One-One Mapping of Reals and Sequences
Is there a "canonical" one-to-one mapping between the reals on the
interval [0,1) and the set of semi-infinite binary sequences, taking
account of the fact that some reals have two representations?
W. Schneeberger mentioned in email that, since each real corresponding
to a terminating binary expansion has two redundant non-terminating
expansions, we can map these two representations down to the next
level. Thus, the sequences {x0111...} and {x1000...} are mapped to
the distinct binary reals {.x01000...} and {.x11000...} respectively.
The first few levels are
1
01 11
001 011 101 111
0001 0011 0101 0111 1001 1011 1101 1111
This is particularly convenient for binary sequences because each
"level" of terminating representations has twice as many members
as the preceding level, so it's easy to define a mapping that absorbs
the double representations. This seems like a good candidate for the
canonical one-to-one mapping.
For other bases it's not quite as natural, but I suppose a similar
method could be applied. In base B, the first level has B-1 members,
and each successive level has B times the previous level. Therefore,
it's necessary to carry down (2)(B-1) from the first to the second
level, and (2+B)(B-1) from the second to the third level, and
(2+B+B^2)(B-1) from the third to the fourth level, and so on. At
the (n+1)th level we need to carry (B^n + B - 2) sequences. This
leaves just B-1 unmapped reals (from the first level), which shouldn't
be too difficult to absorb.
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