  We interpret geometrically the torsion of the symmetric algebra of the ideal
sheaf of a zero-dimensional scheme Z defined by $n+1$ equations in an
$n$-dimensional variety. This leads us to generalise a formula of A.Dimca and
S.Papadima in positive characteristic for a rational transformation with finite
base locus. Among other applications, we construct an explicit example of a
homaloidal curve of degree $5$ in characteristic $3$, answering negatively a
question of A.V.D\'oria, S.H.Hassanzadeh and A.Simis.
