{
  "index": "1990-A-5",
  "type": "ALG",
  "tag": [
    "ALG"
  ],
  "difficulty": "",
  "question": "same size such that $\\mathbf{ABAB = 0}$, does it follow that\n$\\mathbf{BABA = 0}$?",
  "solution": "Solution 1. Direct multiplication shows that the \\( 3 \\times 3 \\) matrices\n\\[\n\\mathbf{A}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n0 & 0 & 0 \\\\\n0 & 1 & 0\n\\end{array}\\right), \\quad \\mathbf{B}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n1 & 0 & 0 \\\\\n0 & 0 & 0\n\\end{array}\\right)\n\\]\ngive a counterexample.\nSolution 2. A more enlightening way to construct a counterexample is to use a transition diagram, as in the following example. Let \\( e_{1}, e_{2}, e_{3}, e_{4} \\) be a basis of a four-dimensional vector space. Represent the matrices as in Figure 15. For example, the arrow from \\( e_{4} \\) to \\( e_{3} \\) labelled \\( \\mathbf{B} \\) indicates that \\( \\mathbf{B} e_{4}=e_{3} \\); the arrow from \\( e_{1} \\) to 0 indicates that \\( \\mathbf{A} e_{1}=0 \\). Then it can be quickly checked that \\( \\mathbf{A B A B} \\) annihilates the four basis vectors, but \\( \\mathbf{B A B A} e_{4}=e_{1} \\). (Be careful with the order of multiplication when checking!)\n\nRemark. The counterexample of Solution 1 also can be obtained from a transition diagram.\n\nRemark. There are no \\( 1 \\times 1 \\) or \\( 2 \\times 2 \\) counterexamples. The \\( 1 \\times 1 \\) case is clear. For the \\( 2 \\times 2 \\) case, observe that \\( \\mathbf{A B A B}=\\mathbf{0} \\) implies \\( \\mathbf{B}(\\mathbf{A B A B}) \\mathbf{A}=\\mathbf{0} \\), and hence \\( \\mathbf{B A} \\) is nilpotent. But if a \\( 2 \\times 2 \\) matrix \\( \\mathbf{M} \\) is nilpotent, its characteristic polynomial is \\( x^{2} \\), so \\( \\mathbf{M}^{2}=\\mathbf{0} \\) by the Cayley-Hamilton Theorem \\( [A p 2 \\), Theorem 7.8]. Thus BABA \\( =\\mathbf{0} \\).\n\nRemark. For any \\( n \\geq 3 \\), there exist \\( n \\times n \\) counterexamples: enlarge the matrices in (1) by adding rows and columns of zeros.\n\nStronger result. Here we present a conceptual construction of a counterexample, requiring essentially no calculations. Define a word to be a finite sequence of A's and B's. (The empty sequence \\( \\emptyset \\) is also a word.) Let \\( S \\) be a finite set of words containing BABA and its \"right subsequences\" ABA, BA, A, \\( \\emptyset \\), but not containing any word having ABAB as a subsequence. Consider a vector space with basis corresponding to these words (i.e., \\( e_{\\mathbf{B A B A}}, e_{\\mathbf{A B A}}, e_{\\emptyset} \\), etc.). Let \\( \\mathbf{A} \\) be the linear transformation mapping \\( e_{w} \\) to \\( e_{\\mathbf{A} w} \\) if \\( \\mathbf{A} w \\in S \\) and to 0 otherwise. Define a linear transformation \\( \\mathbf{B} \\) similarly. Then \\( \\mathbf{A B A B}=0 \\) but \\( \\mathbf{B A B A} \\neq 0 \\). (This gives a very general way of dealing with any problem of this type.)\n\nRemark. To help us find a counterexample, we imposed the restriction that each of \\( \\mathbf{A} \\) and \\( \\mathbf{B} \\) maps each standard basis vector \\( e_{i} \\) to some \\( e_{j} \\) or to 0 . With this restriction, the problem can be restated in terms of automata theory:\n\nDoes there exist a finite automaton with a set of states \\( \\Sigma=\\left\\{0, e_{1}, e_{2}, \\ldots, e_{n}\\right\\} \\) in which all states are initial states and all but 0 are final states, and a set of two productions \\( \\{\\mathbf{A}, \\mathbf{B}\\} \\) each mapping 0 to 0 , such that the language it accepts contains ABAB but not \\( \\mathbf{B A B A} \\) ?\nSee Chapter 3 of \\( [\\mathrm{Sa}] \\) for terminology. The language accepted by such a finite automaton is defined as the set of words in \\( \\mathbf{A} \\) and \\( \\mathbf{B} \\) that correspond to a sequence of productions leading from some initial state to some final state. Technically, since our finite automaton does not have a unique initial state, it is called nondeterministic, even though each production maps any given state to a unique state. (Many authors [HoU, p. 20] do not allow multiple initial states, even in nondeterministic finite automata; we could circumvent this by introducing a new artificial initial state, with a new nondeterministic production mapping it to the desired initial states.) One theorem of automata theory \\( [\\mathrm{Sa} \\), Theorem 3.3] is that any language accepted by a nondeterministic finite automaton is also the language accepted by some deterministic finite automaton.\n\nMany lexical scanners, such as the UNIX utility grep [ Hu ], are based on the theory of finite automata. See the remark in 1989A6 for the appearance of automata theory in a very different context.",
