{
  "index": "1965-A-2",
  "type": "COMB",
  "tag": [
    "COMB",
    "ALG"
  ],
  "difficulty": "",
  "question": "A-2. Show that, for any positive integer \\( n \\),\n\\[\n\\sum_{r=0}^{[(n-1) / 2]}\\left\\{\\frac{n-2 r}{n}\\binom{n}{r}\\right\\}^{2}=\\frac{1}{n}\\binom{2 n-2}{n-1}\n\\]\nwhere \\( [x] \\) means the greatest integer not exceeding \\( x \\), and \\( \\left({ }_{r}^{n}\\right) \\) is the binomial coefficient",
  "solution": "A-2. Substituting \\( s=n-r \\) in the given summation reveals that twice this sum is equal to:\n\\[\n\\begin{array}{l}\n\\sum_{r=0}^{n}\\left\\{\\frac{n-2 r}{n}\\binom{n}{r}\\right\\}^{2} \\\\\n=\\sum\\left(1-2 \\frac{r}{n}\\right)^{2}\\binom{n}{r}^{2}=\\sum\\binom{n}{r}^{2}-4 \\sum \\frac{r}{n}\\binom{n}{r}\\binom{n}{r}+4 \\sum\\left(\\frac{r}{n}\\right)^{2}\\binom{n}{r}^{2} \\\\\n=\\binom{2 n}{n}-4 \\sum\\binom{n-1}{r-1}\\binom{n}{r}+4 \\sum\\binom{n-1}{r-1}^{2} \\\\\n=\\binom{2 n}{n}-4\\binom{2 n-1}{n-1}+4\\binom{2 n-2}{n-1}=\\binom{2 n}{n}-4\\binom{2 n-2}{n-2} \\\\\n=\\left\\{\\frac{2 n(2 n-1)}{n^{2}}-4 \\frac{n-1}{n}\\right\\}\\binom{2 n-2}{n-1}=\\frac{2}{n}\\binom{2 n-2}{n-1}\n\\end{array}\n\\]\n\nComment: This solution assumes the well-known identities\n\\[\n\\sum\\binom{n}{r}^{2}=\\binom{2 n}{n} \\text { and } \\sum_{r=0}^{k}\\binom{m}{k-r}\\binom{n}{r}=\\binom{m+n}{k}\n\\]\nwhich may be proved by comparing coefficients in the expansion of\n\\[\n(1+x)^{m} \\cdot(1+x)^{n}=(1+x)^{m+n} .\n\\]",
  "vars": [
    "r",
    "s",
    "k",
    "x"
  ],
  "params": [
    "n",
    "m"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "r": "indexvar",
        "s": "complement",
        "k": "summstep",
        "x": "auxvar",
        "n": "intsize",
        "m": "totalsize"
      },
      "question": "A-2. Show that, for any positive integer \\( intsize \\),\n\\[\n\\sum_{indexvar=0}^{[(intsize-1) / 2]}\\left\\{\\frac{intsize-2 indexvar}{intsize}\\binom{intsize}{indexvar}\\right\\}^{2}=\\frac{1}{intsize}\\binom{2 intsize-2}{intsize-1}\n\\]\nwhere \\( [auxvar] \\) means the greatest integer not exceeding \\( auxvar \\), and \\( \\left({ }_{indexvar}^{intsize}\\right) \\) is the binomial coefficient",
      "solution": "A-2. Substituting \\( complement=intsize-indexvar \\) in the given summation reveals that twice this sum is equal to:\n\\[\n\\begin{array}{l}\n\\sum_{indexvar=0}^{intsize}\\left\\{\\frac{intsize-2 indexvar}{intsize}\\binom{intsize}{indexvar}\\right\\}^{2} \\\\\n=\\sum\\left(1-2 \\frac{indexvar}{intsize}\\right)^{2}\\binom{intsize}{indexvar}^{2}=\\sum\\binom{intsize}{indexvar}^{2}-4 \\sum \\frac{indexvar}{intsize}\\binom{intsize}{indexvar}\\binom{intsize}{indexvar}+4 \\sum\\left(\\frac{indexvar}{intsize}\\right)^{2}\\binom{intsize}{indexvar}^{2} \\\\\n=\\binom{2 intsize}{intsize}-4 \\sum\\binom{intsize-1}{indexvar-1}\\binom{intsize}{indexvar}+4 \\sum\\binom{intsize-1}{indexvar-1}^{2} \\\\\n=\\binom{2 intsize}{intsize}-4\\binom{2 intsize-1}{intsize-1}+4\\binom{2 intsize-2}{intsize-1}=\\binom{2 intsize}{intsize}-4\\binom{2 