22 lines
1.6 KiB
Markdown
22 lines
1.6 KiB
Markdown
First we need the number of proper reduced fractions for a denominator `d`.
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This is the amount of numbers `d<n` with `gcd(d,n) == 1`.
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Coincidentally this is again Eulers Phi-Function.
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Now we need to find the sum over all phi values from `1` to `1 000 000`.
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First we get all the prime numbers from `2` to `limit/2` (with an [Sieve of Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)).
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Then we iterate through all possible prime combinations smaller than the limit (practically this is prime factorization).
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While we do this we calculate on the side the phi value of these numbers (with the prime factorization this is trivial) and sum these values together.
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For all values bigger than `LIMIT/2` where we haven't got a factorization we use `phi(p) = p-1`.
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Because these *must* be prime. *(see [Eulers totient function](https://en.wikipedia.org/wiki/Euler's_totient_function))*
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The rest is a bit of clever optimization, so this all does not take too long.
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- First we assume every number is prime and calculate the sum of the phi function of all these primes.
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Then when we find a number (that is not a prime) we subtract the old (wrong) value from the result and add the new (correct) value.
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- We abort our loops early when we run into a number `x > limit`. Because every other factor will only increase `x`.
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- The value of phi is always calculated from the last phi value. So we don't need to totally recalculate it in every step.
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After looking a little bit around this is not ***the** fastest solution to this problem.
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But it's the one I found and I think it's reasonable fast. |