{
  "index": "1958-2-B-4",
  "type": "ANA",
  "tag": [
    "ANA"
  ],
  "difficulty": "",
  "question": "4. Let \\( C \\) be a real number, and let \\( f \\) be a function such that\n\\[\n\\lim _{x \\rightarrow \\infty} f(x)=C, \\quad \\lim _{x \\rightarrow \\infty} f^{\\prime \\prime \\prime}(x)=0\n\\]\n\nProve that\n\\[\n\\lim _{x \\rightarrow \\infty} f^{\\prime}(x)=0 \\quad \\text { and } \\quad \\lim _{x \\rightarrow \\infty} f^{\\prime \\prime}(x)=0\n\\]\nwhere superscripts denote derivatives.",
  "solution": "Solution. By Taylor's theorem (finite form, or extended law of the mean), for any \\( x \\) there exist numbers \\( \\xi(x) \\) and \\( \\eta(x) \\) between 0 and 1 such that\n\\[\n\\begin{array}{l}\nf(x+1)=f(x)+f^{\\prime}(x)+\\frac{1}{2} f^{\\prime \\prime}(x)+\\frac{1}{6} f^{\\prime \\prime}(x+\\xi(x)) \\\\\nf(x-1)=f(x)-f^{\\prime}(x)+\\frac{1}{2} f^{\\prime \\prime}(x)-\\frac{1}{6} f^{\\prime \\prime \\prime}(x-\\eta(x))\n\\end{array}\n\\]\n\nAdding these relations and transposing terms, we get\n\\[\n\\begin{aligned}\nf^{\\prime \\prime}(x)=f(x+1)-2 f(x)+f(x-1) & \\\\\n& -\\frac{1}{6} f^{\\prime \\prime \\prime}(x+\\xi(x))+\\frac{1}{6} f^{\\prime \\prime}(x-\\eta(x)) .\n\\end{aligned}\n\\]\n\nSubtracting (2) from (1) and transposing terms, we get\n\\[\n2 f^{\\prime}(x)=f(x+1)-f(x-1)-\\frac{1}{6} f^{m \\prime}(x+\\xi(x))-\\frac{1}{6} f^{\\prime \\prime}(x-\\eta(x)) .\n\\]\n\nAs \\( x \\rightarrow \\infty \\), so do \\( x-\\eta(x) \\) and \\( x+\\xi(x) \\), so all terms on the right of (3) and (4) have limits, and\n\\[\n\\lim _{x \\rightarrow \\infty} f^{\\prime \\prime}(x)=C-2 C+C-\\frac{1}{6} \\cdot 0+\\frac{1}{6} \\cdot 0=0\n\\]\nand\n\\[\n\\lim _{x \\rightarrow \\infty} f^{\\prime}(x)=\\frac{1}{2}\\left[C-C-\\frac{1}{6} \\cdot 0-\\frac{1}{6} \\cdot 0\\right]=0\n\\]\n\nRemarks. The hypothesis can be weakened; it is sufficient to assume that \\( f^{m} \\) is bounded. Moreover, if we assume \\( \\lim _{x-\\infty} f(x) \\) exists and \\( f^{(n)} \\) is bounded, we can prove that \\( \\lim _{x \\rightarrow \\infty} f^{(k)}(x)=0 \\) for \\( 0<k<n \\). This was published by Littlewood in \"The Converse of Abel's Theorem,\" Proceedings of the London Mathematical Society (2), vol. 9 (1910-11), pages 434-448. For results concerning precise bounds for \\( f^{(k)}(x) \\) given bounds for \\( f(x) \\) and \\( f^{(n)}(x) \\) where \\( 0<k<n \\), see I. J. Schoenberg, \"The Elementary Cases of Landau's Problem of Inequalities Between Derivatives,\" American Mathematical Monthly, vol. 80 (1973), pages 121-158.",
  "vars": [
    "x",
    "\\\\xi",
    "\\\\eta"
  ],
  "params": [
    "C",
    "f"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "variable",
        "\\xi": "xiparam",
        "\\eta": "etaparam",
        "C": "limitreal",
        "f": "funcsymbol"
      },
      "question": "4. Let limitreal be a real number, and let funcsymbol be a function such that\n\\[\n\\lim _{variable \\rightarrow \\infty} funcsymbol(variable)=limitreal, \\quad \\lim _{variable \\rightarrow \\infty} funcsymbol^{\\prime \\prime \\prime}(variable)=0\n\\]\n\nProve that\n\\[\n\\lim _{variable \\rightarrow \\infty} funcsymbol^{\\prime}(variable)=0 \\quad \\text { and } \\quad \\lim _{variable \\rightarrow \\infty} funcsymbol^{\\prime \\prime}(variable)=0\n\\]\nwhere superscripts denote derivatives.",