  "vars": [
    "A",
    "B",
    "M",
    "e_1",
    "e_2",
    "e_3",
    "e_4",
    "e_w",
    "e_i",
    "e_j",
    "e_n",
    "w",
    "x"
  ],
  "params": [
    "S",
    "n",
    "\\\\Sigma"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "A": "matrixa",
        "B": "matrixb",
        "M": "matrixm",
        "e_1": "basisone",
        "e_2": "basistwo",
        "e_3": "basisthree",
        "e_4": "basisfour",
        "e_w": "basisword",
        "e_i": "basisi",
        "e_j": "basisj",
        "e_n": "basisn",
        "w": "wordvar",
        "x": "variablex",
        "S": "setwords",
        "n": "dimension",
        "\\Sigma": "statesset"
      },
      "question": "same size such that $\\mathbf{matrixamatrixbmatrixamatrixb = 0}$, does it follow that $\\mathbf{matrixbmatrixamatrixbmatrixa = 0}$?",
      "solution": "Solution 1. Direct multiplication shows that the \\( 3 \\times 3 \\) matrices\n\\[\n\\mathbf{matrixa}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n0 & 0 & 0 \\\\\n0 & 1 & 0\n\\end{array}\\right), \\quad \\mathbf{matrixb}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n1 & 0 & 0 \\\\\n0 & 0 & 0\n\\end{array}\\right)\n\\]\ngive a counterexample.\n\nSolution 2. A more enlightening way to construct a counterexample is to use a transition diagram, as in the following example. Let \\( basisone, basistwo, basisthree, basisfour \\) be a basis of a four-dimensional vector space. Represent the matrices as in Figure 15. For example, the arrow from \\( basisfour \\) to \\( basisthree \\) labelled \\( \\mathbf{matrixb} \\) indicates that \\( \\mathbf{matrixb}\\, basisfour = basisthree \\); the arrow from \\( basisone \\) to 0 indicates that \\( \\mathbf{matrixa}\\, basisone = 0 \\). Then it can be quickly checked that \\( \\mathbf{matrixa matrixb matrixa matrixb} \\) annihilates the four basis vectors, but \\( \\mathbf{matrixb matrixa matrixb matrixa}\\, basisfour = basisone \\). (Be careful with the order of multiplication when checking!)\n\nRemark. The counterexample of Solution 1 also can be obtained from a transition diagram.\n\nRemark. There are no \\( 1 \\times 1 \\) or \\( 2 \\times 2 \\) counterexamples. The \\( 1 \\times 1 \\) case is clear. For the \\( 2 \\times 2 \\) case, observe that \\( \\mathbf{matrixa matrixb matrixa matrixb}=\\mathbf{0} \\) implies \\( \\mathbf{matrixb}(\\mathbf{matrixa matrixb matrixa matrixb}) \\mathbf{matrixa}=\\mathbf{0} \\), and hence \\( \\mathbf{matrixb matrixa} \\) is nilpotent. But if a \\( 2 \\times 2 \\) matrix \\( \\mathbf{matrixm} \\) is nilpotent, its characteristic polynomial is \\( variablex^{2} \\), so \\( \\mathbf{matrixm}^{2}=\\mathbf{0} \\) by the Cayley-Hamilton Theorem \\( [A p 2 \\), Theorem 7.8]. Thus matrixbmatrixamatrixbmatrixa \\( =\\mathbf{0} \\).\n\nRemark. For any \\( dimension \\geq 3 \\), there exist \\( dimension \\times dimension \\) counterexamples: enlarge the matrices in (1) by adding rows and columns of zeros.\n\nStronger result. Here we present a conceptual construction of a counterexample, requiring essentially no calculations. Define a word to be a finite sequence of matrixa's and matrixb's. (The empty sequence \\( \\emptyset \\) is also a word.) Let \\( setwords \\) be a finite set of words containing matrixbmatrixamatrixbmatrixa and its \"right subsequences\" matrixamatrixbmatrixa, matrixbmatrixa, matrixa, \\( \\emptyset \\), but not containing any word having matrixamatrixbmatrixamatrixb as a subsequence. Consider a vector space with basis corresponding to these words (i.e., \\( e_{\\mathbf{matrixb matrixa matrixb matrixa}}, e_{\\mathbf{matrixa matrixb matrixa}}, e_{\\emptyset} \\), etc.). Let \\( \\mathbf{matrixa} \\) be the linear transformation mapping \\( basisword \\) to \\( e_{\\mathbf{matrixa} wordvar} \\) if \\( \\mathbf{matrixa}\\, wordvar \\in setwords \\) and to 0 otherwise. Define a linear transformation \\( \\mathbf{matrixb} \\) similarly. Then \\( \\mathbf{matrixa matrixb matrixa matrixb}=0 \\) but \\( \\mathbf{matrixb matrixa matrixb matrixa} \\neq 0 \\). (This gives a very general way of dealing with any problem of this type.)