One list is provided on the source for function __unimplemented.m__, subfunction missing_functions; it can be edited in the Octave GUI or browsed at [http://hg.savannah.gnu.org/hgweb/octave/file/d63878346099/scripts/help/__unimplemented__.m#l530].
Lists are also kept for the [[Image package|Image]], [[Mapping package|Mapping]], [[Optimization package|Optimization]],
and [[Signal package|Signal]] packages.
It is also possible to look at existing [[Wikipedia:Free and open-source software|FOSS]] implementations, from FreeMat and Scilab (for more closely compatible languages) to R or Scipy or Julia (for less compatible versions). Obviously, it is NOT OK to look at the Matlab implementation since this is not [[Wikipedia:Free software|free software]]!