      "solution": "Solution. By Taylor's theorem (finite form, or extended law of the mean), for any variable there exist numbers xiparam(variable) and etaparam(variable) between 0 and 1 such that\n\\[\n\\begin{array}{l}\nfuncsymbol(variable+1)=funcsymbol(variable)+funcsymbol^{\\prime}(variable)+\\frac{1}{2} funcsymbol^{\\prime \\prime}(variable)+\\frac{1}{6} funcsymbol^{\\prime \\prime}(variable+xiparam(variable)) \\\\\nfuncsymbol(variable-1)=funcsymbol(variable)-funcsymbol^{\\prime}(variable)+\\frac{1}{2} funcsymbol^{\\prime \\prime}(variable)-\\frac{1}{6} funcsymbol^{\\prime \\prime \\prime}(variable-etaparam(variable))\n\\end{array}\n\\]\n\nAdding these relations and transposing terms, we get\n\\[\n\\begin{aligned}\nfuncsymbol^{\\prime \\prime}(variable)=funcsymbol(variable+1)-2 funcsymbol(variable)+funcsymbol(variable-1) & \\\\\n& -\\frac{1}{6} funcsymbol^{\\prime \\prime \\prime}(variable+xiparam(variable))+\\frac{1}{6} funcsymbol^{\\prime \\prime}(variable-etaparam(variable)) .\n\\end{aligned}\n\\]\n\nSubtracting (2) from (1) and transposing terms, we get\n\\[\n2 funcsymbol^{\\prime}(variable)=funcsymbol(variable+1)-funcsymbol(variable-1)-\\frac{1}{6} funcsymbol^{\\prime \\prime \\prime}(variable+xiparam(variable))-\\frac{1}{6} funcsymbol^{\\prime \\prime}(variable-etaparam(variable)) .\n\\]\n\nAs \\( variable \\rightarrow \\infty \\), so do \\( variable-etaparam(variable) \\) and \\( variable+xiparam(variable) \\), so all terms on the right of (3) and (4) have limits, and\n\\[\n\\lim _{variable \\rightarrow \\infty} funcsymbol^{\\prime \\prime}(variable)=limitreal-2 limitreal+limitreal-\\frac{1}{6} \\cdot 0+\\frac{1}{6} \\cdot 0=0\n\\]\nand\n\\[\n\\lim _{variable \\rightarrow \\infty} funcsymbol^{\\prime}(variable)=\\frac{1}{2}\\left[limitreal-limitreal-\\frac{1}{6} \\cdot 0-\\frac{1}{6} \\cdot 0\\right]=0\n\\]\n\nRemarks. The hypothesis can be weakened; it is sufficient to assume that \\( funcsymbol^{m} \\) is bounded. Moreover, if we assume \\( \\lim _{variable-\\infty} funcsymbol(variable) \\) exists and \\( funcsymbol^{(n)} \\) is bounded, we can prove that \\( \\lim _{variable \\rightarrow \\infty} funcsymbol^{(k)}(variable)=0 \\) for \\( 0<k<n \\). This was published by Littlewood in \"The Converse of Abel's Theorem,\" Proceedings of the London Mathematical Society (2), vol. 9 (1910-11), pages 434-448. For results concerning precise bounds for \\( funcsymbol^{(k)}(variable) \\) given bounds for \\( funcsymbol(variable) \\) and \\( funcsymbol^{(n)}(variable) \\) where \\( 0<k<n \\), see I. J. Schoenberg, \"The Elementary Cases of Landau's Problem of Inequalities Between Derivatives,\" American Mathematical Monthly, vol. 80 (1973), pages 121-158."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "mountain",
        "\\xi": "hurricane",
        "\\eta": "pineapple",
        "C": "waterfall",
        "f": "monument"
      },
