“As if pulling a rabbit out of a hat”——On a work of Nanxian Chen

A brief review is presented of two inverse problems concerning the specific heat of phonons and black body radiation, and the solution given by Nanxian Chen using the M¨obius inversion formula in number theory. The page-long comment of John Maddox in Nature on Chen’s work played an important role in promoting this area of research in the subsequent twenty years.