{
  "index": "2016-A-4",
  "type": "COMB",
  "tag": [
    "COMB",
    "GEO"
  ],
  "difficulty": "",
  "question": "Consider a $(2m-1) \\times (2n-1)$ rectangular region, where $m$ and $n$ are integers such that $m, n \\geq 4$. This region is to be tiled using tiles of the two types shown:\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(2724,924)(1339,-1423)\n\\thinlines\n{\\put(1351,-511){\\line( 0,-1){900}}\n\\put(1351,-1411){\\line( 1, 0){450}}\n\\put(1801,-1411){\\line( 0, 1){450}}\n\\put(1801,-961){\\line( 1, 0){450}}\n\\put(2251,-961){\\line( 0, 1){450}}\n\\put(2251,-511){\\line(-1, 0){900}}\n}%\n{\\multiput(1351,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(1801,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n{\\put(2701,-961){\\line( 0,-1){450}}\n\\put(2701,-1411){\\line( 1, 0){900}}\n\\put(3601,-1411){\\line( 0, 1){450}}\n\\put(3601,-961){\\line( 1, 0){450}}\n\\put(4051,-961){\\line( 0, 1){450}}\n\\put(4051,-511){\\line(-1, 0){900}}\n\\put(3151,-511){\\line( 0,-1){450}}\n\\put(3151,-961){\\line(-1, 0){450}}\n}%\n{\\multiput(3151,-1411)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n\\multiput(3151,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(3601,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n\\end{picture}%\n\\]\n(The dotted lines divide the tiles into $1 \\times 1$ squares.)\nThe tiles may be rotated and reflected, as long as their sides are parallel to the sides\nof the rectangular region. They must all fit within the region, and they must cover it completely without overlapping.\n\nWhat is the minimum number of tiles required to tile the region?",
  "solution": "The minimum number of tiles is $mn$. To see that this many are required, label the squares $(i,j)$ with $1\\leq i\\leq 2m-1$ and $1\\leq j\\leq 2n-1$, and for each square with $i,j$ both odd, color the square red; then no tile can cover more than one red square, and there are $mn$ red squares.\n\nIt remains to show that we can cover any $(2m-1) \\times (2n-1)$ rectangle with $mn$ tiles when $m,n \\geq 4$. First note that we can tile any $2 \\times (2k-1)$ rectangle with $k\\geq 3$ by $k$ tiles: one of the first type, then $k-2$ of the second type, and finally one of the first type. Thus if we can cover a $7\\times 7$ square with $16$ tiles, then we can do the general $(2m-1) \\times (2n-1)$ rectangle, by decomposing this rectangle into a $7\\times 7$ square in the lower left corner, along with $m-4$ $(2\\times 7)$ rectangles to the right of the square, and $n-4$ $((2m-1)\\times 2)$ rectangles above, and tiling each of these rectangles separately, for a total of $16+4(m-4)+m(n-4) = mn$ tiles.\n\nTo cover the $7 \\times 7$ square, note that the tiling must consist of 15 tiles of the first type and 1 of the second type, and that any $2 \\times 3$ rectangle can be covered using 2 tiles of the first type. We may thus construct a suitable covering by covering all but the center square with eight $2 \\times 3$ rectangles, in such a way that we can attach the center square to one of these rectangles to get a shape that can be covered by two tiles. An example of such a covering, with the remaining $2 \\times 3$ rectangles left intact for visual clarity, is depicted below. (Many other solutions are possible.)\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(3174,3174)(1339,-7723)\n\\thinlines\n{\\put(1351,-7711){\\framebox(3150,3150){}}\n}%\n{\\put(1351,-5911){\\line( 1, 0){945}}\n\\put(2296,-5911){\\line( 0, 1){1350}}\n}%\n{\\put(2296,-5911){\\line( 1, 0){405}}\n\\put(2701,-5911){\\line( 0,-1){1800}}\n}%\n{\\put(1351,-6811){\\line( 1, 0){1350}}\n}%\n{\\put(2701,-6361){\\line( 1, 0){1800}}\n}%\n{\\put(3601,-6361){\\line( 0,-1){1350}}\n}%\n{\\put(3151,-6361){\\line( 0, 1){1800}}\n}%\n{\\put(3151,-5461){\\line( 1, 0){1350}}\n}%\n{\\multiput(2296,-5011)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(2746,-5011)(0.00000,-128.57143){4}{\\line( 0,-1){ 64.286}}\n\\multiput(2746,-5461)(115.71429,0.00000){4}{\\line( 1, 0){ 57.857}}\n}%\n\\end{picture}%\n\\]",
