## Problem 689

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harrowing_experience
Posts: 2
Joined: Sun Nov 10, 2019 1:56 pm

### Problem 689

I was convinced I understood this until my answer was rejected. I have solved it 3 different ways and keep landing at the same place.

Trying not to give any spoilers, am I correct in saying
f(3/8) = f(0.011 (base 2)) = 13/36,
f(7/8) = f(0.111 (base 2)) = 49/36, and
f(0.1111...(base 2)) = pi2/6 ?

Also, the question asks for the answer rounded to 8 places after the decimal, so does format matter? That is, could I give a correct answer formatted as either 0.xxxxxxxx or .xxxxxxxx and be accepted?
RobertStanforth
Posts: 1459
Joined: Mon Dec 30, 2013 11:25 pm

### Re: Problem 689

Your values of $f$ are correct (albeit with the interpretation of the $\pi^2/6$ value as a limiting case, because $f(x)$ is only defined if $x$ is strictly less than 1).

harrowing_experience
Posts: 2
Joined: Sun Nov 10, 2019 1:56 pm

### Re: Problem 689

Thanks for the clarification. I'm still worried that f(x) may not be clearly defined.

For example, the decimal fraction 1/2 has two binary representations: 0.1000... and 0.0111.... And those lead to
f(1/2) = f(0.1000.. (base 2) ) = 1, and
f(1/2) = f(0.0111.. (base 2) ) = π2/6 - 1.
While these are clearly different expressions, the different values of each seem to contradict a clean definition of f(x) in the problem. For other values of x, this leads me to situations where f(x) > 0.5 by one definition and f(x) < 0.5 for the other.

Can someone help me spot a flaw in my logic?
jaap
Posts: 555
Joined: Tue Mar 25, 2008 3:57 pm
Contact:

### Re: Problem 689

harrowing_experience wrote: Wed Nov 20, 2019 3:21 am Thanks for the clarification. I'm still worried that f(x) may not be clearly defined.

For example, the decimal fraction 1/2 has two binary representations: 0.1000... and 0.0111.... And those lead to
f(1/2) = f(0.1000.. (base 2) ) = 1, and
f(1/2) = f(0.0111.. (base 2) ) = π2/6 - 1.
While these are clearly different expressions, the different values of each seem to contradict a clean definition of f(x) in the problem. For other values of x, this leads me to situations where f(x) > 0.5 by one definition and f(x) < 0.5 for the other.

Can someone help me spot a flaw in my logic?
The only values of x where the binary representation is not unique are those which are rational with denominator equal to a power of 2. These values of x form a set of measure zero, so the probability of choosing such a value of x is zero.

So while you are right that f is not well defined for all x, these values have no effect on p(a), and p(a) is well defined.