      "question": "4. Let \\( waterfall \\) be a real number, and let \\( monument \\) be a function such that\n\\[\n\\lim _{mountain \\rightarrow \\infty} monument(mountain)=waterfall, \\quad \\lim _{mountain \\rightarrow \\infty} monument^{\\prime \\prime \\prime}(mountain)=0\n\\]\n\nProve that\n\\[\n\\lim _{mountain \\rightarrow \\infty} monument^{\\prime}(mountain)=0 \\quad \\text { and } \\quad \\lim _{mountain \\rightarrow \\infty} monument^{\\prime \\prime}(mountain)=0\n\\]\nwhere superscripts denote derivatives.",
      "solution": "Solution. By Taylor's theorem (finite form, or extended law of the mean), for any \\( mountain \\) there exist numbers \\( hurricane(mountain) \\) and \\( pineapple(mountain) \\) between 0 and 1 such that\n\\[\n\\begin{array}{l}\nmonument(mountain+1)=monument(mountain)+monument^{\\prime}(mountain)+\\frac{1}{2} monument^{\\prime \\prime}(mountain)+\\frac{1}{6} monument^{\\prime \\prime}(mountain+hurricane(mountain)) \\\\\nmonument(mountain-1)=monument(mountain)-monument^{\\prime}(mountain)+\\frac{1}{2} monument^{\\prime \\prime}(mountain)-\\frac{1}{6} monument^{\\prime \\prime \\prime}(mountain-pineapple(mountain))\n\\end{array}\n\\]\n\nAdding these relations and transposing terms, we get\n\\[\n\\begin{aligned}\nmonument^{\\prime \\prime}(mountain)=monument(mountain+1)-2 monument(mountain)+monument(mountain-1) & \\\\\n& -\\frac{1}{6} monument^{\\prime \\prime \\prime}(mountain+hurricane(mountain))+\\frac{1}{6} monument^{\\prime \\prime}(mountain-pineapple(mountain)) .\n\\end{aligned}\n\\]\n\nSubtracting (2) from (1) and transposing terms, we get\n\\[\n2 monument^{\\prime}(mountain)=monument(mountain+1)-monument(mountain-1)-\\frac{1}{6} monument^{m \\prime}(mountain+hurricane(mountain))-\\frac{1}{6} monument^{\\prime \\prime}(mountain-pineapple(mountain)) .\n\\]\n\nAs \\( mountain \\rightarrow \\infty \\), so do \\( mountain-pineapple(mountain) \\) and \\( mountain+hurricane(mountain) \\), so all terms on the right of (3) and (4) have limits, and\n\\[\n\\lim _{mountain \\rightarrow \\infty} monument^{\\prime \\prime}(mountain)=waterfall-2 waterfall+waterfall-\\frac{1}{6} \\cdot 0+\\frac{1}{6} \\cdot 0=0\n\\]\nand\n\\[\n\\lim _{mountain \\rightarrow \\infty} monument^{\\prime}(mountain)=\\frac{1}{2}\\left[waterfall-waterfall-\\frac{1}{6} \\cdot 0-\\frac{1}{6} \\cdot 0\\right]=0\n\\]\n\nRemarks. The hypothesis can be weakened; it is sufficient to assume that \\( monument^{m} \\) is bounded. Moreover, if we assume \\( \\lim _{mountain-\\infty} monument(mountain) \\) exists and \\( monument^{(n)} \\) is bounded, we can prove that \\( \\lim _{mountain \\rightarrow \\infty} monument^{(k)}(mountain)=0 \\) for \\( 0<k<n \\). This was published by Littlewood in \"The Converse of Abel's Theorem,\" Proceedings of the London Mathematical Society (2), vol. 9 (1910-11), pages 434-448. For results concerning precise bounds for \\( monument^{(k)}(mountain) \\) given bounds for \\( monument(mountain) \\) and \\( monument^{(n)}(mountain) \\) where \\( 0<k<n \\), see I. J. Schoenberg, \"The Elementary Cases of Landau's Problem of Inequalities Between Derivatives,\" American Mathematical Monthly, vol. 80 (1973), pages 121-158."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "finitevalue",
        "\\xi": "infinite",
        "\\eta": "boundless",
        "C": "variable",
        "f": "constant"
      },
