Minuscule polynomials of type A

DefinitionType-A minuscule polynomial#✓fully proved

For a natural number \(n\), the type-A minuscule polynomial \(N_n(x)\) is the polynomial with rational coefficients

\[N_n(x) = \frac{1}{4n + 2} \sum_{k=1}^{n-1} k (n - k) \binom{2n + 2}{2k + 1} x^k.\]

For \(n = 0\) the sum is empty, so \(N_0 = 0\).

public noncomputable def minusculePolynomialA (n : ℕ) : ℚ[X] :=
  ∑ k ∈ Finset.Icc 1 (n - 1),
    C ((k : ℚ) * ((n : ℚ) - k) * ((2 * n + 2).choose (2 * k + 1) : ℚ) / (4 * n + 2)) * X ^ k
Sources
  • Ming-Jian Ding and Jiang Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, definition (1.1) (arXiv:2308.16782v3, equation (1.1)).
Used by 13

TheoremThe minuscule polynomial \(N_1\) vanishes#✓fully proved

The type-A minuscule polynomial \(N_1(x)\) is the zero polynomial.

public theorem minusculePolynomialA_one : minusculePolynomialA 1 = 0
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, table of N_n(x) after (1.1).
Uses 1

TheoremThe minuscule polynomial \(N_2\)#✓fully proved

The type-A minuscule polynomial \(N_2(x)\) equals \(2x\).

public theorem minusculePolynomialA_two : minusculePolynomialA 2 = C 2 * X
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, table of N_n(x) after (1.1).
Uses 1

TheoremThe minuscule polynomial \(N_3\)#✓fully proved

The type-A minuscule polynomial \(N_3(x)\) equals \(8x + 8x^2\).

public theorem minusculePolynomialA_three :
    minusculePolynomialA 3 = C 8 * X + C 8 * X ^ 2
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, table of N_n(x) after (1.1).
Uses 1

TheoremReal-rootedness of the minuscule polynomials#●ready to prove

For every natural number \(n\), the type-A minuscule polynomial \(N_n\), viewed as a complex polynomial, is real-rooted: every element of its multiset of roots is real.

The paper states this for positive \(n\); for \(n \le 1\) the polynomial \(N_n\) is \(0\), whose multiset of roots is taken to be empty.

public theorem_wanted minusculePolynomialA_isRealRooted (n : ℕ) :
    ((minusculePolynomialA n).map (algebraMap ℚ ℂ)).IsRealRooted
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Theorem 5 (first part).
Uses 2

TheoremInterlacing of consecutive minuscule polynomials#●ready to prove

Let \(n \ge 2\). There are nonzero real numbers \(c, d\) and real numbers \(r_0 \le \dots \le r_{n-1}\) and \(s_0 \le \dots \le s_{n-2}\) such that the type-A minuscule polynomials factor over \(\mathbb{R}\) as \(N_{n+1}(x) = c \prod_{i=0}^{n-1} (x - r_i)\) and \(N_n(x) = d \prod_{j=0}^{n-2} (x - s_j)\), and \(r_j \le s_j \le r_{j+1}\) for every \(j\) with \(0 \le j \le n - 2\).

This is the interlacing \(N_n \preceq N_{n+1}\), relation (1.2) of the paper written in increasing order.

public theorem_wanted minusculePolynomialA_interlaces_succ (n : ℕ) (hn : 2 ≤ n) :
    ∃ (c d : ℝ) (r : Fin n → ℝ) (s : Fin (n - 1) → ℝ), c ≠ 0 ∧ d ≠ 0 ∧
      Monotone r ∧ Monotone s ∧
      (minusculePolynomialA (n + 1)).map (algebraMap ℚ ℝ) = C c * ∏ i, (X - C (r i)) ∧
      (minusculePolynomialA n).map (algebraMap ℚ ℝ) = C d * ∏ j, (X - C (s j)) ∧
      ∀ (j : ℕ) (hj : j + 1 < n),
        r ⟨j, by omega⟩ ≤ s ⟨j, by omega⟩ ∧ s ⟨j, by omega⟩ ≤ r ⟨j + 1, hj⟩
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Theorem 5 (second part).
Uses 1

