  A Waring decomposition of a (homogeneous) polynomial f is a minimal sum of
powers of linear forms expressing f. Under certain conditions, such a
decomposition is unique. We discuss some algorithms to compute the Waring
decomposition, which are linked to the equation of certain secant varieties and
to eigenvectors of tensors. In particular we explicitly decompose a general
cubic polynomial in three variables as the sum of five cubes (Sylvester
Pentahedral Theorem).