      "question": "4. Let \\( variable \\) be a real number, and let \\( constant \\) be a function such that\n\\[\n\\lim _{finitevalue \\rightarrow \\infty} constant(finitevalue)=variable, \\quad \\lim _{finitevalue \\rightarrow \\infty} constant^{\\prime \\prime \\prime}(finitevalue)=0\n\\]\n\nProve that\n\\[\n\\lim _{finitevalue \\rightarrow \\infty} constant^{\\prime}(finitevalue)=0 \\quad \\text { and } \\quad \\lim _{finitevalue \\rightarrow \\infty} constant^{\\prime \\prime}(finitevalue)=0\n\\]\nwhere superscripts denote derivatives.",
      "solution": "Solution. By Taylor's theorem (finite form, or extended law of the mean), for any \\( finitevalue \\) there exist numbers \\( infinite(finitevalue) \\) and \\( boundless(finitevalue) \\) between 0 and 1 such that\n\\[\n\\begin{array}{l}\nconstant(finitevalue+1)=constant(finitevalue)+constant^{\\prime}(finitevalue)+\\frac{1}{2} constant^{\\prime \\prime}(finitevalue)+\\frac{1}{6} constant^{\\prime \\prime}(finitevalue+infinite(finitevalue)) \\\\\nconstant(finitevalue-1)=constant(finitevalue)-constant^{\\prime}(finitevalue)+\\frac{1}{2} constant^{\\prime \\prime}(finitevalue)-\\frac{1}{6} constant^{\\prime \\prime \\prime}(finitevalue-boundless(finitevalue))\n\\end{array}\n\\]\n\nAdding these relations and transposing terms, we get\n\\[\n\\begin{aligned}\nconstant^{\\prime \\prime}(finitevalue)=constant(finitevalue+1)-2 constant(finitevalue)+constant(finitevalue-1) & \\\\\n& -\\frac{1}{6} constant^{\\prime \\prime \\prime}(finitevalue+infinite(finitevalue))+\\frac{1}{6} constant^{\\prime \\prime}(finitevalue-boundless(finitevalue)) .\n\\end{aligned}\n\\]\n\nSubtracting (2) from (1) and transposing terms, we get\n\\[\n2 constant^{\\prime}(finitevalue)=constant(finitevalue+1)-constant(finitevalue-1)-\\frac{1}{6} constant^{m \\prime}(finitevalue+infinite(finitevalue))-\\frac{1}{6} constant^{\\prime \\prime}(finitevalue-boundless(finitevalue)) .\n\\]\n\nAs \\( finitevalue \\rightarrow \\infty \\), so do \\( finitevalue-boundless(finitevalue) \\) and \\( finitevalue+infinite(finitevalue) \\), so all terms on the right of (3) and (4) have limits, and\n\\[\n\\lim _{finitevalue \\rightarrow \\infty} constant^{\\prime \\prime}(finitevalue)=variable-2 variable+variable-\\frac{1}{6} \\cdot 0+\\frac{1}{6} \\cdot 0=0\n\\]\nand\n\\[\n\\lim _{finitevalue \\rightarrow \\infty} constant^{\\prime}(finitevalue)=\\frac{1}{2}\\left[variable-variable-\\frac{1}{6} \\cdot 0-\\frac{1}{6} \\cdot 0\\right]=0\n\\]\n\nRemarks. The hypothesis can be weakened; it is sufficient to assume that \\( constant^{m} \\) is bounded. Moreover, if we assume \\( \\lim _{finitevalue-\\infty} constant(finitevalue) \\) exists and \\( constant^{(n)} \\) is bounded, we can prove that \\( \\lim _{finitevalue \\rightarrow \\infty} constant^{(k)}(finitevalue)=0 \\) for \\( 0<k<n \\). This was published by Littlewood in \"The Converse of Abel's Theorem,\" Proceedings of the London Mathematical Society (2), vol. 9 (1910-11), pages 434-448. For results concerning precise bounds for \\( constant^{(k)}(finitevalue) \\) given bounds for \\( constant(finitevalue) \\) and \\( constant^{(n)}(finitevalue) \\) where \\( 0<k<n \\), see I. J. Schoenberg, \"The Elementary Cases of Landau's Problem of Inequalities Between Derivatives,\" American Mathematical Monthly, vol. 80 (1973), pages 121-158."