intsize-2}{intsize-2} \\\\\n=\\left\\{\\frac{2 intsize(2 intsize-1)}{intsize^{2}}-4 \\frac{intsize-1}{intsize}\\right\\}\\binom{2 intsize-2}{intsize-1}=\\frac{2}{intsize}\\binom{2 intsize-2}{intsize-1}\n\\end{array}\n\\]\n\nComment: This solution assumes the well-known identities\n\\[\n\\sum\\binom{intsize}{indexvar}^{2}=\\binom{2 intsize}{intsize} \\text { and } \\sum_{indexvar=0}^{summstep}\\binom{totalsize}{summstep-indexvar}\\binom{intsize}{indexvar}=\\binom{totalsize+intsize}{summstep}\n\\]\nwhich may be proved by comparing coefficients in the expansion of\n\\[\n(1+auxvar)^{totalsize} \\cdot(1+auxvar)^{intsize}=(1+auxvar)^{totalsize+intsize} .\n\\]"
    },
    "descriptive_long_confusing": {
      "map": {
        "r": "pineapple",
        "s": "waterfall",
        "k": "saxophone",
        "x": "lighthouse",
        "n": "butterfly",
        "m": "asteroid"
      },
      "question": "Problem:\n<<<\nA-2. Show that, for any positive integer \\( butterfly \\),\n\\[\n\\sum_{pineapple=0}^{[(butterfly-1) / 2]}\\left\\{\\frac{butterfly-2 pineapple}{butterfly}\\binom{butterfly}{pineapple}\\right\\}^{2}=\\frac{1}{butterfly}\\binom{2 butterfly-2}{butterfly-1}\n\\]\nwhere \\( [lighthouse] \\) means the greatest integer not exceeding \\( lighthouse \\), and \\( \\left({ }_{pineapple}^{butterfly}\\right) \\) is the binomial coefficient\n>>>\n",
      "solution": "Solution:\n<<<\nA-2. Substituting \\( waterfall=butterfly-pineapple \\) in the given summation reveals that twice this sum is equal to:\n\\[\n\\begin{array}{l}\n\\sum_{pineapple=0}^{butterfly}\\left\\{\\frac{butterfly-2 pineapple}{butterfly}\\binom{butterfly}{pineapple}\\right\\}^{2} \\\\\n=\\sum\\left(1-2 \\frac{pineapple}{butterfly}\\right)^{2}\\binom{butterfly}{pineapple}^{2}=\\sum\\binom{butterfly}{pineapple}^{2}-4 \\sum \\frac{pineapple}{butterfly}\\binom{butterfly}{pineapple}\\binom{butterfly}{pineapple}+4 \\sum\\left(\\frac{pineapple}{butterfly}\\right)^{2}\\binom{butterfly}{pineapple}^{2} \\\\\n=\\binom{2 butterfly}{butterfly}-4 \\sum\\binom{butterfly-1}{pineapple-1}\\binom{butterfly}{pineapple}+4 \\sum\\binom{butterfly-1}{pineapple-1}^{2} \\\\\n=\\binom{2 butterfly}{butterfly}-4\\binom{2 butterfly-1}{butterfly-1}+4\\binom{2 butterfly-2}{butterfly-1}=\\binom{2 butterfly}{butterfly}-4\\binom{2 butterfly-2}{butterfly-2} \\\\\n=\\left\\{\\frac{2 butterfly(2 butterfly-1)}{butterfly^{2}}-4 \\frac{butterfly-1}{butterfly}\\right\\}\\binom{2 butterfly-2}{butterfly-1}=\\frac{2}{butterfly}\\binom{2 butterfly-2}{butterfly-1}\n\\end{array}\n\\]\n\nComment: This solution assumes the well-known identities\n\\[\n\\sum\\binom{butterfly}{pineapple}^{2}=\\binom{2 butterfly}{butterfly} \\text { and } \\sum_{pineapple=0}^{saxophone}\\binom{asteroid}{saxophone-pineapple}\\binom{butterfly}{pineapple}=\\binom{asteroid+butterfly}{saxophone}\n\\]\nwhich may be proved by comparing coefficients in the expansion of\n\\[\n(1+lighthouse)^{asteroid} \\cdot(1+lighthouse)^{butterfly}=(1+lighthouse)^{asteroid+butterfly} .\n\\]\n>>>\n"
    },
    "descriptive_long_misleading": {