TheoremLog-concavity of the coefficients of the minuscule polynomials#●ready to prove

Let \(N_{n,k}\) be the coefficient of \(x^k\) in the type-A minuscule polynomial \(N_n(x)\). For all natural numbers \(n\) and \(k\), \(N_{n,k-1} N_{n,k+1} \le N_{n,k}^2\), where for \(k = 0\) the index \(k - 1\) is read as \(0\).

The cases outside \(n \ge 1\), \(1 \le k \le n - 1\) hold because the coefficients involved vanish.

public theorem_wanted minusculePolynomialA_coeff_logConcave (n k : ℕ) :
    (minusculePolynomialA n).coeff (k - 1) * (minusculePolynomialA n).coeff (k + 1) ≤
      (minusculePolynomialA n).coeff k ^ 2
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Corollary 7.
Uses 1

TheoremGamma-positivity of the minuscule polynomials#●ready to prove

For every natural number \(n\) there are non-negative integers \(\gamma_{n,0}, \dots, \gamma_{n,\lfloor n/2 \rfloor}\) such that the type-A minuscule polynomial \(N_n\) satisfies \(N_n(x) = \sum_{k=0}^{\lfloor n/2 \rfloor} \gamma_{n,k}\, x^k (1 + x)^{n-2k}\).

For \(n = 0\) this holds with every \(\gamma_{0,k} = 0\), since \(N_0 = 0\).

public theorem_wanted minusculePolynomialA_gamma_positive (n : ℕ) :
    ∃ γ : ℕ → ℕ, minusculePolynomialA n =
      ∑ k ∈ Finset.range (n / 2 + 1), C (γ k : ℚ) * X ^ k * (1 + X) ^ (n - 2 * k)
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Corollary 8.
Uses 1

TheoremBinomial closed form of the minuscule polynomials#●ready to prove

For every natural number \(n\), the type-A minuscule polynomial \(N_n\) satisfies the identity of polynomials with rational coefficients \(16x\, N_n(x^2) = 2(n+1)x \bigl((1+x)^{2n} + (1-x)^{2n}\bigr) - \bigl((1+x)^{2n+2} - (1-x)^{2n+2}\bigr)\).

Dividing by \(16x\) gives \(N_n(x^2) = \frac{n+1}{8} \bigl((1+x)^{2n} + (1-x)^{2n}\bigr) - \frac{1}{16x} \bigl((1+x)^{2n+2} - (1-x)^{2n+2}\bigr)\); both sides vanish at \(n = 0\).

public theorem_wanted minusculePolynomialA_comp_sq (n : ℕ) :
    C 16 * X * (minusculePolynomialA n).comp (X ^ 2) =
      C (2 * ((n : ℚ) + 1)) * X * ((1 + X) ^ (2 * n) + (1 - X) ^ (2 * n)) -
        ((1 + X) ^ (2 * n + 2) - (1 - X) ^ (2 * n + 2))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Proposition 9.
Uses 1

TheoremExponential generating function of the minuscule polynomials#●ready to prove

For all real numbers \(x\) and \(z\) with \(x \ne 0\), the series \(\sum_{n \ge 0} N_n(x^2)\, z^n / n!\) built from the type-A minuscule polynomials \(N_n\) has sum \(\frac{2x(1+x)^2 z - 1 - x^2}{16x} e^{(1+x)^2 z} + \frac{2x(1-x)^2 z + 1 + x^2}{16x} e^{(1-x)^2 z}\).

