For strictly positive, linear, and centrally symmetric functionals in
two dimensions the existence of cubature formulas attaining the known
lower bounds is equivalent to the solvability of certain matrix equati
ons under some constraints. Any solution generates a real ideal the co
mmon roots of which are the nodes of the cubature formula. These resul
ts are applied to construct an infinite number of minimal positive cub
ature formulas of an arbitrary degree of erectness for one special, bu
t classical, integral.