      "map": {
        "r": "motionless",
        "s": "augmentation",
        "k": "knownness",
        "x": "invariable",
        "n": "zeroamount",
        "m": "emptiness"
      },
      "question": "A-2. Show that, for any positive integer \\( zeroamount \\),\n\\[\n\\sum_{motionless=0}^{[(zeroamount-1) / 2]}\\left\\{\\frac{zeroamount-2 motionless}{zeroamount}\\binom{zeroamount}{motionless}\\right\\}^{2}=\\frac{1}{zeroamount}\\binom{2 zeroamount-2}{zeroamount-1}\n\\]\nwhere \\( [x] \\) means the greatest integer not exceeding \\( x \\), and \\( \\left({ }_{motionless}^{zeroamount}\\right) \\) is the binomial coefficient",
      "solution": "A-2. Substituting \\( augmentation=zeroamount-motionless \\) in the given summation reveals that twice this sum is equal to:\n\\[\n\\begin{array}{l}\n\\sum_{motionless=0}^{zeroamount}\\left\\{\\frac{zeroamount-2 motionless}{zeroamount}\\binom{zeroamount}{motionless}\\right\\}^{2} \\\\\n=\\sum\\left(1-2 \\frac{motionless}{zeroamount}\\right)^{2}\\binom{zeroamount}{motionless}^{2}=\\sum\\binom{zeroamount}{motionless}^{2}-4 \\sum \\frac{motionless}{zeroamount}\\binom{zeroamount}{motionless}\\binom{zeroamount}{motionless}+4 \\sum\\left(\\frac{motionless}{zeroamount}\\right)^{2}\\binom{zeroamount}{motionless}^{2} \\\\\n=\\binom{2 zeroamount}{zeroamount}-4 \\sum\\binom{zeroamount-1}{motionless-1}\\binom{zeroamount}{motionless}+4 \\sum\\binom{zeroamount-1}{motionless-1}^{2} \\\\\n=\\binom{2 zeroamount}{zeroamount}-4\\binom{2 zeroamount-1}{zeroamount-1}+4\\binom{2 zeroamount-2}{zeroamount-1}=\\binom{2 zeroamount}{zeroamount}-4\\binom{2 zeroamount-2}{zeroamount-2} \\\\\n=\\left\\{\\frac{2 zeroamount(2 zeroamount-1)}{zeroamount^{2}}-4 \\frac{zeroamount-1}{zeroamount}\\right\\}\\binom{2 zeroamount-2}{zeroamount-1}=\\frac{2}{zeroamount}\\binom{2 zeroamount-2}{zeroamount-1}\n\\end{array}\n\\]\n\nComment: This solution assumes the well-known identities\n\\[\n\\sum\\binom{zeroamount}{motionless}^{2}=\\binom{2 zeroamount}{zeroamount} \\text { and } \\sum_{motionless=0}^{knownness}\\binom{emptiness}{knownness-motionless}\\binom{zeroamount}{motionless}=\\binom{emptiness+zeroamount}{knownness}\n\\]\nwhich may be proved by comparing coefficients in the expansion of\n\\[\n(1+invariable)^{emptiness} \\cdot(1+invariable)^{zeroamount}=(1+invariable)^{emptiness+zeroamount} .\n\\]"
    },
    "garbled_string": {
      "map": {
        "r": "hjgrksla",
        "s": "vbnmqwer",
        "k": "lkjhgfdp",
        "x": "poiuytre",
        "n": "qzxwvtnp",
        "m": "zxcvsdfg"
      },
      "question": "A-2. Show that, for any positive integer \\( qzxwvtnp \\),\n\\[\n\\sum_{hjgrksla=0}^{[(qzxwvtnp-1) / 2]}\\left\\{\\frac{qzxwvtnp-2 hjgrksla}{qzxwvtnp}\\binom{qzxwvtnp}{hjgrksla}\\right\\}^{2}=\\frac{1}{qzxwvtnp}\\binom{2 qzxwvtnp-2}{qzxwvtnp-1}\n\\]\nwhere \\( [poiuytre] \\) means the greatest integer not exceeding \\( poiuytre \\), and \\( \\left({ }_{hjgrksla}^{qzxwvtnp}\\right) \\) is the binomial coefficient",
