mjrpes3
Golden Member
http://arxiv.org/PS_cache/arxi...f/0811/0811.0164v8.pdf
In the matter of teaching decimal notation, we would like to obtain
some reaction from educators to the following proposal concerning the
problem of the unital evaluation of .999 . . .. A student may ask:
What does the teacher mean to happen exactly after
nine, nine, nine when he writes dot, dot, dot?
How is a teacher to handle such a question? Experience shows that
toeing the standard line on the unital evaluation of .999 . . . possesses
a high-frustration factor in the classroom. Rather than ba?ing the
student with such a categorical claim, a teacher could proceed by pre-
senting the following ten points, based on the material outlined in
Section 11:
(1) the reals are not, as the rationals are not, the maximal number
system;
(2) there exist larger number systems, containing in?nitesimals;
(3) in such larger systems, the interval [0, 1] contains many numbers
in?nitely close to 1;
(4) in a particular larger system called the hyperreal numbers, there
is a generalized notion of decimal expansion for such numbers,
starting in each case with an unbounded number of digits ?9?;
(5) all such numbers therefore have an arguable claim to the no-
tation ?.999 . . .? which is patently ambiguous (the meaning of
the ellipsis ?. . .? requires disambiguation);
(6) all but one of them are strictly smaller than 1;
(7) the convention adopted by most professional mathematicians
is to interpret the symbol ?.999 . . .? as referring to the largest
such number, namely 1 itself;
(8) thus, the students? intuition that .999 . . . falls just short of 1
can be justi?ed in a mathematically rigorous fashion;
(9) the said extended number system is mostly relevant in in?nites-
imal calculus (also known as di?erential and integral calculus);
(10) if you would like to learn more about the hyperreals, go to your
teacher so he can give you further references.