    },
    "garbled_string": {
      "map": {
        "x": "pqlmznvr",
        "\\xi": "hdjslqmn",
        "\\eta": "vbncxzlk",
        "C": "rtyuioqk",
        "f": "lkjhgfdsa"
      },
      "question": "4. Let \\( rtyuioqk \\) be a real number, and let \\( lkjhgfdsa \\) be a function such that\n\\[\n\\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa(pqlmznvr)=rtyuioqk, \\quad \\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa^{\\prime \\prime \\prime}(pqlmznvr)=0\n\\]\n\nProve that\n\\[\n\\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa^{\\prime}(pqlmznvr)=0 \\quad \\text { and } \\quad \\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa^{\\prime \\prime}(pqlmznvr)=0\n\\]\nwhere superscripts denote derivatives.",
      "solution": "Solution. By Taylor's theorem (finite form, or extended law of the mean), for any \\( pqlmznvr \\) there exist numbers \\( hdjslqmn(pqlmznvr) \\) and \\( vbncxzlk(pqlmznvr) \\) between 0 and 1 such that\n\\[\n\\begin{array}{l}\nlkjhgfdsa(pqlmznvr+1)=lkjhgfdsa(pqlmznvr)+lkjhgfdsa^{\\prime}(pqlmznvr)+\\frac{1}{2} lkjhgfdsa^{\\prime \\prime}(pqlmznvr)+\\frac{1}{6} lkjhgfdsa^{\\prime \\prime}(pqlmznvr+hdjslqmn(pqlmznvr)) \\\\\nlkjhgfdsa(pqlmznvr-1)=lkjhgfdsa(pqlmznvr)-lkjhgfdsa^{\\prime}(pqlmznvr)+\\frac{1}{2} lkjhgfdsa^{\\prime \\prime}(pqlmznvr)-\\frac{1}{6} lkjhgfdsa^{\\prime \\prime \\prime}(pqlmznvr-vbncxzlk(pqlmznvr))\n\\end{array}\n\\]\n\nAdding these relations and transposing terms, we get\n\\[\n\\begin{aligned}\nlkjhgfdsa^{\\prime \\prime}(pqlmznvr)=lkjhgfdsa(pqlmznvr+1)-2 lkjhgfdsa(pqlmznvr)+lkjhgfdsa(pqlmznvr-1) & \\\\\n& -\\frac{1}{6} lkjhgfdsa^{\\prime \\prime \\prime}(pqlmznvr+hdjslqmn(pqlmznvr))+\\frac{1}{6} lkjhgfdsa^{\\prime \\prime}(pqlmznvr-vbncxzlk(pqlmznvr)) .\n\\end{aligned}\n\\]\n\nSubtracting (2) from (1) and transposing terms, we get\n\\[\n2 lkjhgfdsa^{\\prime}(pqlmznvr)=lkjhgfdsa(pqlmznvr+1)-lkjhgfdsa(pqlmznvr-1)-\\frac{1}{6} lkjhgfdsa^{m \\prime}(pqlmznvr+hdjslqmn(pqlmznvr))-\\frac{1}{6} lkjhgfdsa^{\\prime \\prime}(pqlmznvr-vbncxzlk(pqlmznvr)) .\n\\]\n\nAs \\( pqlmznvr \\rightarrow \\infty \\), so do \\( pqlmznvr-vbncxzlk(pqlmznvr) \\) and \\( pqlmznvr+hdjslqmn(pqlmznvr) \\), so all terms on the right of (3) and (4) have limits, and\n\\[\n\\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa^{\\prime \\prime}(pqlmznvr)=rtyuioqk-2 rtyuioqk+rtyuioqk-\\frac{1}{6} \\cdot 0+\\frac{1}{6} \\cdot 0=0\n\\]\nand\n\\[\n\\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa^{\\prime}(pqlmznvr)=\\frac{1}{2}\\left[rtyuioqk-rtyuioqk-\\frac{1}{6} \\cdot 0-\\frac{1}{6} \\cdot 0\\right]=0\n\\]\n\nRemarks. The hypothesis can be weakened; it is sufficient to assume that \\( lkjhgfdsa^{m} \\) is bounded. Moreover, if we assume \\( \\lim _{pqlmznvr-\\infty} lkjhgfdsa(pqlmznvr) \\) exists and \\( lkjhgfdsa^{(n)} \\) is bounded, we can prove that \\( \\lim _{pqlmznvr \\rightarrow \\infty} lkjhgfdsa^{(k)}(pqlmznvr)=0 \\) for \\( 0<k<n \\). This was published by Littlewood in \"The Converse of Abel's Theorem,\" Proceedings of the London Mathematical Society (2), vol. 9 (1910-11), pages 434-448. For results concerning precise bounds for \\( lkjhgfdsa^{(k)}(pqlmznvr) \\) given bounds for \\( lkjhgfdsa(pqlmznvr) \\) and \\( lkjhgfdsa^{(n)}(pqlmznvr) \\) where \\( 0<k<n \\), see I. J. Schoenberg, \"The Elementary Cases of Landau's Problem of Inequalities Between Derivatives,\" American Mathematical Monthly, vol. 80 (1973), pages 121-158."