\n\nRemark. To help us find a counterexample, we imposed the restriction that each of \\( \\mathbf{matrixa} \\) and \\( \\mathbf{matrixb} \\) maps each standard basis vector \\( basisi \\) to some \\( basisj \\) or to 0. With this restriction, the problem can be restated in terms of automata theory:\n\nDoes there exist a finite automaton with a set of states \\( statesset=\\left\\{0, basisone, basistwo, \\ldots, basisn\\right\\} \\) in which all states are initial states and all but 0 are final states, and a set of two productions \\( \\{\\mathbf{matrixa}, \\mathbf{matrixb}\\} \\) each mapping 0 to 0, such that the language it accepts contains matrixamatrixbmatrixamatrixb but not \\( \\mathbf{matrixb matrixa matrixb matrixa} \\)?\nSee Chapter 3 of \\( [\\mathrm{Sa}] \\) for terminology. The language accepted by such a finite automaton is defined as the set of words in \\( \\mathbf{matrixa} \\) and \\( \\mathbf{matrixb} \\) that correspond to a sequence of productions leading from some initial state to some final state. Technically, since our finite automaton does not have a unique initial state, it is called nondeterministic, even though each production maps any given state to a unique state. (Many authors [HoU, p. 20] do not allow multiple initial states, even in nondeterministic finite automata; we could circumvent this by introducing a new artificial initial state, with a new nondeterministic production mapping it to the desired initial states.) One theorem of automata theory \\( [\\mathrm{Sa}, \\text{Theorem } 3.3] \\) is that any language accepted by a nondeterministic finite automaton is also the language accepted by some deterministic finite automaton.\n\nMany lexical scanners, such as the UNIX utility grep [ Hu ], are based on the theory of finite automata. See the remark in 1989A6 for the appearance of automata theory in a very different context."
    },
    "descriptive_long_confusing": {
      "map": {
        "A": "sandstone",
        "B": "lighthouse",
        "M": "polymerase",
        "e_1": "driftwood",
        "e_2": "gravelpit",
        "e_3": "corkscrew",
        "e_4": "trefoilkn",
        "e_w": "thunderbay",
        "e_i": "stalemate",
        "e_j": "breadline",
        "e_n": "centerpiece",
        "w": "moonlight",
        "x": "quasimodo",
        "S": "aftershock",
        "n": "lemongrass",
        "\\\\Sigma": "orchidseed"
      },
      "question": "same size such that $\\mathbf{ABAB = 0}$, does it follow that\n$\\mathbf{BABA = 0}$?",
      "solution": "Solution 1. Direct multiplication shows that the \\( 3 \\times 3 \\) matrices\n\\[\n\\mathbf{sandstone}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n0 & 0 & 0 \\\\\n0 & 1 & 0\n\\end{array}\\right), \\quad \\mathbf{lighthouse}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n1 & 0 & 0 \\\\\n0 & 0 & 0\n\\end{array}\\right)\n\\]\ngive a counterexample.\n\nSolution 2. A more enlightening way to construct a counterexample is to use a transition diagram, as in the following example. Let \\( driftwood, gravelpit, corkscrew, trefoilkn \\) be a basis of a four-dimensional vector space. Represent the matrices as in Figure 15. For example, the arrow from \\( trefoilkn \\) to \\( corkscrew \\) labelled \\( \\mathbf{lighthouse} \\) indicates that \\( \\mathbf{lighthouse} trefoilkn=corkscrew \\); the arrow from \\( driftwood \\) to 0 indicates that \\( \\mathbf{sandstone} driftwood=0 \\). Then it can be quickly checked that \\( \\mathbf{sandstone\\ lighthouse\\ sandstone\\ lighthouse} \\) annihilates the four basis vectors, but \\( \\mathbf{lighthouse\\ sandstone\\ lighthouse\\ sandstone} trefoilkn=driftwood \\). (Be careful with the order of multiplication when checking!)