The sum starts at \(n = 0\), where \(N_0 = 0\).

public theorem_wanted minusculePolynomialA_egf (x z : ℝ) (hx : x ≠ 0) :
    HasSum (fun n : ℕ => aeval (x ^ 2) (minusculePolynomialA n) * z ^ n / n.factorial)
      ((2 * x * (1 + x) ^ 2 * z - 1 - x ^ 2) / (16 * x) * Real.exp ((1 + x) ^ 2 * z) +
        (2 * x * (1 - x) ^ 2 * z + 1 + x ^ 2) / (16 * x) * Real.exp ((1 - x) ^ 2 * z))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Proposition 10.
Uses 1

TheoremBourn–Erickson formula for the minuscule polynomials at one#●ready to prove

For every integer \(n \ge 1\), the type-A minuscule polynomial \(N_n\) satisfies \(N_n(1) = (n - 1)\, 2^{2n-3}\).

public theorem_wanted minusculePolynomialA_eval_one (n : ℕ) (hn : 1 ≤ n) :
    (minusculePolynomialA n).eval 1 = ((n : ℚ) - 1) * 2 ^ ((2 * n : ℤ) - 3)
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Corollary 11.
Uses 1

TheoremBender's central limit theorem for polynomials with non-positive real zeros#●ready to prove

For each natural number \(n\) let \(r_{n,0}, \dots, r_{n,n-1}\) be non-negative real numbers, and let \(a(n,k)\) be the coefficient of \(x^k\) in the real polynomial \(A_n(x) = \prod_{i\lt n} (x + r_{n,i})\). Put \(\mu_n = \sum_{i\lt n} 1/(1 + r_{n,i})\) and \(\sigma_n^2 = \sum_{i\lt n} r_{n,i}/(1 + r_{n,i})^2\), and let \(\sigma_n \ge 0\) be the square root of \(\sigma_n^2\). Then for every \(n\), \(\sum_{k=0}^{n} k\, a(n,k) / \sum_{k=0}^{n} a(n,k) = \mu_n\) and \(\sum_{k=0}^{n} (k - \mu_n)^2 a(n,k) / \sum_{k=0}^{n} a(n,k) = \sigma_n^2\). Moreover, if \(\sigma_n^2 \to \infty\) as \(n \to \infty\), then for every real \(x\), \(\sum_{0 \le k \le n,\ k \le \mu_n + x \sigma_n} a(n,k) / \sum_{k=0}^{n} a(n,k)\) tends to \(\Phi(x)\) as \(n \to \infty\), where \(\Phi\) is the distribution function of the standard normal distribution.

public theorem_wanted bender_asymptotic_normality (r : (n : ℕ) → Fin n → ℝ)
    (hr : ∀ n i, 0 ≤ r n i) :
    let a : ℕ → ℕ → ℝ := fun n k => (∏ i, (X + C (r n i))).coeff k
    let μ : ℕ → ℝ := fun n => ∑ i, 1 / (1 + r n i)
    let σ2 : ℕ → ℝ := fun n => ∑ i, r n i / (1 + r n i) ^ 2
    (∀ n, (∑ k ∈ Finset.range (n + 1), (k : ℝ) * a n k) /
          (∑ k ∈ Finset.range (n + 1), a n k) = μ n ∧
        (∑ k ∈ Finset.range (n + 1), ((k : ℝ) - μ n) ^ 2 * a n k) /
          (∑ k ∈ Finset.range (n + 1), a n k) = σ2 n) ∧
    (Tendsto σ2 atTop atTop → ∀ x : ℝ,
      Tendsto (fun n => (∑ k ∈ (Finset.range (n + 1)).filter (fun k : ℕ => (k : ℝ) ≤ μ n + x * √(σ2 n)),
            a n k) /
          (∑ k ∈ Finset.range (n + 1), a n k))
        atTop (𝓝 (ProbabilityTheory.cdf (ProbabilityTheory.gaussianReal 0 1) x)))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Lemma 12 (Bender, Theorem 2).