    },
    "kernel_variant": {
      "question": "Fix integers d ≥ 1 and m ≥ 3.  \nLet M > 0 and let  \n\n  f : { x ∈ ℝᵈ : |x| > M } → ℝ ,  f ∈ C^{m}\n\nbe given.  Assume\n\n(A) lim_{|x|→∞} f(x) = L ∈ ℝ ,                                       \n(B) lim_{|x|→∞} D^{α}f(x) = 0 for every multi-index α with |α| = m.\n\nShow that for every multi-index β with  \n  1 ≤ |β| ≤ m − 1                                                       \nwe also have  \n\n  lim_{|x|→∞} D^{β}f(x) = 0 .                                                        \n\n(Thus every derivative of order strictly between 0 and m tends to 0 at infinity.)",
      "solution": "Step 0 – Reduction to the case L = 0.  \nReplace f by f − L.  Assumptions (A)–(B) and the desired conclusion are invariant under this\ntranslation, so from now on  \n\n  lim_{|x|→∞} f(x) = 0.                                            (0.1)\n\n\n\nStep 1 – A local Landau–Kolmogorov inequality in arbitrary dimension.  \n\nLemma 1 (one–variable, scale-invariant).  \nLet g ∈ C^{m}([−1,1]).  For k = 1,…,m − 1\n\n  |g^{(k)}(0)| ≤ C(m,k) ‖g‖_{∞}^{1−k/m} ‖g^{(m)}‖_{∞}^{k/m}.         (1.1)\n\n(The constant C(m,k) depends only on m and k.)\n\nProof.  This is the classical Landau–Kolmogorov inequality; a short proof is obtained by optimising\nTaylor’s formula with integral remainder.\n\nLemma 2 (pointwise interpolation in ℝᵈ).  \nLet m ≥ 3 and 1 ≤ k ≤ m − 1.  There exists a constant C(d,m,β) such that for every\nmulti-index β with |β| = k, every x ∈ ℝᵈ, every ρ > 0 and every\nG ∈ C^{m}(B(x,ρ)) one has  \n\n  |D^{β}G(x)| ≤ C(d,m,β) · A(x,ρ)^{1−k/m} · B(x,ρ)^{k/m},            (1.2)  \n\nwhere  \n\n  A(x,ρ) := sup_{|y−x|≤ρ} |G(y)|,  \n  B(x,ρ) := max_{|α|=m} sup_{|y−x|≤ρ} |D^{α}G(y)| .\n\nProof.  \nFix a unit vector θ ∈ 𝕊^{d−1} and set h(t) = G(x + ρ t θ) for t ∈ [−1,1].\nApplying (1.1) to h and rescaling gives  \n\n  |∂_{θ}^{k}G(x)| ≤ C(m,k) A(x,ρ)^{1−k/m} B(x,ρ)^{k/m}.          (1.3)\n\nBecause ∂_{θ}^{k}G(x) = Σ_{|β|=k} θ^{β} D^{β}G(x), choose a collection Θ ⊂ 𝕊^{d−1}\nwith |Θ| = N(d,k) so that the matrix (θ^{β})_{θ∈Θ,|β|=k} is invertible.\nSolving the resulting linear system bounds every |D^{β}G(x)| by the right–hand\nside of (1.3) up to a factor depending only on d,m,β. ∎\n\n\n\nStep 2 – Application to f.  \n\nFix a multi-index β, 1 ≤ |β| = k ≤ m − 1, and let C = C(d,m,β) be as in (1.2).  \nFor any x with |x| > 2M set  \n\n  ρ(x) := |x|/2        (so B(x,ρ(x)) ⊂ { |y| > M }).\n\nDefine  \n\n  A(x) := sup_{|y−x|≤ρ(x)} |f(y)|,  \n  B(x) := max_{|α|=m} sup_{|y−x|≤ρ(x)} |D^{α}f(y)|.                (2.1)\n\nBecause |y| ≥ |x| − ρ(x) = |x|/2 for every y in this ball,\nassumptions (0.1) and (B) imply  \n\n  A(x)  → 0,  B(x) → 0  as |x| → ∞.                        (2.2)\n\nApplying (1.2) with G = f and ρ = ρ(x) gives the pointwise bound  \n\n  |D^{β}f(x)| ≤ C · A(x)^{1−k/m} · B(x)^{k/m}.                (2.3)\n\nLet ε > 0 be arbitrary.  \nChoose R so large that A(x) < ε and B(x) < ε whenever |x| ≥ R\n(allowed by (2.2)).  Then (2.3) yields  \n\n  |D^{β}f(x)| ≤ C ε^{1−k/m} ε^{k/m} = C ε         (|x| ≥ R).\n\nLetting ε ↓ 0 proves  \n\n  lim_{|x|→∞} D^{β}f(x) = 0.                            (2.4)\n\nSince β was arbitrary with 1 ≤ |β| ≤ m − 1, the theorem is established. ∎",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.497959",