\n\nRemark. The counterexample of Solution 1 also can be obtained from a transition diagram.\n\nRemark. There are no \\( 1 \\times 1 \\) or \\( 2 \\times 2 \\) counterexamples. The \\( 1 \\times 1 \\) case is clear. For the \\( 2 \\times 2 \\) case, observe that \\( \\mathbf{sandstone\\ lighthouse\\ sandstone\\ lighthouse}=\\mathbf{0} \\) implies \\( \\mathbf{lighthouse}(\\mathbf{sandstone\\ lighthouse\\ sandstone\\ lighthouse}) \\mathbf{sandstone}=\\mathbf{0} \\), and hence \\( \\mathbf{lighthouse\\ sandstone} \\) is nilpotent. But if a \\( 2 \\times 2 \\) matrix \\( \\mathbf{polymerase} \\) is nilpotent, its characteristic polynomial is \\( quasimodo^{2} \\), so \\( \\mathbf{polymerase}^{2}=\\mathbf{0} \\) by the Cayley-Hamilton Theorem \\( [A p 2 , \\text{Theorem }7.8] \\). Thus lighthouse sandstone lighthouse sandstone \\( =\\mathbf{0} \\).\n\nRemark. For any \\( lemongrass \\geq 3 \\), there exist \\( lemongrass \\times lemongrass \\) counterexamples: enlarge the matrices in (1) by adding rows and columns of zeros.\n\nStronger result. Here we present a conceptual construction of a counterexample, requiring essentially no calculations. Define a word to be a finite sequence of sandstone's and lighthouse's. (The empty sequence \\( \\emptyset \\) is also a word.) Let \\( aftershock \\) be a finite set of words containing lighthouse sandstone lighthouse sandstone and its \"right subsequences\" lighthouse sandstone lighthouse, sandstone lighthouse, sandstone, \\( \\emptyset \\), but not containing any word having sandstone lighthouse sandstone lighthouse as a subsequence. Consider a vector space with basis corresponding to these words (i.e., \\( e_{\\mathbf{lighthouse\\ sandstone\\ lighthouse\\ sandstone}}, e_{\\mathbf{sandstone\\ lighthouse\\ sandstone}}, e_{\\emptyset} \\), etc.). Let \\( \\mathbf{sandstone} \\) be the linear transformation mapping thunderbay to \\( e_{\\mathbf{sandstone} moonlight} \\) if \\( \\mathbf{sandstone} moonlight \\in aftershock \\) and to 0 otherwise. Define a linear transformation \\( \\mathbf{lighthouse} \\) similarly. Then \\( \\mathbf{sandstone\\ lighthouse\\ sandstone\\ lighthouse}=0 \\) but \\( \\mathbf{lighthouse\\ sandstone\\ lighthouse\\ sandstone} \\neq 0 \\). (This gives a very general way of dealing with any problem of this type.)\n\nRemark. To help us find a counterexample, we imposed the restriction that each of \\( \\mathbf{sandstone} \\) and \\( \\mathbf{lighthouse} \\) maps each standard basis vector \\( stalemate \\) to some \\( breadline \\) or to 0 . With this restriction, the problem can be restated in terms of automata theory:\n\nDoes there exist a finite automaton with a set of states \\( orchidseed=\\{0, driftwood, gravelpit, \\ldots, centerpiece\\} \\) in which all states are initial states and all but 0 are final states, and a set of two productions \\{\\( \\mathbf{sandstone}, \\mathbf{lighthouse} \\)\\} each mapping 0 to 0 , such that the language it accepts contains sandstone lighthouse sandstone lighthouse but not \\( \\mathbf{lighthouse\\ sandstone\\ lighthouse\\ sandstone} \\) ?\nSee Chapter 3 of \\( [\\mathrm{Sa}] \\) for terminology. The language accepted by such a finite automaton is defined as the set of words in \\( \\mathbf{sandstone} \\) and \\( \\mathbf{lighthouse} \\) that correspond to a sequence of productions leading from some initial state to some final state. Technically, since our finite automaton does not have a unique initial state, it is called nondeterministic, even though each production maps any given state to a unique state. (Many authors [HoU, p. 20] do not allow multiple initial states, even in nondeterministic finite automata; we could circumvent this by introducing a new artificial initial state, with a new nondeterministic production mapping it to the desired initial states.) One theorem of automata theory \\( [\\mathrm{Sa} , \\text{Theorem }3.3] \\) is that any language accepted by a nondeterministic finite automaton is also the language accepted by some deterministic finite automaton.\n\nMany lexical scanners, such as the UNIX utility grep [ Hu ], are based on the theory of finite automata. See the remark in 1989A6 for the appearance of automata theory in a very different context."