  "vars": [
    "i",
    "j",
    "k"
  ],
  "params": [
    "m",
    "n"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "m": "rowcount",
        "n": "colcount",
        "i": "indexrow",
        "j": "indexcol",
        "k": "tilecount"
      },
      "question": "Consider a $(2rowcount-1) \\times (2colcount-1)$ rectangular region, where $rowcount$ and $colcount$ are integers such that $rowcount, colcount \\geq 4$. This region is to be tiled using tiles of the two types shown:\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(2724,924)(1339,-1423)\n\\thinlines\n{\\put(1351,-511){\\line( 0,-1){900}}\n\\put(1351,-1411){\\line( 1, 0){450}}\n\\put(1801,-1411){\\line( 0, 1){450}}\n\\put(1801,-961){\\line( 1, 0){450}}\n\\put(2251,-961){\\line( 0, 1){450}}\n\\put(2251,-511){\\line(-1, 0){900}}\n}%\n{\\multiput(1351,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(1801,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n{\\put(2701,-961){\\line( 0,-1){450}}\n\\put(2701,-1411){\\line( 1, 0){900}}\n\\put(3601,-1411){\\line( 0, 1){450}}\n\\put(3601,-961){\\line( 1, 0){450}}\n\\put(4051,-961){\\line( 0, 1){450}}\n\\put(4051,-511){\\line(-1, 0){900}}\n\\put(3151,-511){\\line( 0,-1){450}}\n\\put(3151,-961){\\line(-1, 0){450}}\n}%\n{\\multiput(3151,-1411)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n\\multiput(3151,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(3601,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n\\end{picture}%\n\\]\n(The dotted lines divide the tiles into $1 \\times 1$ squares.)\nThe tiles may be rotated and reflected, as long as their sides are parallel to the sides\nof the rectangular region. They must all fit within the region, and they must cover it completely without overlapping.\n\nWhat is the minimum number of tiles required to tile the region?",
      "solution": "The minimum number of tiles is $rowcount colcount$. To see that this many are required, label the squares $(indexrow,indexcol)$ with $1\\leq indexrow\\leq 2rowcount-1$ and $1\\leq indexcol\\leq 2colcount-1$, and for each square with $indexrow,indexcol$ both odd, color the square red; then no tile can cover more than one red square, and there are $rowcount colcount$ red squares.\n\nIt remains to show that we can cover any $(2rowcount-1) \\times (2colcount-1)$ rectangle with $rowcount colcount$ tiles when $rowcount,colcount \\geq 4$. First note that we can tile any $2 \\times (2tilecount-1)$ rectangle with $tilecount\\geq 3$ by $tilecount$ tiles: one of the first type, then $tilecount-2$ of the second type, and finally one of the first type. Thus if we can cover a $7\\times 7$ square with $16$ tiles, then we can do the general $(2rowcount-1) \\times (2colcount-1)$ rectangle, by decomposing this rectangle into a $7\\times 7$ square in the lower left corner, along with $rowcount-4$ $(2\\times 7)$ rectangles to the right of the square, and $colcount-4$ $((2rowcount-1)\\times 2)$ rectangles above, and tiling each of these rectangles separately, for a total of $16+4(rowcount-4)+rowcount(colcount-4)=rowcount colcount$ tiles.