Problem 198

Ambiguous Numbers

A best approximation to a real number `x` for the denominator bound `d` is a rational number `r`/`s` (in reduced form) with `s` `d`, so that any rational number `p`/`q` which is closer to `x` than `r`/`s` has `q` `d`.

Usually the best approximation to a real number is uniquely
determined for all denominator bounds. However, there are some
exceptions, e.g. 9/40 has the two best approximations 1/4 and 1/5 for
the denominator bound 6.
We shall call a real number `x` *ambiguous*, if there is at least one denominator bound for which `x` possesses two best approximations. Clearly, an ambiguous number is necessarily rational.

How many ambiguous numbers `x` = `p`/`q`,
0 `x` 1/100, are there whose denominator `q` does not exceed 10^{8}?

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These problems are part of
Project Euler
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CC BY-NC-SA 2.0 UK
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