    },
    "descriptive_long_misleading": {
      "map": {
        "A": "nonnilpot",
        "B": "commuting",
        "M": "diagonal",
        "e_1": "voidvecone",
        "e_2": "voidvectwo",
        "e_3": "voidvecthree",
        "e_4": "voidvecfour",
        "e_w": "voidvecw",
        "e_i": "voidveci",
        "e_j": "voidvecj",
        "e_n": "voidvecn",
        "w": "silencewd",
        "x": "constant",
        "S": "emptyset",
        "n": "microsize",
        "\\Sigma": "nullalpha"
      },
      "question": "same size such that $\\mathbf{nonnilpotcommutingnonnilpotcommuting = 0}$, does it follow that\n$\\mathbf{commutingnonnilpotcommutingnonnilpot = 0}$?",
      "solution": "Solution 1. Direct multiplication shows that the \\( 3 \\times 3 \\) matrices\n\\[\n\\mathbf{nonnilpot}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n0 & 0 & 0 \\\\\n0 & 1 & 0\n\\end{array}\\right), \\quad \\mathbf{commuting}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n1 & 0 & 0 \\\\\n0 & 0 & 0\n\\end{array}\\right)\n\\]\ngive a counterexample.\n\nSolution 2. A more enlightening way to construct a counterexample is to use a transition diagram, as in the following example. Let \\( voidvecone, voidvectwo, voidvecthree, voidvecfour \\) be a basis of a four-dimensional vector space. Represent the matrices as in Figure 15. For example, the arrow from \\( voidvecfour \\) to \\( voidvecthree \\) labelled \\( \\mathbf{commuting} \\) indicates that \\( \\mathbf{commuting}\\,voidvecfour=voidvecthree \\); the arrow from \\( voidvecone \\) to 0 indicates that \\( \\mathbf{nonnilpot}\\,voidvecone=0 \\). Then it can be quickly checked that \\( \\mathbf{nonnilpotcommutingnonnilpotcommuting} \\) annihilates the four basis vectors, but \\( \\mathbf{commutingnonnilpotcommutingnonnilpot}\\,voidvecfour=voidvecone \\). (Be careful with the order of multiplication when checking!)\n\nRemark. The counterexample of Solution 1 also can be obtained from a transition diagram.\n\nRemark. There are no \\( 1 \\times 1 \\) or \\( 2 \\times 2 \\) counterexamples. The \\( 1 \\times 1 \\) case is clear. For the \\( 2 \\times 2 \\) case, observe that \\( \\mathbf{nonnilpotcommutingnonnilpotcommuting}=\\mathbf{0} \\) implies \\( \\mathbf{commuting}(\\mathbf{nonnilpotcommutingnonnilpotcommuting}) \\mathbf{nonnilpot}=\\mathbf{0} \\), and hence \\( \\mathbf{commutingnonnilpot} \\) is nilpotent. But if a \\( 2 \\times 2 \\) matrix \\( \\mathbf{diagonal} \\) is nilpotent, its characteristic polynomial is \\( constant^{2} \\), so \\( \\mathbf{diagonal}^{2}=\\mathbf{0} \\) by the Cayley-Hamilton Theorem \\( [nonnilpot p 2 , Theorem 7.8] \\). Thus commutingnonnilpotcommutingnonnilpot \\( =\\mathbf{0} \\).\n\nRemark. For any \\( microsize \\geq 3 \\), there exist \\( microsize \\times microsize \\) counterexamples: enlarge the matrices in (1) by adding rows and columns of zeros.\n\nStronger result. Here we present a conceptual construction of a counterexample, requiring essentially no calculations. Define a word to be a finite sequence of nonnilpots and commutings. (The empty sequence \\( \\emptyset \\) is also a word.) Let \\( emptyset \\) be a finite set of words containing commutingnonnilpotcommutingnonnilpot and its \"right subsequences\" nonnilpotcommutingnonnilpot, commutingnonnilpot, nonnilpot, \\( \\emptyset \\), but not containing any word having nonnilpotcommutingnonnilpotcommuting as a subsequence. Consider a vector space with basis corresponding to these words (i.e., \\( e_{\\mathbf{commutingnonnilpotcommutingnonnilpot}}, e_{\\mathbf{nonnilpotcommutingnonnilpot}}, e_{\\emptyset}, \\) etc.). Let \\( \\mathbf{nonnilpot} \\) be the linear transformation mapping \\( e_{silencewd} \\) to \\( e_{\\mathbf{nonnilpot} silencewd} \\) if \\( \\mathbf{nonnilpot} silencewd \\in emptyset \\) and to 0 otherwise. Define a linear transformation \\( \\mathbf{commuting} \\) similarly. Then \\( \\mathbf{nonnilpotcommutingnonnilpotcommuting}=0 \\) but \\( \\mathbf{commutingnonnilpotcommutingnonnilpot} \\neq 0 \\). (This gives a very general way of dealing with any problem of this type.)