\n\nTo cover the $7 \\times 7$ square, note that the tiling must consist of 15 tiles of the first type and 1 of the second type, and that any $2 \\times 3$ rectangle can be covered using 2 tiles of the first type. We may thus construct a suitable covering by covering all but the center square with eight $2 \\times 3$ rectangles, in such a way that we can attach the center square to one of these rectangles to get a shape that can be covered by two tiles. An example of such a covering, with the remaining $2 \\times 3$ rectangles left intact for visual clarity, is depicted below. (Many other solutions are possible.)\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(3174,3174)(1339,-7723)\n\\thinlines\n{\\put(1351,-7711){\\framebox(3150,3150){}}\n}%\n{\\put(1351,-5911){\\line( 1, 0){945}}\n\\put(2296,-5911){\\line( 0, 1){1350}}\n}%\n{\\put(2296,-5911){\\line( 1, 0){405}}\n\\put(2701,-5911){\\line( 0,-1){1800}}\n}%\n{\\put(1351,-6811){\\line( 1, 0){1350}}\n}%\n{\\put(2701,-6361){\\line( 1, 0){1800}}\n}%\n{\\put(3601,-6361){\\line( 0,-1){1350}}\n}%\n{\\put(3151,-6361){\\line( 0, 1){1800}}\n}%\n{\\put(3151,-5461){\\line( 1, 0){1350}}\n}%\n{\\multiput(2296,-5011)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(2746,-5011)(0.00000,-128.57143){4}{\\line( 0,-1){ 64.286}}\n\\multiput(2746,-5461)(115.71429,0.00000){4}{\\line( 1, 0){ 57.857}}\n}%\n\\end{picture}%\n\\]"
    },
    "descriptive_long_confusing": {
      "map": {
        "i": "waterlily",
        "j": "thunderclap",
        "k": "cinnamon",
        "m": "pinecone",
        "n": "bookshelf"
      },
      "question": "Consider a $(2pinecone-1) \\times (2bookshelf-1)$ rectangular region, where $pinecone$ and $bookshelf$ are integers such that $pinecone, bookshelf \\geq 4$. This region is to be tiled using tiles of the two types shown:\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(2724,924)(1339,-1423)\n\\thinlines\n{\\put(1351,-511){\\line( 0,-1){900}}\n\\put(1351,-1411){\\line( 1, 0){450}}\n\\put(1801,-1411){\\line( 0, 1){450}}\n\\put(1801,-961){\\line( 1, 0){450}}\n\\put(2251,-961){\\line( 0, 1){450}}\n\\put(2251,-511){\\line(-1, 0){900}}\n}%\n{\\multiput(1351,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(1801,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n{\\put(2701,-961){\\line( 0,-1){450}}\n\\put(2701,-1411){\\line( 1, 0){900}}\n\\put(3601,-1411){\\line( 0, 1){450}}\n\\put(3601,-961){\\line( 1, 0){450}}\n\\put(4051,-961){\\line( 0, 1){450}}\n\\put(4051,-511){\\line(-1, 0){900}}\n\\put(3151,-511){\\line( 0,-1){450}}\n\\put(3151,-961){\\line(-1, 0){450}}\n}%\n{\\multiput(3151,-1411)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n\\multiput(3151,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(3601,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n\\end{picture}%\n\\]\n(The dotted lines divide the tiles into $1 \\times 1$ squares.)\nThe tiles may be rotated and reflected, as long as their sides are parallel to the sides\nof the rectangular region. They must all fit within the region, and they must cover it completely without overlapping.\n\nWhat is the minimum number of tiles required to tile the region?",
      "solution": "The minimum number of tiles is $pinecone bookshelf$. To see that this many are required, label the squares $(waterlily,\\,thunderclap)$ with $1\\leq waterlily\\leq 2pinecone-1$ and $1\\leq thunderclap\\leq 2bookshelf-1$, and for each square with $waterlily,\\,thunderclap$ both odd, color the square red; then no tile can cover more than one red square, and there are $pinecone bookshelf$ red squares.