{
  "index": "1995-A-1",
  "type": "ALG",
  "tag": [
    "ALG"
  ],
  "difficulty": "",
  "question": "multiplication (that is, if $a$ and $b$ are in $S$, then so is $ab$).\nLet $T$ and $U$ be disjoint subsets of $S$ whose union is $S$. Given\nthat the product of any {\\em three} (not necessarily distinct)\nelements of $T$ is in $T$ and that the product of any three elements\nof $U$ is in $U$, show that at least one of the two subsets $T,U$ is\nclosed under multiplication.",
  "solution": "Suppose on the contrary that there exist $t_{1}, t_{2} \\in T$\nwith $t_{1}t_{2} \\in U$ and $u_{1}, u_{2} \\in U$ with $u_{1}u_{2} \\in\nT$. Then $(t_{1}t_{2})u_{1}u_{2} \\in U$ while\n$t_{1}t_{2}(u_{1}u_{2}) \\in T$, contradiction.",
  "vars": [
    "a",
    "b",
    "t_1",
    "t_2",
    "u_1",
    "u_2"
  ],
  "params": [
    "S",
    "T",
    "U"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "a": "elemone",
        "b": "elemtwo",
        "t_1": "telemone",
        "t_2": "telemtwo",
        "u_1": "uelemone",
        "u_2": "uelemtwo",
        "S": "bigset",
        "T": "subsett",
        "U": "subsetu"
      },
      "question": "multiplication (that is, if $elemone$ and $elemtwo$ are in $bigset$, then so is $elemone elemtwo$).\nLet $subsett$ and $subsetu$ be disjoint subsets of $bigset$ whose union is $bigset$. Given\nthat the product of any {\\em three} (not necessarily distinct)\nelements of $subsett$ is in $subsett$ and that the product of any three elements\nof $subsetu$ is in $subsetu$, show that at least one of the two subsets $subsett,subsetu$ is\nclosed under multiplication.",
      "solution": "Suppose on the contrary that there exist $telemone, telemtwo \\in subsett$\nwith $telemone telemtwo \\in subsetu$ and $uelemone, uelemtwo \\in subsetu$ with $uelemone uelemtwo \\in\nsubsett$. Then $(telemone telemtwo) uelemone uelemtwo \\in subsetu$ while\n$telemone telemtwo(uelemone uelemtwo) \\in subsett$, contradiction."
    },
    "descriptive_long_confusing": {
      "map": {
        "a": "moonlight",
        "b": "sandstone",
        "t_1": "driftwood",
        "t_2": "hazelroot",
        "u_1": "glasswind",
        "u_2": "riverstone",
        "S": "meadowlark",
        "T": "cottonseed",
        "U": "nightshade"
      },
      "question": "multiplication (that is, if $moonlight$ and $sandstone$ are in $meadowlark$, then so is $moonlight sandstone$).\nLet $cottonseed$ and $nightshade$ be disjoint subsets of $meadowlark$ whose union is $meadowlark$. Given\nthat the product of any {\\em three} (not necessarily distinct)\nelements of $cottonseed$ is in $cottonseed$ and that the product of any three elements\nof $nightshade$ is in $nightshade$, show that at least one of the two subsets $cottonseed, nightshade$ is\nclosed under multiplication.",
      "solution": "Suppose on the contrary that there exist $driftwood, hazelroot \\in cottonseed$\nwith $driftwood hazelroot \\in nightshade$ and $glasswind, riverstone \\in nightshade$ with $glasswind riverstone \\in\ncottonseed$. Then $(driftwood hazelroot)glasswind riverstone \\in nightshade$ while\n$driftwood hazelroot(glasswind riverstone) \\in cottonseed$, contradiction."
    },
    "descriptive_long_misleading": {
      "map": {
        "a": "nonmember",
        "b": "outsider",
        "t_1": "exteriorone",
        "t_2": "exteriortwo",
        "u_1": "interiorone",
        "u_2": "interiortwo",
        "S": "emptyset",
        "T": "openclass",
        "U": "intersection"
      },
      "question": "multiplication (that is, if $nonmember$ and $outsider$ are in $emptyset$, then so is $nonmemberoutsider$).\nLet $openclass$ and $intersection$ be disjoint subsets of $emptyset$ whose union is $emptyset$. Given\nthat the product of any {\\em three} (not necessarily distinct)\nelements of $openclass$ is in $openclass$ and that the product of any three elements\nof $intersection$ is in $intersection$, show that at least one of the two subsets $openclass,intersection$ is\nclosed under multiplication.",
      "solution": "Suppose on the contrary that there exist $exteriorone, exteriortwo \\in openclass$ with $exterioroneexteriortwo \\in intersection$ and $interiorone, interiortwo \\in intersection$ with $interioroneinteriortwo \\in openclass$. Then $(exterioroneexteriortwo)interioroneinteriortwo \\in intersection$ while\n$exterioroneexteriortwo(interioroneinteriortwo) \\in openclass$, contradiction."
