2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/69750We classify all irreducible projective threefolds $X$ which are $k$-defective, i.e. some $k$-secant variety of $X$ has dimension less than the expected value. This results extends the classical Scorza's classification of the case $k=1$.AMSLaTeX, 30 pagesAlgebraic Geometry14J30text