  Establishing the sphere packing bound for block codes on the discrete
stationary product channels with feedback ---which are commonly called the
discrete memoryless channels with feedback--- was considered to be an open
problem until recently, notwithstanding the proof sketch provided by Augustin
in 1978. A complete proof following Augustin's proof sketch is presented, to
demonstrate its adequacy and to draw attention to two novel ideas it employs.
These novel ideas (i.e., the Augustin's averaging and the use of subblocks) are
likely to be applicable in other communication problems for establishing
impossibility results.
