{
  "index": "1974-A-1",
  "type": "NT",
  "tag": [
    "NT",
    "COMB"
  ],
  "difficulty": "",
  "question": "A-1. Call a set of positive integers \"conspiratorial\" if no three of them are pairwise relatively prime. (A set of integers is \"pairwise relatively prime\" if no pair of them has a common divisor greater than 1 .) What is the largest number of elements in any \"conspiratorial\" subset of the integers 1 through 16?",
  "solution": "A-1.\nA conspiratorial subset (CS) of \\( \\{1,2, \\cdots, 16\\} \\) has at most two numbers from the pairwise relatively prime set \\( \\{1,2,3,5,7,11,13\\} \\) and so has at most \\( 16-(7-2)=11 \\) numbers. But\n\\[\n\\{2,3,4,6,8,9,10,12,14,15,16\\}\n\\]\nis a CS with 11 elements; hence the answer is 11 .",
  "vars": [],
  "params": [
    "n"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "n": "posintegercount"
      },
      "question": "A-1. Call a set of positive integers \"conspiratorial\" if no three of them are pairwise relatively prime. (A set of integers is \"pairwise relatively prime\" if no pair of them has a common divisor greater than 1 .) What is the largest number of elements in any \"conspiratorial\" subset of the integers 1 through 16?",
      "solution": "A-1.\nA conspiratorial subset (CS) of \\( \\{1,2, \\cdots, 16\\} \\) has at most two numbers from the pairwise relatively prime set \\( \\{1,2,3,5,7,11,13\\} \\) and so has at most \\( 16-(7-2)=11 \\) numbers. But\n\\[\n\\{2,3,4,6,8,9,10,12,14,15,16\\}\n\\]\nis a CS with 11 elements; hence the answer is 11 ."
    },
    "descriptive_long_confusing": {
      "map": {
        "n": "ballooner"
      },
      "question": "A-1. Call a set of positive integers \"conspiratorial\" if no three of them are pairwise relatively prime. (A set of integers is \"pairwise relatively prime\" if no pair of them has a common divisor greater than 1.) What is the largest number of elements in any \"conspiratorial\" subset of the integers 1 through 16?",
      "solution": "A-1.\nA conspiratorial subset (CS) of \\( \\{1,2, \\cdots, 16\\} \\) has at most two numbers from the pairwise relatively prime set \\( \\{1,2,3,5,7,11,13\\} \\) and so has at most \\( 16-(7-2)=11 \\) numbers. But\n\\[\n\\{2,3,4,6,8,9,10,12,14,15,16\\}\n\\]\nis a CS with 11 elements; hence the answer is 11."
    },
    "descriptive_long_misleading": {
      "map": {
        "n": "constantvalue"
      },
      "question": "A-1. Call a set of positive integers \"conspiratorial\" if no three of them are pairwise relatively prime. (A set of integers is \"pairwise relatively prime\" if no pair of them has a common divisor greater than 1 .) What is the largest number of elements in any \"conspiratorial\" subset of the integers 1 through 16?",
      "solution": "A-1.\nA conspiratorial subset (CS) of \\( \\{1,2, \\cdots, 16\\} \\) has at most two numbers from the pairwise relatively prime set \\( \\{1,2,3,5,7,11,13\\} \\) and so has at most \\( 16-(7-2)=11 \\) numbers. But\n\\[\n\\{2,3,4,6,8,9,10,12,14,15,16\\}\n\\]\nis a CS with 11 elements; hence the answer is 11 ."
