On Kohn’s sums of squares of complex vector fields

This is a survey of some recent alternative way of proving a subelliptic estimate, first proven by J. J. Kohn, for certain sums of squares of complex vector fields. My approach here makes it possible to extend the result also to more general families of complex vector fields, to perturbations of sums of squares operators by a first-order complex term and furthermore to a pseudodifferential setting.