An alternate algorithm for FLASH loadings updates

Introduction

If the expression for the KL divergence derived in the previous note is correct, then it seems likely that the FLASH objective can be optimized in a more direct fashion.

Notation

It will simplify matters if I parametrize the posteriors for, respectively, the \(i\)th element of the \(k\)th loading and the \(j\)th element of the \(k\)th factor as \[ q_{l_i} \sim (1 - w_i^{(l)}) \delta_0 + w_i^{(l)} N(\mu_i^{(l)}, \sigma_i^{2(l)}) \] and \[ q_{f_j} \sim (1 - w_j^{(f)}) \delta_0 + w_j^{(f)} N(\mu_j^{(f)}, \sigma_j^{2(f)}) \] I parametrize the priors as \[ g_{l_i} \sim \pi_0^{(l)} \delta_0 + (1 - \pi_0^{(l)}) N(0, 1/a_l) \] and \[ g_{f_j} \sim \pi_0^{(f)} \delta_0 + (1 - \pi_0^{(f)}) N(0, 1/a_f) \]

Objective

Using the expression for KL divergence derived in the previous note, the objective can be written: \[\begin{aligned} \sum_{i, j} \left[ \frac{1}{2} \log \frac{\tau_{ij}}{2 \pi} - \frac{\tau_{ij}}{2} \left( (R_{ij}^{-k})^2 - 2 R_{ij}^{-k} w_i^{(l)} \mu_i^{(l)} w_j^{(f)} \mu_j^{(f)} + w_i^{(l)} (\mu_i^{(l)2} + \sigma_i^{2(l)}) w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)}) \right) \right] \\ +\sum_i \left[ (1 - w_i^{(l)}) \log \frac{\pi_0^{(l)}}{1 - w_i^{(l)}} + w_i^{(l)} \log \frac{1 - \pi_0^{(l)}}{w_i^{(l)}} + \frac{w_i^{(l)}}{2} \left( \log(a_l \sigma_i^{2(l)}) + 1 \right) - \frac{w_i^{(l)} a_l}{2}(\mu_i^{(l)2} + \sigma_i^{2(l)}) \right] \\ +\sum_i \left[ (1 - w_j^{(f)}) \log \frac{\pi_0^{(f)}}{1 - w_j^{(f)}} + w_j^{(f)} \log \frac{1 - \pi_0^{(f)}}{w_j^{(f)}} + \frac{w_j^{(f)}}{2} \left( \log(a_f \sigma_j^{2(f)}) + 1 \right) - \frac{w_j^{(f)} a_f}{2}(\mu_j^{(f)2} + \sigma_j^{2(f)}) \right] \end{aligned} \]

Prior parameter updates

I derive an algorithm for loadings updates by differentiating with respect to each variable (\(a_l\), \(\pi_0^{(l)}\), \(\mu_1^{(l)}, \ldots, \mu_n^{(l)}\), \(\sigma_1^{2(l)}, \ldots, \sigma_n^{2(l)}\), and \(w_1^{(l)}, \ldots, w_n^{(l)}\)) and setting each result equal to zero.

The updates for the prior parameters \(a_l\) and \(\pi_0^{(l)}\) turn out to be very simple. First, differentiating with respect to \(a_l\) gives \[ \sum_i \left[ \frac{w_i^{(l)}}{2a_l} - \frac{w_i^{(l)}}{2}(\mu_i^{(l)2} + \sigma_i^{2(l)}) \right] \] Setting this equal to zero gives \[ a_l = \frac{\sum_i w_i^{(l)}}{\sum_i w_i^{(l)} (\mu_i^{(l)2} + \sigma_i^{2(l)})} = \frac{\sum_i w_i^{(l)}}{\sum_i E_ql_i^2} \]

Next, differentiating with respect to \(\pi_0^{(l)}\) gives \[ \sum_i \left[ \frac{1 - w_i^{(l)}}{\pi_0^{(l)}} - \frac{w_i^{(l)}}{1 - \pi_0^{(l)}} \right] \] Setting this equal to zero gives \[\begin{aligned} \pi_0^{(l)} \sum_i w_i^{(l)} &= (1 - \pi_0^{(l)}) \sum_i (1 - w_i^{(l)}) \\ \pi_0^{(l)} &= \frac{1}{n} \sum_i (1 - w_i^{(l)}) \end{aligned}\]

