Weihnachten customs held upon returning from church leading to the gift giving vary across the German-speaking countries. One of the most common situations sees the returning children wait to enter into their (locked) living room until a little bell rings. This bell marks the departure of the one delivering gifts. In the more Catholic regions - primarily South Germany and Switzerland - this is considered the Christ Child ("Christkind"), while those in North Germany commonly consider it to be the "Weihnachtsmann" (Saint Nicholas or Santa Claus) who is exiting. The children then enter to see the decorated Weihnachtsbaum (Christmas tree), with
Allen Tannenbaum thesis advisor Heisuke Hironaka at the Harvard University in 1976. Tannenbaum has done research in numerous areas including robust control, computer vision, and biomedical imaging, having almost 500 publications. He pioneered the field of robust control with the solution of the gain margin and phase margin problems using techniques from Nevanlinna–Pick interpolation theory, which was the first H-infinity type control problem solved. Tannenbaum used techniques from elliptic curves to show that the reachability does not imply pole assignability for systems defined over polynomial rings in two or more variables over an arbitrary field. He pioneered the use of partial differential