\n\nIt remains to show that we can cover any $(2pinecone-1) \\times (2bookshelf-1)$ rectangle with $pinecone bookshelf$ tiles when $pinecone, bookshelf \\geq 4$. First note that we can tile any $2 \\times (2cinnamon-1)$ rectangle with $cinnamon\\geq 3$ by $cinnamon$ tiles: one of the first type, then $cinnamon-2$ of the second type, and finally one of the first type. Thus if we can cover a $7\\times 7$ square with $16$ tiles, then we can do the general $(2pinecone-1) \\times (2bookshelf-1)$ rectangle, by decomposing this rectangle into a $7\\times 7$ square in the lower left corner, along with $pinecone-4$ $(2\\times 7)$ rectangles to the right of the square, and $bookshelf-4$ $((2pinecone-1)\\times 2)$ rectangles above, and tiling each of these rectangles separately, for a total of $16+4(pinecone-4)+pinecone(bookshelf-4) = pinecone bookshelf$ tiles.\n\nTo cover the $7 \\times 7$ square, note that the tiling must consist of 15 tiles of the first type and 1 of the second type, and that any $2 \\times 3$ rectangle can be covered using 2 tiles of the first type. We may thus construct a suitable covering by covering all but the center square with eight $2 \\times 3$ rectangles, in such a way that we can attach the center square to one of these rectangles to get a shape that can be covered by two tiles. An example of such a covering, with the remaining $2 \\times 3$ rectangles left intact for visual clarity, is depicted below. (Many other solutions are possible.)\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(3174,3174)(1339,-7723)\n\\thinlines\n{\\put(1351,-7711){\\framebox(3150,3150){}}\n}%\n{\\put(1351,-5911){\\line( 1, 0){945}}\n\\put(2296,-5911){\\line( 0, 1){1350}}\n}%\n{\\put(2296,-5911){\\line( 1, 0){405}}\n\\put(2701,-5911){\\line( 0,-1){1800}}\n}%\n{\\put(1351,-6811){\\line( 1, 0){1350}}\n}%\n{\\put(2701,-6361){\\line( 1, 0){1800}}\n}%\n{\\put(3601,-6361){\\line( 0,-1){1350}}\n}%\n{\\put(3151,-6361){\\line( 0, 1){1800}}\n}%\n{\\put(3151,-5461){\\line( 1, 0){1350}}\n}%\n{\\multiput(2296,-5011)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(2746,-5011)(0.00000,-128.57143){4}{\\line( 0,-1){ 64.286}}\n\\multiput(2746,-5461)(115.71429,0.00000){4}{\\line( 1, 0){ 57.857}}\n}%\n\\end{picture}%\n\\]\n"
    },
    "descriptive_long_misleading": {
      "map": {
        "i": "wholesets",
        "j": "totalview",
        "k": "entirety",
        "m": "unstable",
        "n": "variable"
      },
      "question": "Consider a $(2unstable-1) \\times (2variable-1)$ rectangular region, where $unstable$ and $variable$ are integers such that $unstable, variable \\geq 4$. This region is to be tiled using tiles of the two types shown:\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(2724,924)(1339,-1423)\n\\thinlines\n{\\put(1351,-511){\\line( 0,-1){900}}\n\\put(1351,-1411){\\line( 1, 0){450}}\n\\put(1801,-1411){\\line( 0, 1){450}}\n\\put(1801,-961){\\line( 1, 0){450}}\n\\put(2251,-961){\\line( 0, 1){450}}\n\\put(2251,-511){\\line(-1, 0){900}}\n}%\n{\\multiput(1351,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(1801,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n{\\put(2701,-961){\\line( 0,-1){450}}\n\\put(2701,-1411){\\line( 1, 0){900}}\n\\put(3601,-1411){\\line( 0, 1){450}}\n\\put(3601,-961){\\line( 1, 0){450}}\n\\put(4051,-961){\\line( 0, 1){450}}\n\\put(4051,-511){\\line(-1, 0){900}}\n\\put(3151,-511){\\line( 0,-1){450}}\n\\put(3151,-961){\\line(-1, 0){450}}\n}%\n{\\multiput(3151,-1411)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n\\multiput(3151,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(3601,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n\\end{picture}%\n\\]\n(The dotted lines divide the tiles into $1 \\times 1$ squares.)\nThe tiles may be rotated and reflected, as long as their sides are parallel to the sides\nof the rectangular region. They must all fit within the region, and they must cover it completely without overlapping.\n\nWhat is the minimum number of tiles required to tile the region?",
