This paper studies numerals,
natural numbers objects and, more generally, free actions, in a topos.
A pre-numeral is a poset with a constant, 0, and a unary
operation, s, such that:
x\leq y implies sx \leq sy
x\leq sx
A numeral is a minimal pre-numeral.
Keywords: numerals, topoi
2000 MSC: 18B25
Theory and Applications of Categories,
Vol. 16, 2006,
No. 19, pp 522-528.
http://www.tac.mta.ca/tac/volumes/16/19/16-19.dvi
http://www.tac.mta.ca/tac/volumes/16/19/16-19.ps
http://www.tac.mta.ca/tac/volumes/16/19/16-19.pdf
ftp://ftp.tac.mta.ca/pub/tac/html/volumes/16/19/16-19.dvi
ftp://ftp.tac.mta.ca/pub/tac/html/volumes/16/19/16-19.ps