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"name": "langford_n9__v7",
"problem_type": "langford",
"params": {
"n": 9
},
"prompt": "Construct a Langford sequence L(2,9): a sequence of length 18 containing exactly 2 copies of each integer from 1 to 9, such that the two occurrences of each value v are exactly v+1 positions apart (so copies of 1 are 1 position apart, copies of 2 are 2 positions apart, etc.). Example: L(2,3) has solution [2, 3, 1, 2, 1, 3].\n\nReturn seq as a list of 18 integers in 1..9, or state \"UNSATISFIABLE\" if no such sequence exists.",
"satisfiable": false,
"solution": null,
"difficulty": {
"solve_time_ms": 2919.6,
"search_space": 6402373705728000,
"num_variables": 18,
"num_constraints": 176,
"num_edges": -1,
"backend": "pycsp",
"solve_tier": "hard",
"solve_pct_global": 96.05,
"solve_pct_type": 93.75
},
"partial_assignment": null
}
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