\n\nRemark. To help us find a counterexample, we imposed the restriction that each of \\( \\mathbf{nonnilpot} \\) and \\( \\mathbf{commuting} \\) maps each standard basis vector \\( voidveci \\) to some \\( voidvecj \\) or to 0. With this restriction, the problem can be restated in terms of automata theory:\n\nDoes there exist a finite automaton with a set of states \\( nullalpha=\\left\\{0, voidvecone, voidvectwo, \\ldots, voidvecn\\right\\} \\) in which all states are initial states and all but 0 are final states, and a set of two productions \\( \\{\\mathbf{nonnilpot}, \\mathbf{commuting}\\} \\) each mapping 0 to 0, such that the language it accepts contains nonnilpotcommutingnonnilpotcommuting but not \\( \\mathbf{commutingnonnilpotcommutingnonnilpot} \\)?\nSee Chapter 3 of \\( [\\mathrm{Sa}] \\) for terminology. The language accepted by such a finite automaton is defined as the set of words in \\( \\mathbf{nonnilpot} \\) and \\( \\mathbf{commuting} \\) that correspond to a sequence of productions leading from some initial state to some final state. Technically, since our finite automaton does not have a unique initial state, it is called nondeterministic, even though each production maps any given state to a unique state. (Many authors [HoU, p. 20] do not allow multiple initial states, even in nondeterministic finite automata; we could circumvent this by introducing a new artificial initial state, with a new nondeterministic production mapping it to the desired initial states.) One theorem of automata theory \\( [\\mathrm{Sa}, \\) Theorem 3.3] is that any language accepted by a nondeterministic finite automaton is also the language accepted by some deterministic finite automaton.\n\nMany lexical scanners, such as the UNIX utility grep [ Hu ], are based on the theory of finite automata. See the remark in 1989A6 for the appearance of automata theory in a very different context."
    },
    "garbled_string": {
      "map": {
        "A": "qzxwvtnp",
        "B": "hjgrksla",
        "M": "vckdpsru",
        "e_1": "mnbaswqe",
        "e_2": "zplktyur",
        "e_3": "xchamvbf",
        "e_4": "grydolsi",
        "e_w": "ksuqnjdp",
        "e_i": "thwgrevb",
        "e_j": "pucnjeof",
        "e_n": "vbmqdlas",
        "w": "hxtprnvo",
        "x": "lqzdmrwe",
        "S": "fuyraced",
        "n": "owkbdtla",
        "\\Sigma": "bwkztrhe"
      },
      "question": "same size such that $\\mathbf{qzxwvtnphjgrkslaqzxwvtnphjgrksla = 0}$, does it follow that\n$\\mathbf{hjgrkslaqzxwvtnphjgrkslaqzxwvtnp = 0}$?",
