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D. L. Burk­hold­er: “Com­ments on (5), (14), (22), (30), and (31),” pp. 553–​558 in Se­lec­ted pa­pers of Al­berto P. Calderón with com­ment­ary. Edi­ted by A. Bel­low, C. E. Kenig, and P. Malliavin. AMS Col­lec­ted Works 21. Amer­ic­an Math­ema­tial So­ci­ety (Provid­ence, RI), 2008. MR 2435333 incollection