For large-dimensional tensor time series, dimension reduction plays a pivotal role. Tensor factor model depicts tensor-valued time series through a low-dimensional projection on a space of common factors, thereby achieving great dimension reduction and having a wide range of applications in economics and finance. In this paper, we propose a simple iterative least squares algorithm for estimating tensor factor model. We first estimate the latent common factors by using deterministic mode- projection matrices and then estimate the loading matrices by minimizing the squared Frobenius loss function under certain identifiability conditions. The estimated loading matrices are further taken as new mode- projection matrices, and the above update procedures are iteratively executed until convergence. We also propose a novel eigenvalue ratio method for estimating the number of factors and show the consistency of the estimators. Given the true number of factors, we theoretically establish the convergence rates of the estimated loading matrices and signal components at the th iteration for any . Thorough numerical studies are conducted to investigate the finite-sample performance of the proposed method. Analyses of import-export transport networks and lung cancer histopathological image datasets illustrate the empirical usefulness of the proposed method.
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