$ sudo apt-get update
$ sudo apt-get install octave-specfun 

octave-specfun is installed in your system.

Make ensure the octave-specfun package were installed using the commands given below,

$ sudo dpkg-query -l | grep octave-specfun *

You will get with octave-specfun package name, version, architecture and description in a table

octave-specfun package basic information: