1. [C1] M. Christ, Some nonanalytic-hypoelliptic sums of squares of vector fields, Bull. Amer. Math. Soc. (N.S.) 26 (1992), no. 1, 137–140.
2. [C2] M. Christ, Certain sums of squares of vector fields fail to be analytic hypoelliptic, Comm. Partial Differential Equations 16 (1991), no. 10, 1695–1707.
3. [C3] M. Christ, Analytic hypoellipticity breaks down for weakly pseudoconvex Reinhardt domains, Internat. Math. Res. Notices (1991), no. 3, 31–40.
4. [C4] M. Christ, Nonexistence of invariant analytic hypoelliptic differential operators on nilpotent groups of step greater than two, to appear in proceedings of symposium in honor of E. M. Stein, Princeton University Press.
5. [C5] M. Christ, A class of hypoelliptic PDE admitting nonanalytic solutions, The Madison Symposium on Complex Analysis (Madison, WI, 1991), Contemp. Math., vol. 137, Amer. Math. Soc., Providence, RI, 1992, pp. 155–167.