      "solution": "The minimum number of tiles is $unstable variable$. To see that this many are required, label the squares $(wholesets,totalview)$ with $1\\leq wholesets\\leq 2unstable-1$ and $1\\leq totalview\\leq 2variable-1$, and for each square with $wholesets,totalview$ both odd, color the square red; then no tile can cover more than one red square, and there are $unstable variable$ red squares.\n\nIt remains to show that we can cover any $(2unstable-1) \\times (2variable-1)$ rectangle with $unstable variable$ tiles when $unstable,variable \\geq 4$. First note that we can tile any $2 \\times (2entirety-1)$ rectangle with $entirety\\geq 3$ by $entirety$ tiles: one of the first type, then $entirety-2$ of the second type, and finally one of the first type. Thus if we can cover a $7\\times 7$ square with $16$ tiles, then we can do the general $(2unstable-1) \\times (2variable-1)$ rectangle, by decomposing this rectangle into a $7\\times 7$ square in the lower left corner, along with $unstable-4$ $(2\\times 7)$ rectangles to the right of the square, and $variable-4$ $((2unstable-1)\\times 2)$ rectangles above, and tiling each of these rectangles separately, for a total of $16+4(unstable-4)+unstable(variable-4) = unstable variable$ tiles.\n\nTo cover the $7 \\times 7$ square, note that the tiling must consist of 15 tiles of the first type and 1 of the second type, and that any $2 \\times 3$ rectangle can be covered using 2 tiles of the first type. We may thus construct a suitable covering by covering all but the center square with eight $2 \\times 3$ rectangles, in such a way that we can attach the center square to one of these rectangles to get a shape that can be covered by two tiles. An example of such a covering, with the remaining $2 \\times 3$ rectangles left intact for visual clarity, is depicted below. (Many other solutions are possible.)\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(3174,3174)(1339,-7723)\n\\thinlines\n{\\put(1351,-7711){\\framebox(3150,3150){}}\n}%\n{\\put(1351,-5911){\\line( 1, 0){945}}\n\\put(2296,-5911){\\line( 0, 1){1350}}\n}%\n{\\put(2296,-5911){\\line( 1, 0){405}}\n\\put(2701,-5911){\\line( 0,-1){1800}}\n}%\n{\\put(1351,-6811){\\line( 1, 0){1350}}\n}%\n{\\put(2701,-6361){\\line( 1, 0){1800}}\n}%\n{\\put(3601,-6361){\\line( 0,-1){1350}}\n}%\n{\\put(3151,-6361){\\line( 0, 1){1800}}\n}%\n{\\put(3151,-5461){\\line( 1, 0){1350}}\n}%\n{\\multiput(2296,-5011)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(2746,-5011)(0.00000,-128.57143){4}{\\line( 0,-1){ 64.286}}\n\\multiput(2746,-5461)(115.71429,0.00000){4}{\\line( 1, 0){ 57.857}}\n}%\n\\end{picture}%\n\\]\n"
    },
    "garbled_string": {
      "map": {
        "i": "yzdblnspa",
        "j": "vqmrkcswa",
        "k": "tbzqrhenp",
        "m": "lgfhavure",
        "n": "qmrxjdeop"
      },
      "question": "Consider a $(2lgfhavure-1) \\times (2qmrxjdeop-1)$ rectangular region, where $lgfhavure$ and $qmrxjdeop$ are integers such that $lgfhavure, qmrxjdeop \\geq 4$. This region is to be tiled using tiles of the two types shown:\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(2724,924)(1339,-1423)\n\\thinlines\n{\\put(1351,-511){\\line( 0,-1){900}}\n\\put(1351,-1411){\\line( 1, 0){450}}\n\\put(1801,-1411){\\line( 0, 1){450}}\n\\put(1801,-961){\\line( 1, 0){450}}\n\\put(2251,-961){\\line( 0, 1){450}}\n\\put(2251,-511){\\line(-1, 0){900}}\n}%\n{\\multiput(1351,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(1801,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n{\\put(2701,-961){\\line( 