Let be the set . Consider a sequence of elements from of length that forms a closed loop where the last element equals the first (). The sequence must satisfy the condition that any two elements are distinct if they are adjacent or separated by exactly one position. This restriction also applies to the boundary, specifically requiring that . \nLet denote the total number of such sequences of length .\nDetermine the exact integer value of .
[THOUGHT]
We need to count the number of proper colorings of a cycle of length with 11 colors such that no two vertices at distance 1 or 2 share the same color.
This can be modeled as a walk on the state graph where a state is an ordered pair of distinct colors .
From we may go to iff and .
The number of valid sequences of length forming a closed loop equals the trace of the adjacency matrix of this state graph raised to the ‑th power: .
We will:
- Enumerate all ordered pairs of distinct colors from (11 colors).
- Build the adjacency matrix where iff .
- Compute .
[/THOUGHT] [PYTHON] import numpy as np
number of colors
C = 11
generate all ordered pairs (a,b) with a != b
pairs = [(a, b) for a in range(C) for b in range(C) if a != b] N = len(pairs) # 110
map pair to index
pair_to_idx = {pair: i for i, pair in enumerate(pairs)}
build adjacency matrix
M = np.zeros((N, N), dtype=int)
for i, (a, b) in enumerate(pairs): for c in range(C): if c != a and c != b: j = pair_to_idx[(b, c)] M[i, j] = 1
compute trace of M^L for L = 11
L = 11 M_L = np.linalg.matrix_power(M, L) trace = int(np.trace(M_L))
trace [/PYTHON]
[THOUGHT] The Python computation confirms that the number of valid sequences is 31384473120. [/THOUGHT]