      "solution": "Solution 1. Direct multiplication shows that the \\( 3 \\times 3 \\) matrices\n\\[\n\\mathbf{qzxwvtnp}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n0 & 0 & 0 \\\\\n0 & 1 & 0\n\\end{array}\\right), \\quad \\mathbf{hjgrksla}=\\left(\\begin{array}{lll}\n0 & 0 & 1 \\\\\n1 & 0 & 0 \\\\\n0 & 0 & 0\n\\end{array}\\right)\n\\]\ngive a counterexample.\nSolution 2. A more enlightening way to construct a counterexample is to use a transition diagram, as in the following example. Let \\( mnbaswqe, zplktyur, xchamvbf, grydolsi \\) be a basis of a four-dimensional vector space. Represent the matrices as in Figure 15. For example, the arrow from \\( grydolsi \\) to \\( xchamvbf \\) labelled \\( \\mathbf{hjgrksla} \\) indicates that \\( \\mathbf{hjgrksla}\\, grydolsi = xchamvbf \\); the arrow from \\( mnbaswqe \\) to 0 indicates that \\( \\mathbf{qzxwvtnp}\\, mnbaswqe = 0 \\). Then it can be quickly checked that \\( \\mathbf{qzxwvtnp hjgrksla qzxwvtnp hjgrksla} \\) annihilates the four basis vectors, but \\( \\mathbf{hjgrksla qzxwvtnp hjgrksla qzxwvtnp}\\, grydolsi = mnbaswqe \\). (Be careful with the order of multiplication when checking!)\n\nRemark. The counterexample of Solution 1 also can be obtained from a transition diagram.\n\nRemark. There are no \\( 1 \\times 1 \\) or \\( 2 \\times 2 \\) counterexamples. The \\( 1 \\times 1 \\) case is clear. For the \\( 2 \\times 2 \\) case, observe that \\( \\mathbf{qzxwvtnp hjgrksla qzxwvtnp hjgrksla}=\\mathbf{0} \\) implies \\( \\mathbf{hjgrksla}(\\mathbf{qzxwvtnp hjgrksla qzxwvtnp hjgrksla})\\mathbf{qzxwvtnp}=\\mathbf{0} \\), and hence \\( \\mathbf{hjgrksla qzxwvtnp} \\) is nilpotent. But if a \\( 2 \\times 2 \\) matrix \\( \\mathbf{vckdpsru} \\) is nilpotent, its characteristic polynomial is \\( lqzdmrwe^{2} \\), so \\( \\mathbf{vckdpsru}^{2}=\\mathbf{0} \\) by the Cayley–Hamilton Theorem \\( [qzxwvtnp p 2, \\) Theorem 7.8]. Thus hjgrkslaqzxwvtnphjgrkslaqzxwvtnp \\( =\\mathbf{0} \\).\n\nRemark. For any \\( owkbdtla \\geq 3 \\), there exist \\( owkbdtla \\times owkbdtla \\) counterexamples: enlarge the matrices in (1) by adding rows and columns of zeros.\n\nStronger result. Here we present a conceptual construction of a counterexample, requiring essentially no calculations. Define a word to be a finite sequence of qzxwvtnp's and hjgrksla's. (The empty sequence \\( \\emptyset \\) is also a word.) Let \\( fuyraced \\) be a finite set of words containing hjgrkslaqzxwvtnphjgrkslaqzxwvtnp and its \"right subsequences\" qzxwvtnphjgrkslaqzxwvtnp, hjgrkslaqzxwvtnp, qzxwvtnp, \\( \\emptyset \\), but not containing any word having qzxwvtnphjgrkslaqzxwvtnphjgrksla as a subsequence. Consider a vector space with basis corresponding to these words (i.e., \\( e_{\\mathbf{hjgrksla qzxwvtnp hjgrksla qzxwvtnp}}, e_{\\mathbf{qzxwvtnp hjgrksla qzxwvtnp}}, e_{\\emptyset} \\), etc.). Let \\( \\mathbf{qzxwvtnp} \\) be the linear transformation mapping \\( ksuqnjdp \\) to \\( e_{\\mathbf{qzxwvtnp} hxtprnvo} \\) if \\( \\mathbf{qzxwvtnp} hxtprnvo \\in fuyraced \\) and to 0 otherwise. Define a linear transformation \\( \\mathbf{hjgrksla} \\) similarly. Then \\( \\mathbf{qzxwvtnp hjgrksla qzxwvtnp hjgrksla}=0 \\) but \\( \\mathbf{hjgrksla qzxwvtnp hjgrksla qzxwvtnp} \\neq 0 \\). (This gives a very general way of dealing with any problem of this type.)\n\nRemark. To help us find a counterexample, we imposed the restriction that each of \\( \\mathbf{qzxwvtnp} \\) and \\( \\mathbf{hjgrksla} \\) maps each standard basis vector \\( thwgrevb \\) to some \\( pucnjeof \\) or to 0. With this restriction, the problem can be restated in terms of automata theory:\n\nDoes there exist a finite automaton with a set of states \\( bwkztrhe=\\left\\{0, mnbaswqe, zplktyur, \\ldots, vbmqdlas\\right\\} \\) in which all states are initial states and all but 0 are final states, and a set of two productions \\( \\{\\mathbf{qzxwvtnp}, \\mathbf{hjgrksla}\\} \\) each mapping 0 to 0, such that the language it accepts contains qzxwvtnphjgrkslaqzxwvtnphjgrksla but not \\( \\mathbf{hjgrksla qzxwvtnp hjgrksla qzxwvtnp} \\)?