    },
    "garbled_string": {
      "map": {
        "n": "qzxwvtnp"
      },
      "question": "A-1. Call a set of positive integers \"conspiratorial\" if no three of them are pairwise relatively prime. (A set of integers is \"pairwise relatively prime\" if no pair of them has a common divisor greater than 1 .) What is the largest number of elements in any \"conspiratorial\" subset of the integers 1 through 16?",
      "solution": "A-1.\nA conspiratorial subset (CS) of \\( \\{1,2, \\cdots, 16\\} \\) has at most two numbers from the pairwise relatively prime set \\( \\{1,2,3,5,7,11,13\\} \\) and so has at most \\( 16-(7-2)=11 \\) numbers. But\n\\[\n\\{2,3,4,6,8,9,10,12,14,15,16\\}\n\\]\nis a CS with 11 elements; hence the answer is 11 ."
    },
    "kernel_variant": {
      "question": "Call a subset \\(T\\subset\\{1,2,\\ldots ,30\\}\\)\\emph{quadruple–free} if it contains no four elements that are pairwise relatively prime.  (Equivalently, there is no four–element subcollection of \\(T\\) whose members have greatest common divisor 1 in every pair.)  What is the greatest possible size of a quadruple–free subset of \\(\\{1,2,\\ldots ,30\\}\\)?",
      "solution": "Let k=4; we must forbid the occurrence of any k=4 pairwise–coprime numbers.\n\nStep 1.  Build a maximal pairwise–coprime subset.\n\nTake\nS={1,2,3,5,7,11,13,17,19,23,29},\nconsisting of 1 together with all primes not exceeding 30.  Any integer between 1 and 30 is divisible by at least one of the primes in S; therefore no further element can be adjoined without destroying pairwise coprimality.  Hence S is maximal and |S|=11.\n\nStep 2.  At most k−1=3 members of S may lie in a quadruple–free set, for any four distinct elements of S are automatically pairwise coprime.  Consequently every quadruple–free set must omit at least |S|−(k−1)=11−3=8 elements of the universe.  Thus |T|≤30−8=22.\n\nStep 3.  Exhibit a quadruple–free set of size 22.\nChoose the three smallest elements of S, namely 2, 3, and 5, and discard the other eight members of S.  Adjoin every remaining integer up to 30.  Explicitly,\nT={2,3,5,4,6,8,9,10,12,14,15,16,18,20,21,22,24,25,26,27,28,30}.\nThis set contains 22 numbers.\n\nStep 4.  Verify that T is quadruple–free.\nBesides the three primes 2, 3, 5, every element of T is divisible by at least one of 2, 3, 5.  Given any four members of T, the pigeon-hole principle forces two of them to share the same divisor among 2, 3, 5, so those two are not coprime.  Hence no four elements of T can be pairwise relatively prime, and T is indeed quadruple-free.\n\nStep 5.  Conclusion.\nWe have produced a quadruple-free subset of size 22, and Step 2 showed that no larger such set exists.  Therefore the maximum possible size is\n22.",
      "_meta": {
        "core_steps": [
          "Pick a maximal pairwise-relatively-prime subset S of the ambient set.",
          "Note that a conspiratorial set can include at most (k−1) elements of S, where k is the prohibited size of a coprime sub-collection (here k=3).",
          "Apply simple counting: |CS| ≤ |Universe| − (|S| − (k−1)).",
          "Exhibit a conspiratorial set whose size hits this upper bound."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Prohibited size k of a pairwise-coprime subcollection (‘no k of them are pairwise relatively prime’).",
            "original": 3
          },
          "slot2": {
            "description": "Size of the ambient interval 1,…,N.",
            "original": 16
          },
          "slot3": {
            "description": "Concrete maximal pairwise-coprime subset S that is used in the argument.",
            "original": [
              1,
              2,
              3,
              5,
              7,
              11,
              13
            ]
          },
          "slot4": {
            "description": "Cardinality |S| of that maximal pairwise-coprime subset.",
            "original": 7
          },
          "slot5": {
            "description": "Explicit conspiratorial set that attains the upper bound.",
            "original": [
              2,
              3,
              4,
              6,
              8,
              9,
              10,
              12,
              14,
              15,
              16
            ]
          },
          "slot6": {
            "description": "Resulting maximal size of a conspiratorial set.",
            "original": 11
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}