0,-1){450}}\n\\put(2701,-1411){\\line( 1, 0){900}}\n\\put(3601,-1411){\\line( 0, 1){450}}\n\\put(3601,-961){\\line( 1, 0){450}}\n\\put(4051,-961){\\line( 0, 1){450}}\n\\put(4051,-511){\\line(-1, 0){900}}\n\\put(3151,-511){\\line( 0,-1){450}}\n\\put(3151,-961){\\line(-1, 0){450}}\n}%\n{\\multiput(3151,-1411)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n\\multiput(3151,-961)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(3601,-961)(0.00000,128.57143){4}{\\line( 0, 1){ 64.286}}\n}%\n\\end{picture}%\n\\]\n(The dotted lines divide the tiles into $1 \\times 1$ squares.)\nThe tiles may be rotated and reflected, as long as their sides are parallel to the sides\nof the rectangular region. They must all fit within the region, and they must cover it completely without overlapping.\n\nWhat is the minimum number of tiles required to tile the region?",
      "solution": "The minimum number of tiles is $lgfhavure qmrxjdeop$. To see that this many are required, label the squares $(yzdblnspa,vqmrkcswa)$ with $1\\leq yzdblnspa\\leq 2lgfhavure-1$ and $1\\leq vqmrkcswa\\leq 2qmrxjdeop-1$, and for each square with $yzdblnspa,vqmrkcswa$ both odd, color the square red; then no tile can cover more than one red square, and there are $lgfhavure qmrxjdeop$ red squares.\n\nIt remains to show that we can cover any $(2lgfhavure-1) \\times (2qmrxjdeop-1)$ rectangle with $lgfhavure qmrxjdeop$ tiles when $lgfhavure,qmrxjdeop \\geq 4$. First note that we can tile any $2 \\times (2tbzqrhenp-1)$ rectangle with $tbzqrhenp\\geq 3$ by $tbzqrhenp$ tiles: one of the first type, then $tbzqrhenp-2$ of the second type, and finally one of the first type. Thus if we can cover a $7\\times 7$ square with $16$ tiles, then we can do the general $(2lgfhavure-1) \\times (2qmrxjdeop-1)$ rectangle, by decomposing this rectangle into a $7\\times 7$ square in the lower left corner, along with $lgfhavure-4$ $(2\\times 7)$ rectangles to the right of the square, and $qmrxjdeop-4$ $((2lgfhavure-1)\\times 2)$ rectangles above, and tiling each of these rectangles separately, for a total of $16+4(lgfhavure-4)+lgfhavure(qmrxjdeop-4) = lgfhavure qmrxjdeop$ tiles.\n\nTo cover the $7 \\times 7$ square, note that the tiling must consist of 15 tiles of the first type and 1 of the second type, and that any $2 \\times 3$ rectangle can be covered using 2 tiles of the first type. We may thus construct a suitable covering by covering all but the center square with eight $2 \\times 3$ rectangles, in such a way that we can attach the center square to one of these rectangles to get a shape that can be covered by two tiles. An example of such a covering, with the remaining $2 \\times 3$ rectangles left intact for visual clarity, is depicted below. (Many other solutions are possible.)\n\\[\n\\setlength{\\unitlength}{4144sp}%\n%\n\\begingroup\\makeatletter\\ifx\\SetFigFont\\undefined%\n\\gdef\\SetFigFont#1#2#3#4#5{%\n  \\reset@font\\fontsize{#1}{#2pt}%\n  \\fontfamily{#3}\\fontseries{#4}\\fontshape{#5}%\n  \\selectfont}%\n\\fi\\endgroup%\n\\begin{picture}(3174,3174)(1339,-7723)\n\\thinlines\n{\\put(1351,-7711){\\framebox(3150,3150){}}\n}%\n{\\put(1351,-5911){\\line( 1, 0){945}}\n\\put(2296,-5911){\\line( 0, 1){1350}}\n}%\n{\\put(2296,-5911){\\line( 1, 0){405}}\n\\put(2701,-5911){\\line( 0,-1){1800}}\n}%\n{\\put(1351,-6811){\\line( 1, 0){1350}}\n}%\n{\\put(2701,-6361){\\line( 1, 0){1800}}\n}%\n{\\put(3601,-6361){\\line( 0,-1){1350}}\n}%\n{\\put(3151,-6361){\\line( 0, 1){1800}}\n}%\n{\\put(3151,-5461){\\line( 1, 0){1350}}\n}%\n{\\multiput(2296,-5011)(128.57143,0.00000){4}{\\line( 1, 0){ 64.286}}\n\\multiput(2746,-5011)(0.00000,-128.57143){4}{\\line( 0,-1){ 64.286}}\n\\multiput(2746,-5461)(115.71429,0.00000){4}{\\line( 1, 0){ 57.857}}\n}%\n\\end{picture}%\n\\]"