Posterior parameter updates

The updates for the posterior parameters \(\mu_i^{(l)}\) and \(\sigma_i^{2(l)}\) also turn out to be quite manageable. Differentiating with respect to \(\mu_i^{(l)}\) gives \[ \sum_j \tau_{ij} \left[ R_{ij}^{-k} w_i^{(l)} w_j^{(f)} \mu_j^{(f)} - w_i^{(l)} \mu_i^{(l)} w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)}) \right] - w_i^{(l)} a_l \mu_i^{(l)} \] Setting this equal to zero gives \[ \mu_i^{(l)} = \frac{\sum_j \tau_{ij} R_{ij}^{-k} w_j^{(f)} \mu_j^{(f)}} {a_l + \sum_j \tau_{ij} w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)})} = \frac{\sum_j \tau_{ij} R_{ij}^{-k} Ef_j} {a_l + \sum_j \tau_{ij} Ef_j^{2}} \]

Next, differentiating with respect to \(\sigma_i^{2(l)}\) gives \[ -\frac{1}{2} \sum_j \tau_{ij} w_i^{(l)} w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)}) + \frac{w_i^{(l)}}{2\sigma_i^{2(l)}} - \frac{w_i^{(l)} a_l}{2} \] Setting this equal to zero gives \[ \sigma_i^{2(l)} = \frac{1}{a_l + \sum_j \tau_{ij} w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)})} = \frac{1}{a_l + \sum_j \tau_{ij} Ef_j^2} \]

It remains to derive the update for \(w_i^{(l)}\). Differentiating gives \[ \begin{aligned} \sum_j \left[ \tau_{ij} R_{ij}^{-k} \mu_i^{(l)} w_j^{(f)} \mu_j^{(f)} - \frac{\tau_{ij}}{2}(\mu_i^{(l)2} + \sigma_i^{2(l)}) w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)}) \right] \\ - \log \frac{\pi_0^{(l)}}{1 - w_i^{(l)}} + \log \frac{1 - \pi_0^{(l)}}{w_i^{(l)}} + \frac{1}{2} \left( \log (a_l \sigma_i^{2(l)}) + 1 \right) - \frac{a_l}{2} (\mu_i^{(l)2} + \sigma_i^{2(l)}) \end{aligned}\] Setting this equal to zero gives \[ \begin{aligned} \log \frac{w_i^{(l)}}{1 - w_i^{(l)}} &= \log \frac{1 - \pi_0^{(l)}}{\pi_0^{(l)}} + \frac{1}{2} \left( \log (a_l \sigma_i^{2(l)}) - a_l (\mu_i^{(l)2} + \sigma_i^{2(l)}) + 1 \right) \\ &+ \sum_j \tau_{ij} \left[R_{ij}^{-k} \mu_i^{(l)} w_j^{(f)} \mu_j^{(f)} - \frac{1}{2}(\mu_i^{(l)2} + \sigma_i^{2(l)}) w_j^{(f)} (\mu_j^{(f)2} + \sigma_j^{2(f)}) \right] \\ &= \log \frac{\pi_0^{(l)}}{1 - \pi_0^{(l)}} + \frac{1}{2} \left( \log (a_l \sigma_i^{2(l)}) - a_l (\mu_i^{(l)2} + \sigma_i^{2(l)}) + 1 \right) \\ &+ \sum_j \tau_{ij} \left[R_{ij}^{-k} \mu_i^{(l)} Ef_j - \frac{1}{2}(\mu_i^{(l)2} + \sigma_i^{2(l)}) Ef_j^2 \right] \end{aligned} \]

Algorithm

I suggest that the loadings could be updated by

0. Choosing starting values for \(a_l\), \(\pi_0^{(l)}\), and \(w_1^{(l)}, \ldots, w_n^{(l)}\),

and then repeating the following two steps until convergence:

1. Update \(\mu_1^{(l)}, \ldots, \mu_n^{(l)}\) and \(\sigma_1^{2(l)}, \ldots, \sigma_n^{2(l)}\), and then update \(w_1^{(l)}, \ldots, w_n^{(l)}\).

2. Update \(a_l\) and \(\pi_0^{(l)}\).