here and why svd decomposition needed.
• In section 3 there was the definition of a dictionary, "we redefine a dictionary as a collection $\mathcal{I}={(i,\widetilde{p_i},q_i)}$, where $i$ is the index of the point $x_i$ stored in the dictionary, $\widetilde{p_i}$ tracks the probability used to sample it, and $q_i$ is the number of copies (multiplicity) of i." - here I couldn't understand the $q_i$
• Overall, I feel, I didn't understand the EXPAND and SHRINK for Algorithm 1 intuitively. It will be a great help if you comment something on this.

It would be greatly appreciated if you could assist me in resolving this matter.