    },
    "garbled_string": {
      "map": {
        "a": "qzxwvtnp",
        "b": "hjgrksla",
        "t_1": "slkdmnva",
        "t_2": "odifjgwe",
        "u_1": "plsnrjkw",
        "u_2": "xvbcnmal",
        "S": "qwerlkjh",
        "T": "asdfghjk",
        "U": "zxcvbnml"
      },
      "question": "multiplication (that is, if $qzxwvtnp$ and $hjgrksla$ are in $qwerlkjh$, then so is $qzxwvtnphjgrksla$).\nLet $asdfghjk$ and $zxcvbnml$ be disjoint subsets of $qwerlkjh$ whose union is $qwerlkjh$. Given\nthat the product of any {\\em three} (not necessarily distinct)\nelements of $asdfghjk$ is in $asdfghjk$ and that the product of any three elements\nof $zxcvbnml$ is in $zxcvbnml$, show that at least one of the two subsets $asdfghjk,zxcvbnml$ is\nclosed under multiplication.",
      "solution": "Suppose on the contrary that there exist $slkdmnva, odifjgwe \\in asdfghjk$\nwith $slkdmnvaodifjgwe \\in zxcvbnml$ and $plsnrjkw, xvbcnmal \\in zxcvbnml$ with $plsnrjkwxvbcnmal \\in\nasdfghjk$. Then $(slkdmnvaodifjgwe)plsnrjkwxvbcnmal \\in zxcvbnml$ while\n$slkdmnvaodifjgwe(plsnrjkwxvbcnmal) \\in asdfghjk$, contradiction."
    },
    "kernel_variant": {
      "question": "Let $S$ be a commutative semigroup (associative, commutative binary operation, no identity required).  Split $S$ into two disjoint subsets $T$ and $U$ whose union is $S$.  Assume that\n\n(1) the product of any six (not necessarily distinct) elements of $T$ lies in $T$, and\n\n(2) the product of any six elements of $U$ lies in $U$.\n\nProve that at least one of the two subsets $T,\\,U$ is closed under multiplication (i.e. is a sub-semigroup of $S$).",
      "solution": "Suppose, toward a contradiction, that neither T nor U is closed under multiplication.\n\nStep 1.  Because T is not closed, pick t₁,t₂∈T such that their product lies outside T; since T and U are disjoint, we have\n   a := t₁ t₂ ∈ U.\nLikewise, because U is not closed, there exist u₁,u₂∈U whose product lies in T; set\n   b := u₁ u₂ ∈ T.\n\nStep 2.  Form the common product\n   P := a·a·u₁·u₁·u₂·u₂\n       = (t₁t₂)(t₁t₂)u₁u₁u₂u₂\n       = t₁t₁t₂t₂(u₁u₂)(u₁u₂)\n       = t₁² t₂² u₁² u₂²,\nwhere commutativity lets us rewrite factors freely.\n\nView P in two different ways:\n •  As a,a,u₁,u₁,u₂,u₂ – six individual elements of U.\n •  As t₁,t₁,t₂,t₂,b,b – six individual elements of T.\n\nStep 3.  By hypothesis (2), the first presentation forces P∈U.  By hypothesis (1), the second presentation forces P∈T.\n\nStep 4.  This is impossible because T and U are disjoint.  Hence our initial assumption is false: at least one of the two sets must in fact be closed under multiplication.\n\nTherefore either T or U is a (multiplicatively) closed subset of S, as required.",
      "_meta": {
        "core_steps": [
          "Assume neither T nor U is closed; pick t1,t2 in T with t1t2 in U, and u1,u2 in U with u1u2 in T.",
          "Form the common product P = t1 t2 u1 u2 and view it in two ways: (t1t2)·u1·u2 (three U‐elements) and t1·t2·(u1u2) (three T‐elements).",
          "Invoke the hypothesis that any triple from U (resp. T) has its product in U (resp. T) to get P in U and P in T.",
          "Since T and U are disjoint, this is impossible; hence at least one of the subsets must be multiplicatively closed."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "The specific count of factors required in the hypothesis (“three”); any integer k ≥ 3 (repetition allowed) works identically.",
            "original": 3
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}