Before revision:  Nevertheless, the challenging problem of estimating the BTD model structure, namely the number of block terms  and their individual ranks,  has only recently started to attract significant attention  .
After revision:  Nevertheless, the challenging problem of estimating the BTD model structure, namely the number of block terms and their individual ranks, has only recently started to attract significant attention , mainly through regularization-based approaches which entail the need to tune the regularization parameter(s) .