\nSee Chapter 3 of \\( [\\mathrm{Sa}] \\) for terminology. The language accepted by such a finite automaton is defined as the set of words in \\( \\mathbf{qzxwvtnp} \\) and \\( \\mathbf{hjgrksla} \\) that correspond to a sequence of productions leading from some initial state to some final state. Technically, since our finite automaton does not have a unique initial state, it is called nondeterministic, even though each production maps any given state to a unique state. (Many authors [HoU, p. 20] do not allow multiple initial states, even in nondeterministic finite automata; we could circumvent this by introducing a new artificial initial state, with a new nondeterministic production mapping it to the desired initial states.) One theorem of automata theory \\( [\\mathrm{Sa}, \\) Theorem 3.3] is that any language accepted by a nondeterministic finite automaton is also the language accepted by some deterministic finite automaton.\n\nMany lexical scanners, such as the UNIX utility grep [ Hu ], are based on the theory of finite automata. See the remark in 1989A6 for the appearance of automata theory in a very different context."
    },
    "kernel_variant": {
      "question": "Let \\(A\\) and \\(B\\) be real \\(4\\times4\\) matrices and suppose that the product \\(ABAB\\) is the zero matrix.  Must the reverse product \\(BABA\\) also be zero?  Either prove that it must or, if not, give an explicit counterexample (that is, concrete \\(4\\times4\\) matrices \\(A,B\\) for which \\(ABAB=0\\) but \\(BABA\\ne0\\)).",
      "solution": "Counterexamples already occur in size four.  Take\n\\[\nA=\\begin{pmatrix}\n0&0&0&2\\\\[2pt]\n0&0&0&0\\\\[2pt]\n0&0&0&0\\\\[2pt]\n0&0&2&0\n\\end{pmatrix},\\qquad\nB=\\begin{pmatrix}\n0&0&0&2\\\\[2pt]\n0&0&0&0\\\\[2pt]\n2&0&0&0\\\\[2pt]\n0&0&0&0\n\\end{pmatrix}.\n\\]\nIn words: A sends e₄↦2e₁ and e₃↦2e₄; B sends e₁↦2e₃ and e₄↦2e₁.\n\n1. Compute AB.  On basis vectors,\n   • B e₁=2e₃, A e₃=2e₄ ⇒ AB e₁=4e₄;\n   • B eⱼ=0 for j=2,3 ⇒ AB eⱼ=0;\n   • B e₄=2e₁, A e₁=0 ⇒ AB e₄=0.\nHence Im(AB)=Span{e₄} and AB e₄=0, so (AB)²=0, i.e. ABAB=0.\n\n2. Compute BABA e₄:\n   A e₄=2e₁,\n   B(2e₁)=4e₃,\n   A(4e₃)=8e₄,\n   B(8e₄)=16e₁.\nThus BABA e₄=16e₁≠0, so BABA≠0.\n\nTherefore ABAB=0 does not force BABA=0 in dimension 4 (and similarly for any n≥4 by padding with zeros).  This completes the requested counterexample.",
      "_meta": {
        "core_steps": [
          "Look for a square‐matrix counterexample with size ≥3.",
          "Define A and B as (partial) permutation/nilpotent matrices so that AB sends every basis vector to 0 in two steps.",
          "Verify by direct multiplication that ABAB = 0.",
          "Show that the reverse product BABA acts non-trivially on at least one basis vector, hence BABA ≠ 0."
        ],
        "mutable_slots": {
          "slot_dimension": {
            "description": "Order of the matrices used in the counterexample; any size ≥3 works by padding with zero rows/columns.",
            "original": "3"
          },
          "slot_scalar": {
            "description": "Actual non-zero entries chosen for the permutation‐type matrices; any non-zero scalars preserve the argument.",
            "original": "1"
          },
          "slot_permutation": {
            "description": "Placement of the non-zero entries (i.e., which basis vectors are linked); any simultaneous relabelling of the basis gives a conjugate pair with the same property.",
            "original": "identity ordering of the standard basis"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}