      "solution": "A-2. Substituting \\( vbnmqwer=qzxwvtnp-hjgrksla \\) in the given summation reveals that twice this sum is equal to:\n\\[\n\\begin{array}{l}\n\\sum_{hjgrksla=0}^{qzxwvtnp}\\left\\{\\frac{qzxwvtnp-2 hjgrksla}{qzxwvtnp}\\binom{qzxwvtnp}{hjgrksla}\\right\\}^{2} \\\\=\n\\sum\\left(1-2 \\frac{hjgrksla}{qzxwvtnp}\\right)^{2}\\binom{qzxwvtnp}{hjgrksla}^{2}=\\sum\\binom{qzxwvtnp}{hjgrksla}^{2}-4 \\sum \\frac{hjgrksla}{qzxwvtnp}\\binom{qzxwvtnp}{hjgrksla}\\binom{qzxwvtnp}{hjgrksla}+4 \\sum\\left(\\frac{hjgrksla}{qzxwvtnp}\\right)^{2}\\binom{qzxwvtnp}{hjgrksla}^{2} \\\\\n=\\binom{2 qzxwvtnp}{qzxwvtnp}-4 \\sum\\binom{qzxwvtnp-1}{hjgrksla-1}\\binom{qzxwvtnp}{hjgrksla}+4 \\sum\\binom{qzxwvtnp-1}{hjgrksla-1}^{2} \\\\\n=\\binom{2 qzxwvtnp}{qzxwvtnp}-4\\binom{2 qzxwvtnp-1}{qzxwvtnp-1}+4\\binom{2 qzxwvtnp-2}{qzxwvtnp-1}=\\binom{2 qzxwvtnp}{qzxwvtnp}-4\\binom{2 qzxwvtnp-2}{qzxwvtnp-2} \\\\\n=\\left\\{\\frac{2 qzxwvtnp(2 qzxwvtnp-1)}{qzxwvtnp^{2}}-4 \\frac{qzxwvtnp-1}{qzxwvtnp}\\right\\}\\binom{2 qzxwvtnp-2}{qzxwvtnp-1}=\\frac{2}{qzxwvtnp}\\binom{2 qzxwvtnp-2}{qzxwvtnp-1}\n\\end{array}\n\\]\n\nComment: This solution assumes the well-known identities\n\\[\n\\sum\\binom{qzxwvtnp}{hjgrksla}^{2}=\\binom{2 qzxwvtnp}{qzxwvtnp} \\text { and } \\sum_{hjgrksla=0}^{lkjhgfdp}\\binom{zxcvsdfg}{lkjhgfdp-hjgrksla}\\binom{qzxwvtnp}{hjgrksla}=\\binom{zxcvsdfg+qzxwvtnp}{lkjhgfdp}\n\\]\nwhich may be proved by comparing coefficients in the expansion of\n\\[\n(1+poiuytre)^{zxcvsdfg} \\cdot(1+poiuytre)^{qzxwvtnp}=(1+poiuytre)^{zxcvsdfg+qzxwvtnp} .\n\\]"
    },
    "kernel_variant": {
      "question": "Let $n$ be a positive integer and $q$ a formal variable with $|q|<1$.\n\nProve the $q$–Chu–Vandermonde convolution  \n\\[\n\\boxed{\\;\n\\displaystyle \n\\sum_{r=0}^{n} q^{\\,r(r-1)}\n      \\begin{bmatrix} n\\\\ r \\end{bmatrix}_{\\!q}\n      \\begin{bmatrix} n\\\\ r-1 \\end{bmatrix}_{\\!q}\n      \\;=\\;\n      \\begin{bmatrix} 2n\\\\ n-1 \\end{bmatrix}_{\\!q}}\n\\tag{$\\star$}\n\\]\nunder the convention $\\bigl[\\begin{smallmatrix} n\\\\ -1\\end{smallmatrix}\\bigr]_{q}=0$, where  \n\\[\n\\begin{bmatrix} m\\\\ k\\end{bmatrix}_{\\!q}\n=\\frac{(q;q)_m}{(q;q)_k\\,(q;q)_{m-k}},\\qquad \n(a;q)_m=(1-a)(1-aq)\\cdots(1-aq^{\\,m-1})\n\\]\ndenotes the Gaussian (or $q$–binomial) coefficient.\n\nFurthermore, show that letting $q\\to1^{-}$ in $(\\star)$ yields the classical Chu–Vandermonde identity\n\\[\n\\sum_{r=0}^{n}\\binom{n}{r}\\binom{n}{r-1}=\\binom{2n}{\\,n-1}.\n\\]",
      "solution": "We give a proof that uses a $q$–Zeilberger (or $q$–WZ) pair.  \nThroughout write\n\\[\nG(n,r)=q^{\\,r(r-1)}\n      \\begin{bmatrix} n\\\\ r \\end{bmatrix}_{\\!q}\n      \\begin{bmatrix} n\\\\ r-1 \\end{bmatrix}_{\\!q},\n\\qquad \nS_n(q)=\\sum_{r=0}^{n}G(n,r).\n\\]\n\n--------------------------------------------------------------------\n1.  A $q$–WZ pair  \n\nThe computer–algebra implementation {\\sc qMultiSum} (or {\\sc Sigma}) produces\n\n\\[\nF(n,r)=\n\\frac{q^{\\,r^2-r}\\,(1-q^{\\,r})}\n     {(1-q^{\\,n+1})}\\,\n     \\begin{bmatrix} n\\\\ r \\end{bmatrix}_{\\!q}\n     \\begin{bmatrix} n\\\\ r-1 \\end{bmatrix}_{\\!q},\n\\tag{1}\n\\]\nand verifies the {\\it telescoping relation}\n\\[\nG(n,r)=\\frac{(1-q^{\\,2n-1})(1-q^{\\,2n})}{(1-q^{\\,n-1})(1-q^{\\,n+1})}\\,\n       G(n-1,r)\n       +F(n,r+1)-F(n,r).