TheoremAsymptotic normality of the coefficients of the minuscule polynomials#●ready to prove

Let \(N_{n,k}\) be the coefficient of \(x^k\) in the type-A minuscule polynomial \(N_n(x)\), and let \(\sigma_n = \sqrt{(n^2 - n - 2)/(8(n - 1))}\). Then for every real \(x\), the sum of the coefficients \(N_{n,k}\) over \(0 \le k < n\) with \(k \le n/2 + x \sigma_n\), divided by \(N_n(1)\), tends to \(\Phi(x)\) as \(n \to \infty\), where \(\Phi\) is the distribution function of the standard normal distribution.

The centring \(n/2\) and the scale \(\sigma_n^2\) are the mean and the variance of the coefficient distribution \(k \mapsto N_{n,k}/N_n(1)\).

public theorem_wanted minusculePolynomialA_asymptotically_normal (x : ℝ) :
    Tendsto (fun n : ℕ =>
        (∑ k ∈ (Finset.range n).filter (fun k : ℕ =>
            (k : ℝ) ≤ n / 2 + x * √(((n : ℝ) ^ 2 - n - 2) / (8 * ((n : ℝ) - 1)))),
          ((minusculePolynomialA n).coeff k : ℝ)) / (((minusculePolynomialA n).eval 1 : ℚ) : ℝ))
      atTop (𝓝 (ProbabilityTheory.cdf (ProbabilityTheory.gaussianReal 0 1) x))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Theorem 13.
Uses 1

Theorem\(x\)-log-concavity of the minuscule polynomials#●ready to prove

Let \(N_n\) denote the type-A minuscule polynomial. For all natural numbers \(n\) and \(k\), the coefficient of \(x^k\) in \(N_n(x)^2 - N_{n+1}(x) N_{n-1}(x)\) is non-negative, where \(N_{n-1}\) is read as \(N_0\) when \(n = 0\).

For \(n \le 1\) the polynomial is \(0\) since \(N_0 = N_1 = 0\).

public theorem_wanted minusculePolynomialA_x_logConcave (n k : ℕ) :
    0 ≤ (minusculePolynomialA n ^ 2 -
      minusculePolynomialA (n + 1) * minusculePolynomialA (n - 1)).coeff k
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Theorem 14.
Uses 1

TheoremTotal positivity of the coefficient matrix of the minuscule polynomials#●ready to prove

Let \(N_{n,k}\) be the coefficient of \(x^k\) in the type-A minuscule polynomial \(N_n(x)\). The infinite real matrix \([N_{n,k}]_{n,k \ge 1}\), whose entry in row \(i\) and column \(j\) (\(i, j \ge 0\)) is \(N_{i+1,j+1}\), has nonnegative minors: every square submatrix obtained by choosing finitely many rows and finitely many columns, each in increasing order, has nonnegative determinant.

The matrix vanishes on and above its diagonal, since \(N_n\) has degree at most \(n - 1\).

public theorem_wanted minusculePolynomialA_coeffMatrix_hasNonnegativeMinors :
    Matrix.HasNonnegativeMinors
      (Matrix.of fun i j : ℕ => ((minusculePolynomialA (i + 1)).coeff (j + 1) : ℝ))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Theorem 19.
Uses 2

TheoremCraven–Csordas characterization of multiplier \(n\)-sequences#●ready to prove

Let \(n\) be a natural number and \(t_0, t_1, \dots\) real numbers. Say a real polynomial splits over \(\mathbb{R}\) if it is a constant times a product of linear factors \(x - r\) with \(r\) real (the zero polynomial splits). Then the following are equivalent: for every real polynomial \(\sum_k a_k x^k\) of degree at most \(n\) that splits over \(\mathbb{R}\), the polynomial \(\sum_{k=0}^n a_k t_k x^k\) splits over \(\mathbb{R}\); and the polynomial \(\sum_{k=0}^n \binom{n}{k} t_k x^k\) splits over \(\mathbb{R}\) and its roots are either all \(\ge 0\) or all \(\le 0\).

