P478
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Mixtures

ℹ️Published on Saturday, 30th August 2014, 07:00 pm; Solved by 220;
Difficulty rating: 100%

Let us consider mixtures of three substances: A, B and C. A mixture can be described by a ratio of the amounts of A, B, and C in it, i.e., $(a : b : c)$. For example, a mixture described by the ratio $(2 : 3 : 5)$ contains $20\%$ A, $30\%$ B and $50\%$ C.

For the purposes of this problem, we cannot separate the individual components from a mixture. However, we can combine different amounts of different mixtures to form mixtures with new ratios.

For example, say we have three mixtures with ratios $(3 : 0 : 2)$, $(3: 6 : 11)$ and $(3 : 3 : 4)$. By mixing $10$ units of the first, $20$ units of the second and $30$ units of the third, we get a new mixture with ratio $(6 : 5 : 9)$, since:
$(10 \cdot \tfrac 3 5$ + $20 \cdot \tfrac 3 {20} + 30 \cdot \tfrac 3 {10} : 10 \cdot \tfrac 0 5 + 20 \cdot \tfrac 6 {20} + 30 \cdot \tfrac 3 {10} : 10 \cdot \tfrac 2 5 + 20 \cdot \tfrac {11} {20} + 30 \cdot \tfrac 4 {10}) = (18 : 15 : 27) = (6 : 5 : 9)$

However, with the same three mixtures, it is impossible to form the ratio $(3 : 2 : 1)$, since the amount of B is always less than the amount of C.

Let $n$ be a positive integer. Suppose that for every triple of integers $(a, b, c)$ with $0 \le a, b, c \le n$ and $\gcd(a, b, c) = 1$, we have a mixture with ratio $(a : b : c)$. Let $M(n)$ be the set of all such mixtures.

For example, $M(2)$ contains the $19$ mixtures with the following ratios:

\begin{align} \{&(0 : 0 : 1), (0 : 1 : 0), (0 : 1 : 1), (0 : 1 : 2), (0 : 2 : 1),\\ &(1 : 0 : 0), (1 : 0 : 1), (1 : 0 : 2), (1 : 1 : 0), (1 : 1 : 1),\\ &(1 : 1 : 2), (1 : 2 : 0), (1 : 2 : 1), (1 : 2 : 2), (2 : 0 : 1),\\ &(2 : 1 : 0), (2 : 1 : 1), (2 : 1 : 2), (2 : 2 : 1)\}. \end{align}

Let $E(n)$ be the number of subsets of $M(n)$ which can produce the mixture with ratio $(1 : 1 : 1)$, i.e., the mixture with equal parts A, B and C.
We can verify that $E(1) = 103$, $E(2) = 520447$, $E(10) \bmod 11^8 = 82608406$ and $E(500) \bmod 11^8 = 13801403$.
Find $E(10\,000\,000) \bmod 11^8$.



Soluzione

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