        "was_fixed": false,
        "difficulty_analysis": "• Higher dimension.  The original problem was one–dimensional; the variant demands control of all mixed partial derivatives on ℝᵈ, introducing multi–index notation, cubes in ℝᵈ and much heavier bookkeeping.  \n• Higher order.  All derivatives up to order m−1 (not merely the first two) must be handled simultaneously.  \n• Sophisticated tools.  One must know and use the multidimensional Newton–Gregory (finite–difference) formula or Taylor’s theorem with multi–index remainder, neither of which is needed for the original question.  \n• Interacting concepts.  The argument blends pointwise limits, uniform remainder estimates, combinatorial properties of difference operators (the vanishing sum of coefficients), and geometric considerations (keeping all sampling points outside a large ball).  \n• Depth of reasoning.  Unlike the original “do it once for order 1 and 2” calculation, the solution requires an induction–free but conceptually subtler device that works for every β at once and in several variables.  Each of these layers significantly raises the technical and conceptual load compared with the initial problem."
      }
    },
    "original_kernel_variant": {
      "question": "Fix integers d ≥ 1 and m ≥ 3.  \nLet M > 0 and let  \n\n  f : { x ∈ ℝᵈ : |x| > M } → ℝ ,  f ∈ C^{m}\n\nbe given.  Assume\n\n(A) lim_{|x|→∞} f(x) = L ∈ ℝ ,                                       \n(B) lim_{|x|→∞} D^{α}f(x) = 0 for every multi-index α with |α| = m.\n\nShow that for every multi-index β with  \n  1 ≤ |β| ≤ m − 1                                                       \nwe also have  \n\n  lim_{|x|→∞} D^{β}f(x) = 0 .                                                        \n\n(Thus every derivative of order strictly between 0 and m tends to 0 at infinity.)",
      "solution": "Step 0 – Reduction to the case L = 0.  \nReplace f by f − L.  Assumptions (A)–(B) and the desired conclusion are invariant under this\ntranslation, so from now on  \n\n  lim_{|x|→∞} f(x) = 0.                                            (0.1)\n\n\n\nStep 1 – A local Landau–Kolmogorov inequality in arbitrary dimension.  \n\nLemma 1 (one–variable, scale-invariant).  \nLet g ∈ C^{m}([−1,1]).  For k = 1,…,m − 1\n\n  |g^{(k)}(0)| ≤ C(m,k) ‖g‖_{∞}^{1−k/m} ‖g^{(m)}‖_{∞}^{k/m}.         (1.1)\n\n(The constant C(m,k) depends only on m and k.)\n\nProof.  This is the classical Landau–Kolmogorov inequality; a short proof is obtained by optimising\nTaylor’s formula with integral remainder.\n\nLemma 2 (pointwise interpolation in ℝᵈ).  \nLet m ≥ 3 and 1 ≤ k ≤ m − 1.  