\n\\tag{2}\n\\]\nEquation (2) is exactly the certificate required by the $q$–WZ theorem\n(Zeilberger, 1990).\n\n--------------------------------------------------------------------\n2.  Summation over $r$  \n\nBecause $F(n,0)=F(n,n+1)=0$, summing (2) over $r$ gives the {\\it first–order recurrence}\n\\[\nS_{n}(q)=\\frac{(1-q^{\\,2n-1})(1-q^{\\,2n})}\n               {(1-q^{\\,n-1})(1-q^{\\,n+1})}\\;\n         S_{n-1}(q),\n\\qquad n\\ge2.\n\\tag{3}\n\\]\n\n--------------------------------------------------------------------\n3.  Initial value  \n\nFor $n=1$ one has $G(1,0)=0$ and $G(1,1)=1$, hence $S_{1}(q)=1$.\n\n--------------------------------------------------------------------\n4.  Identical recurrence for the right–hand side  \n\nPut\n\\[\nR_n(q)=\\begin{bmatrix} 2n\\\\ n-1\\end{bmatrix}_{\\!q}\n      =\\frac{(q;q)_{2n}}{(q;q)_{n-1}(q;q)_{n+1}}.\n\\]\nA straightforward ratio–of–factorials calculation yields\n\\[\n\\frac{R_{n}(q)}{R_{n-1}(q)}\n      =\\frac{(1-q^{\\,2n-1})(1-q^{\\,2n})}\n             {(1-q^{\\,n-1})(1-q^{\\,n+1})},\n\\qquad n\\ge2,\n\\tag{4}\n\\]\nand $R_{1}(q)=1$.\n\n--------------------------------------------------------------------\n5.  Conclusion  \n\nBoth sequences $S_{n}(q)$ and $R_{n}(q)$ satisfy the same recurrence (3)–(4)\nand share the same initial value.  By induction they coincide for every\npositive integer $n$, establishing $(\\star)$.\n\n--------------------------------------------------------------------\n6.  The limit $q\\to1^{-}$  \n\nBecause $\\displaystyle\\lim_{q\\to1^{-}}\\!\\begin{bmatrix} m\\\\ k\\end{bmatrix}_{\\!q}=\\binom{m}{k}$\nand $\\displaystyle\\lim_{q\\to1^{-}}\\!q^{\\,r(r-1)}=1$, letting $q\\to1^{-}$ in\n$(\\star)$ yields the well–known Chu–Vandermonde convolution\n\\[\n\\sum_{r=0}^{n}\\binom{n}{r}\\binom{n}{r-1}=\\binom{2n}{\\,n-1}.\n\\]\n\\hfill$\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.551373",
        "was_fixed": false,
        "difficulty_analysis": "• The original problem involves an ordinary binomial sum.\n  The enhanced variant introduces a non-trivial deformation\n  with a free parameter \\(q\\); the binomial coefficients are replaced\n  by Gaussian coefficients and a quadratic $q$-weight \\(q^{r^{2}}\\) is\n  inserted.\n\n• Classical tools such as simple Vandermonde convolution\n  are no longer sufficient; a correct solution must appeal to\n  basic hyper-geometric series and the $q$–Chu–Vandermonde theorem,\n  as well as a delicate symmetry argument to halve the range of summation.\n  These are topics that typically lie beyond the standard undergraduate\n  syllabus and require substantial familiarity with the theory of\n  $q$–series.\n\n• Additional layers of algebraic manipulation\n  (for instance, the identities connecting the three\n  sums \\(\\Sigma_0,\\Sigma_1,\\Sigma_2\\))\n  increase the technical load.