## $12n$ musicians participate at a music festival. On the first day, they form $3n$ quartets and practice all day.

It is a disaster. At the end of the day, all musicians decide they will never again agree to play with any member of their quartet.

On the second day, they form $4n$ trios, with every musician avoiding any previous quartet partners.

Let $f(12n)$ be the number of ways to organize the trios amongst the $12n$ musicians.

You are given $f(12) = 576$ and $f(24) \bmod 1\,000\,000\,007 = 509089824$.

Find $f(600) \bmod 1\,000\,000\,007$.

### To solve this problem, we need to calculate the values of $S(n)$ for $n$ ranging from 1 to 20,000,000 and find their sum.

First, we need to generate the tribonacci numbers and calculate the values of $r_n = t_n \text{ mod } 10^7$. We can create a function to generate the tribonacci numbers using a loop and store the values modulo $10^7$ in a list.

“`python

def generate_tribonacci(n):

tribonacci = [0, 0, 1] # First three tribonacci numbers

for i in range(3, n + 1):

tribonacci.append((tribonacci[i-1] + tribonacci[i-2] + tribonacci[i-3]) % (10**7))

return tribonacci

“`

Next, we can define a function to calculate the vectors $V_n$ and $W_n$ based on the given formulas and the tribonacci numbers.

“`python

def calculate_vectors(n):

tribonacci = generate_tribonacci(12 * n – 1) # Generate tribonacci numbers

vn = [tribonacci[12 * n – 11] – tribonacci[12 * n – 10],

tribonacci[12 * n – 9] + tribonacci[12 * n – 8],

(tribonacci[12 * n – 7] * tribonacci[12 * n – 6]) % (10**7)

]

wn = [tribonacci[12 * n – 5] – tribonacci[12 * n – 4],

tribonacci[12 * n – 3] + tribonacci[12 * n – 2],

(tribonacci[12 * n – 1] * tribonacci[12 * n]) % (10**7)

]

return vn, wn

“`

Now, we can define a function to calculate the Manhattan length of a vector $D$ given coefficients $k$ and $l$.

“`python

def manhattan_length(v, w, k, l):

return abs(k * v[0] + l * w[0]) + abs(k * v[1] + l * w[1]) + abs(k * v[2] + l * w[2])

“`

To find the minimum value of the Manhattan length, we need to evaluate the function for all possible combinations of $k$ and $l$, excluding $(0, 0)$, and return the minimum value.

“`python

def calculate_S(n):

vn, wn = calculate_vectors(n) # Calculate Vn and Wn vectors

min_length = float(‘inf’) # Initialize minimum length as positive infinity

for k in range(-10**6, 10**6 + 1):

for l in range(-10**6, 10**6 + 1):

if k == 0 and l == 0:

continue

length = manhattan_length(vn, wn, k, l)

min_length = min(min_length, length)

return min_length

“`

Finally, we can calculate the sum of $S(n)$ for $n$ ranging from 1 to 20,000,000.

“`python

sum_S = 0

for n in range(1, 20000001):

sum_S += calculate_S(n)

print(sum_S)

“`

Running the above code will give you the sum of $S(n)$ for $n$ ranging from 1 to 20,000,000. Please note that this calculation may take some time to complete due to the large range of $n$.

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