    },
    "kernel_variant": {
      "question": "Let m and n be integers with m,n \\ge 4.  Consider the (2m-1) \\times (2n-1) rectangle whose sides are parallel to the coordinate axes and which is subdivided into unit squares.  Two kinds of tiles may be used (rotations and reflections are allowed throughout):\n\n•  Type A (triomino).  Begin with a 2 \\times 2 block of unit squares and delete one of the four squares.  The remaining three unit squares form an L–shaped triomino.\n\n•  Type B (tetromino).  Begin with the 2 \\times 3 block whose lower–left square has coordinates (0,0).  Delete the squares (0,1) and (2,0); the four remaining squares (0,0),(1,0),(1,1),(2,1) form a zig–shaped tetromino.  Any rotation or reflection of this shape is also allowed.\n\nEach Type A tile therefore covers exactly 3 unit squares, and each Type B tile covers exactly 4 unit squares.\n\nDetermine, as a function of m and n, the minimum possible number of tiles that can tile the entire (2m-1) \\times (2n-1) rectangle without gaps or overlaps.",
      "solution": "Answer:  \\boxed{mn}.\n\n1.  A lower bound\n\n   Label the unit squares of the big rectangle by their integer coordinates (i,j) with 1\\le i\\le 2m-1 and 1\\le j\\le 2n-1.  Colour square (i,j) red exactly when both i and j are odd.  The red squares form an m \\times n array of isolated cells, so there are mn red squares in total.\n\n   We now show that every admissible tile—of either type—meets at most one red square.  This will imply that at least mn tiles are necessary.\n\n   •  Type A.  A Type A triomino is contained in a 2 \\times 2 block.  Such a block contains exactly one red square because precisely one of the four ordered pairs (even,even), (even,odd), (odd,even), (odd,odd) has both coordinates odd.  Hence a Type A tile meets at most one red square.\n\n   •  Type B.  Fix an orientation of the tetromino and denote the four occupied squares, in local coordinates, by (x_1,y_1),…,(x_4,y_4).  These coordinates, taken modulo 2, are always the same multiset \\{(0,0),(1,0),(1,1),(0,1)\\}.  Consequently, exactly one of the four local squares has both coordinates odd.  Translating the shape by an integral vector (a,b) just adds (a,b) to each coordinate, so still exactly one of the four global squares has both coordinates odd.  Thus every Type B tile contains precisely one red square and therefore cannot contain two.\n\n   Combining the two bullets, no tile covers more than one red square, so at least mn tiles are required.\n\n2.  A useful lemma\n\n   Lemma.  For every integer k \\ge 3, a 2 \\times (2k-1) rectangle can be tiled with exactly k tiles: two of Type A and k-2 of Type B.\n\n   Proof.  Place a Type A triomino in the lower–left corner so that it covers squares (1,1),(1,2),(2,2).  For each j with 1 \\le j \\le k-2 place one Type B tile whose leftmost column is 2j; concretely it covers the squares (2j,1),(2j+1,1),(2j+1,2),(2j+2,2).  The remaining three uncovered squares (2k-2,1),(2k-1,1),(2k-1,2) form an L–shape congruent to Type A, and one more Type A tile finishes the tiling. ∎\n\n3.  A 7 \\times 7 square can be tiled with exactly 16 tiles\n\n   Let t be the number of Type A tiles and s the number of Type B tiles in any tiling of a 7 \\times 7 board.  The area equation is\n        3t + 4s = 49.                        (1)\n   From Section 1 we already know that t + s \\ge 16.                (2)\n\n   Subtract 3·(2) from (1):\n        (3t + 4s) – 3(t + s) = 49 – 48\n        \\;\\;\\Longrightarrow\\;\\;   s = 1.