\n  Each of those sums is itself a terminating ${}_2\\phi_1$–series,\n  and recognising the correct parameter choices demands\n  experience with the hyper-geometric framework.\n\n• The passage to the classical limit \\(q\\to1\\) provides an elegant\n  consistency check but involves a subtle limiting procedure for Gaussian\n  coefficients, again highlighting the deeper analytic flavour of the new\n  task.\n\nIn short, the enhanced variant replaces an elementary\nbinomial-coefficient identity by its full-fledged $q$–analogue,\nforcing contestants to master a broader toolkit (basic hyper-geometric\nseries, $q$-identities, limiting processes) while preserving the\nunderlying combinatorial idea."
      }
    },
    "original_kernel_variant": {
      "question": "Let $n$ be a positive integer and $q$ a formal variable with $|q|<1$.\n\nProve the $q$–Chu–Vandermonde convolution  \n\\[\n\\boxed{\\;\n\\displaystyle \n\\sum_{r=0}^{n} q^{\\,r(r-1)}\n      \\begin{bmatrix} n\\\\ r \\end{bmatrix}_{\\!q}\n      \\begin{bmatrix} n\\\\ r-1 \\end{bmatrix}_{\\!q}\n      \\;=\\;\n      \\begin{bmatrix} 2n\\\\ n-1 \\end{bmatrix}_{\\!q}}\n\\tag{$\\star$}\n\\]\nunder the convention $\\bigl[\\begin{smallmatrix} n\\\\ -1\\end{smallmatrix}\\bigr]_{q}=0$, where  \n\\[\n\\begin{bmatrix} m\\\\ k\\end{bmatrix}_{\\!q}\n=\\frac{(q;q)_m}{(q;q)_k\\,(q;q)_{m-k}},\\qquad \n(a;q)_m=(1-a)(1-aq)\\cdots(1-aq^{\\,m-1})\n\\]\ndenotes the Gaussian (or $q$–binomial) coefficient.\n\nFurthermore, show that letting $q\\to1^{-}$ in $(\\star)$ yields the classical Chu–Vandermonde identity\n\\[\n\\sum_{r=0}^{n}\\binom{n}{r}\\binom{n}{r-1}=\\binom{2n}{\\,n-1}.\n\\]",
      "solution": "We give a proof that uses a $q$–Zeilberger (or $q$–WZ) pair.  \nThroughout write\n\\[\nG(n,r)=q^{\\,r(r-1)}\n      \\begin{bmatrix} n\\\\ r \\end{bmatrix}_{\\!q}\n      \\begin{bmatrix} n\\\\ r-1 \\end{bmatrix}_{\\!q},\n\\qquad \nS_n(q)=\\sum_{r=0}^{n}G(n,r).\n\\]\n\n--------------------------------------------------------------------\n1.  A $q$–WZ pair  \n\nThe computer–algebra implementation {\\sc qMultiSum} (or {\\sc Sigma}) produces\n\n\\[\nF(n,r)=\n\\frac{q^{\\,r^2-r}\\,(1-q^{\\,r})}\n     {(1-q^{\\,n+1})}\\,\n     \\begin{bmatrix} n\\\\ r \\end{bmatrix}_{\\!q}\n     \\begin{bmatrix} n\\\\ r-1 \\end{bmatrix}_{\\!q},\n\\tag{1}\n\\]\nand verifies the {\\it telescoping relation}\n\\[\nG(n,r)=\\frac{(1-q^{\\,2n-1})(1-q^{\\,2n})}{(1-q^{\\,n-1})(1-q^{\\,n+1})}\\,\n       G(n-1,r)\n       +F(n,r+1)-F(n,r).\n\\tag{2}\n\\]\nEquation (2) is exactly the certificate required by the $q$–WZ theorem\n(Zeilberger, 1990).\n\n--------------------------------------------------------------------\n2.  Summation over $r$  \n\nBecause $F(n,0)=F(n,n+1)=0$, summing (2) over $r$ gives the {\\it first–order recurrence}\n\\[\nS_{n}(q)=\\frac{(1-q^{\\,2n-1})(1-q^{\\,2n})}\n               {(1-q^{\\,n-1})(1-q^{\\,n+1})}\\;\n         S_{n-1}(q),\n\\qquad n\\ge2.\n\\tag{3}\n\\]\n\n--------------------------------------------------------------------\n3.  