There exists a constant C(d,m,β) such that for every\nmulti-index β with |β| = k, every x ∈ ℝᵈ, every ρ > 0 and every\nG ∈ C^{m}(B(x,ρ)) one has  \n\n  |D^{β}G(x)| ≤ C(d,m,β) · A(x,ρ)^{1−k/m} · B(x,ρ)^{k/m},            (1.2)  \n\nwhere  \n\n  A(x,ρ) := sup_{|y−x|≤ρ} |G(y)|,  \n  B(x,ρ) := max_{|α|=m} sup_{|y−x|≤ρ} |D^{α}G(y)| .\n\nProof.  \nFix a unit vector θ ∈ 𝕊^{d−1} and set h(t) = G(x + ρ t θ) for t ∈ [−1,1].\nApplying (1.1) to h and rescaling gives  \n\n  |∂_{θ}^{k}G(x)| ≤ C(m,k) A(x,ρ)^{1−k/m} B(x,ρ)^{k/m}.          (1.3)\n\nBecause ∂_{θ}^{k}G(x) = Σ_{|β|=k} θ^{β} D^{β}G(x), choose a collection Θ ⊂ 𝕊^{d−1}\nwith |Θ| = N(d,k) so that the matrix (θ^{β})_{θ∈Θ,|β|=k} is invertible.\nSolving the resulting linear system bounds every |D^{β}G(x)| by the right–hand\nside of (1.3) up to a factor depending only on d,m,β. ∎\n\n\n\nStep 2 – Application to f.  \n\nFix a multi-index β, 1 ≤ |β| = k ≤ m − 1, and let C = C(d,m,β) be as in (1.2).  \nFor any x with |x| > 2M set  \n\n  ρ(x) := |x|/2        (so B(x,ρ(x)) ⊂ { |y| > M }).\n\nDefine  \n\n  A(x) := sup_{|y−x|≤ρ(x)} |f(y)|,  \n  B(x) := max_{|α|=m} sup_{|y−x|≤ρ(x)} |D^{α}f(y)|.                (2.1)\n\nBecause |y| ≥ |x| − ρ(x) = |x|/2 for every y in this ball,\nassumptions (0.1) and (B) imply  \n\n  A(x)  → 0,  B(x) → 0  as |x| → ∞.                        (2.2)\n\nApplying (1.2) with G = f and ρ = ρ(x) gives the pointwise bound  \n\n  |D^{β}f(x)| ≤ C · A(x)^{1−k/m} · B(x)^{k/m}.                (2.3)\n\nLet ε > 0 be arbitrary.  \nChoose R so large that A(x) < ε and B(x) < ε whenever |x| ≥ R\n(allowed by (2.2)).  Then (2.3) yields  \n\n  |D^{β}f(x)| ≤ C ε^{1−k/m} ε^{k/m} = C ε         (|x| ≥ R).\n\nLetting ε ↓ 0 proves  \n\n  lim_{|x|→∞} D^{β}f(x) = 0.                            (2.4)\n\nSince β was arbitrary with 1 ≤ |β| ≤ m − 1, the theorem is established. ∎",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.416828",
        "was_fixed": false,
        "difficulty_analysis": "• Higher dimension.  The original problem was one–dimensional; the variant demands control of all mixed partial derivatives on ℝᵈ, introducing multi–index notation, cubes in ℝᵈ and much heavier bookkeeping.  \n• Higher order.  All derivatives up to order m−1 (not merely the first two) must be handled simultaneously.  \n• Sophisticated tools.  One must know and use the multidimensional Newton–Gregory (finite–difference) formula or Taylor’s theorem with multi–index remainder, neither of which is needed for the original question.  \n• Interacting concepts.  The argument blends pointwise limits, uniform remainder estimates, combinatorial properties of difference operators (the vanishing sum of coefficients), and geometric considerations (keeping all sampling points outside a large ball).  \n• Depth of reasoning.  Unlike the original “do it once for order 1 and 2” calculation, the solution requires an induction–free but conceptually subtler device that works for every β at once and in several variables.  Each of these layers significantly raises the technical and conceptual load compared with the initial problem."
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}