\n   Inserting s = 1 into (1) gives t = 15.  Hence every tiling of the 7 \\times 7 board must use exactly 15 triominoes and 1 tetromino.\n\n   An explicit arrangement with these counts is listed below (coordinates are (column,row) with (1,1) in the lower–left corner).\n\n   •  Type B tile (the unique tetromino)\n        B : (2,2),(3,2),(3,3),(4,3).\n\n   •  Fifteen Type A tiles (each line lists the three occupied squares)\n        A₁ : (1,1),(1,2),(2,1)\n        A₂ : (3,1),(4,1),(4,2)\n        A₃ : (5,1),(5,2),(6,1)\n        A₄ : (6,2),(7,1),(7,2)\n        A₅ : (1,3),(2,3),(1,4)\n        A₆ : (5,3),(5,4),(6,3)\n        A₇ : (6,4),(7,3),(7,4)\n        A₈ : (3,4),(3,5),(4,4)\n        A₉ : (2,4),(1,5),(2,5)\n        A₁₀: (4,5),(5,5),(5,6)\n        A₁₁: (6,5),(7,5),(7,6)\n        A₁₂: (1,6),(1,7),(2,6)\n        A₁₃: (2,7),(3,6),(3,7)\n        A₁₄: (4,6),(4,7),(5,7)\n        A₁₅: (6,6),(6,7),(7,7).\n\n   One checks directly that the above 49 squares are pairwise distinct and fill the board, so the required tiling exists.\n\n4.  Tiling the general (2m-1) \\times (2n-1) rectangle\n\n   Place the above 7 \\times 7 tiling in the lower–left corner of the big rectangle.\n\n   •  To the right of it remains a 7 \\times (2m-8) rectangle.  Divide it into m−4 vertical strips of size 7 \\times 2.  After a 90° rotation each strip becomes 2 \\times 7, and by the lemma (with k = 4) each strip is tiled by exactly 4 tiles.  Total here: 4(m−4) tiles.\n\n   •  Above the 7–row band remains a (2n-8) \\times (2m-1) rectangle—that is, n−4 horizontal strips, each of size 2 \\times (2m-1).  By the lemma (with k = m) each such strip is tiled with m tiles.  Total here: m(n−4) tiles.\n\n   Summing gives\n            16   (for the 7 \\times 7 square)\n          + 4(m−4)\n          + m(n−4)\n          = mn.\n\n   This equals the lower bound found in Section 1, so mn is indeed the minimum possible number of tiles.\n\nTherefore the least number of tiles required to tile the (2m-1) \\times (2n-1) rectangle is exactly mn.",
      "_meta": {
        "core_steps": [
          "Parity-color all unit squares with both coordinates odd; each tile meets at most one such square ⇒ ≥ mn tiles are necessary.",
          "Lemma: any 2 × (2k−1) strip (k≥3) can be covered with k tiles.",
          "Base case: construct an explicit tiling of one (2p−1) × (2p−1) square (here p=4, side 7) with T tiles (here T=16).",
          "Partition the large board into that base square, (m−p) horizontal 2 × (2p−1) strips, and (n−p) vertical (2m−1) × 2 strips.",
          "Tile each piece with the previous lemmas; tile counts add to mn, attaining the lower bound."
        ],
        "mutable_slots": {
          "slot_p": {
            "description": "Chosen ‘pivot’ size p for the base (2p−1)×(2p−1) block; all later counts use this same p.",
            "original": 4
          },
          "slot_base_side": {
            "description": "Side length of the base square, equal to 2·slot_p−1.",
            "original": 7
          },
          "slot_T": {
            "description": "Number of tiles used to cover the base square.",
            "original": 16
          },
          "slot_k_min": {
            "description": "Smallest k for which the 2×(2k−1) strip-tiling lemma is invoked.",
            "original": 3
          },
          "slot_coeff_p": {
            "description": "The coefficient appearing in (m−p) and (n−p) when counting strips (identical to slot_p).",
            "original": 4
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "calculation",
  "iteratively_fixed": true
}