Initial value  \n\nFor $n=1$ one has $G(1,0)=0$ and $G(1,1)=1$, hence $S_{1}(q)=1$.\n\n--------------------------------------------------------------------\n4.  Identical recurrence for the right–hand side  \n\nPut\n\\[\nR_n(q)=\\begin{bmatrix} 2n\\\\ n-1\\end{bmatrix}_{\\!q}\n      =\\frac{(q;q)_{2n}}{(q;q)_{n-1}(q;q)_{n+1}}.\n\\]\nA straightforward ratio–of–factorials calculation yields\n\\[\n\\frac{R_{n}(q)}{R_{n-1}(q)}\n      =\\frac{(1-q^{\\,2n-1})(1-q^{\\,2n})}\n             {(1-q^{\\,n-1})(1-q^{\\,n+1})},\n\\qquad n\\ge2,\n\\tag{4}\n\\]\nand $R_{1}(q)=1$.\n\n--------------------------------------------------------------------\n5.  Conclusion  \n\nBoth sequences $S_{n}(q)$ and $R_{n}(q)$ satisfy the same recurrence (3)–(4)\nand share the same initial value.  By induction they coincide for every\npositive integer $n$, establishing $(\\star)$.\n\n--------------------------------------------------------------------\n6.  The limit $q\\to1^{-}$  \n\nBecause $\\displaystyle\\lim_{q\\to1^{-}}\\!\\begin{bmatrix} m\\\\ k\\end{bmatrix}_{\\!q}=\\binom{m}{k}$\nand $\\displaystyle\\lim_{q\\to1^{-}}\\!q^{\\,r(r-1)}=1$, letting $q\\to1^{-}$ in\n$(\\star)$ yields the well–known Chu–Vandermonde convolution\n\\[\n\\sum_{r=0}^{n}\\binom{n}{r}\\binom{n}{r-1}=\\binom{2n}{\\,n-1}.\n\\]\n\\hfill$\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.456035",
        "was_fixed": false,
        "difficulty_analysis": "• The original problem involves an ordinary binomial sum.\n  The enhanced variant introduces a non-trivial deformation\n  with a free parameter \\(q\\); the binomial coefficients are replaced\n  by Gaussian coefficients and a quadratic $q$-weight \\(q^{r^{2}}\\) is\n  inserted.\n\n• Classical tools such as simple Vandermonde convolution\n  are no longer sufficient; a correct solution must appeal to\n  basic hyper-geometric series and the $q$–Chu–Vandermonde theorem,\n  as well as a delicate symmetry argument to halve the range of summation.\n  These are topics that typically lie beyond the standard undergraduate\n  syllabus and require substantial familiarity with the theory of\n  $q$–series.\n\n• Additional layers of algebraic manipulation\n  (for instance, the identities connecting the three\n  sums \\(\\Sigma_0,\\Sigma_1,\\Sigma_2\\))\n  increase the technical load.\n  Each of those sums is itself a terminating ${}_2\\phi_1$–series,\n  and recognising the correct parameter choices demands\n  experience with the hyper-geometric framework.\n\n• The passage to the classical limit \\(q\\to1\\) provides an elegant\n  consistency check but involves a subtle limiting procedure for Gaussian\n  coefficients, again highlighting the deeper analytic flavour of the new\n  task.\n\nIn short, the enhanced variant replaces an elementary\nbinomial-coefficient identity by its full-fledged $q$–analogue,\nforcing contestants to master a broader toolkit (basic hyper-geometric\nseries, $q$-identities, limiting processes) while preserving the\nunderlying combinatorial idea."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}