Implementations of the k-means, hierarchical agglomerative and 
    DBSCAN clustering methods for functional data which allows for jointly 
    aligning and clustering curves. It supports functional data defined on 
    one-dimensional domains but possibly evaluating in multivariate codomains. 
    It supports functional data defined in arrays but also via the 'fd' and 
    'funData' classes for functional data defined in the 'fda' and 'funData' 
    packages respectively. It currently supports shift, dilation and affine 
    warping functions for functional data defined on the real line and uses the 
    SRVF framework to handle boundary-preserving warping for functional data 
    defined on a specific interval. Main reference for the k-means algorithm: 
    Sangalli L.M., Secchi P., Vantini S., Vitelli V. (2010) "k-mean alignment 
    for curve clustering" <doi:10.1016/j.csda.2009.12.008>. Main reference for 
    the SRVF framework: Tucker, J. D., Wu, W., & Srivastava, A. (2013) 
    "Generative models for functional data using phase and amplitude separation" 
    <doi:10.1016/j.csda.2012.12.001>.

Provides an implementation of concurrent or varying coefficient regression methods for 
    functional data. The implementations are done for both dense and sparsely observed functional
    data. Pointwise confidence bands can be constructed for each case. Further, the influence of
    past predictor values are modeled by a smooth history index function, 
    while the effects on the response are described by smooth varying coefficient functions, 
    which are very useful in analyzing real data such as COVID data.
    References: Yao, F., Müller, H.G., Wang, J.L. (2005) <doi:10.1214/009053605000000660>.
                Sentürk, D., Müller, H.G. (2010) <doi:10.1198/jasa.2010.tm09228>.

An implementation of the methodology described in
    Petersen and Mueller (2016) <doi:10.1214/15-AOS1363> for the functional
    data analysis of samples of density functions.  Densities are first
    transformed to their corresponding log quantile densities, followed by
    ordinary Functional Principal Components Analysis (FPCA).  Transformation
    modes of variation yield improved interpretation of the variability in the
    data as compared to FPCA on the densities themselves.  The standard
    fraction of variance explained (FVE) criterion commonly used for functional
    data is adapted to the transformation setting, also allowing for an
    alternative quantification of variability for density data through the
    Wasserstein metric of optimal transport.