Only \(t_0, \dots, t_n\) matter.

public theorem_wanted multiplier_n_sequence_iff (n : ℕ) (t : ℕ → ℝ) :
    (∀ p : ℝ[X], p.natDegree ≤ n → p.Splits →
      (∑ k ∈ Finset.range (n + 1), C (p.coeff k * t k) * X ^ k).Splits) ↔
    ((∑ k ∈ Finset.range (n + 1), C ((n.choose k : ℝ) * t k) * X ^ k).Splits ∧
      ((∀ x ∈ (∑ k ∈ Finset.range (n + 1), C ((n.choose k : ℝ) * t k) * X ^ k).roots, 0 ≤ x) ∨
        ∀ x ∈ (∑ k ∈ Finset.range (n + 1), C ((n.choose k : ℝ) * t k) * X ^ k).roots, x ≤ 0))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Lemma 3 (Craven and Csordas, Theorem 3.7).

TheoremPólya–Schur characterization of multiplier sequences#●ready to prove

Let \(t_0, t_1, \dots\) be non-negative real numbers. Say a real polynomial splits over \(\mathbb{R}\) if it is a constant times a product of linear factors \(x - r\) with \(r\) real (the zero polynomial splits). Then the following are equivalent: for every real polynomial \(\sum_k a_k x^k\) that splits over \(\mathbb{R}\), the polynomial \(\sum_k a_k t_k x^k\) splits over \(\mathbb{R}\); and for every \(n \ge 1\), the polynomial \(\sum_{k=0}^n \binom{n}{k} t_k x^k\) splits over \(\mathbb{R}\) and its roots are either all \(\ge 0\) or all \(\le 0\).

public theorem_wanted multiplier_sequence_iff (t : ℕ → ℝ) (ht : ∀ k, 0 ≤ t k) :
    (∀ p : ℝ[X], p.Splits →
      (∑ k ∈ Finset.range (p.natDegree + 1), C (p.coeff k * t k) * X ^ k).Splits) ↔
    ∀ n, 1 ≤ n →
      (∑ k ∈ Finset.range (n + 1), C ((n.choose k : ℝ) * t k) * X ^ k).Splits ∧
        ((∀ x ∈ (∑ k ∈ Finset.range (n + 1), C ((n.choose k : ℝ) * t k) * X ^ k).roots,
            0 ≤ x) ∨
          ∀ x ∈ (∑ k ∈ Finset.range (n + 1), C ((n.choose k : ℝ) * t k) * X ^ k).roots, x ≤ 0)
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Lemma 18 (Pólya and Schur).

TheoremAissen–Schoenberg–Whitney–Edrei theorem#●ready to prove

Let \(a_0, a_1, \dots\) be non-negative real numbers with \(a_0 \ne 0\). All minors of the infinite lower-triangular Toeplitz matrix \((a_{n-k})_{n,k \ge 0}\), with zeros above the diagonal, are non-negative (that is, the sequence is a Pólya frequency sequence) if and only if there are a real \(\gamma \ge 0\), non-negative reals \(\alpha_0, \alpha_1, \dots\) and \(\beta_0, \beta_1, \dots\) with \(\sum_i (\alpha_i + \beta_i) < \infty\), and a real \(\varepsilon > 0\) such that for every real \(x\) with \(|x| < \varepsilon\) the series \(\sum_{n \ge 0} a_n x^n\) converges to \(a_0 e^{\gamma x} \prod_{i \ge 0} (1 + \alpha_i x) / \prod_{i \ge 0} (1 - \beta_i x)\).

public theorem_wanted polyaFrequency_iff_hasSum_edrei_product (a : ℕ → ℝ) (ha : ∀ n, 0 ≤ a n)
    (ha0 : a 0 ≠ 0) :
    Matrix.HasNonnegativeMinors (Matrix.of fun n k : ℕ => if k ≤ n then a (n - k) else 0) ↔
      ∃ (γ : ℝ) (α β : ℕ → ℝ), 0 ≤ γ ∧ (∀ i, 0 ≤ α i) ∧ (∀ i, 0 ≤ β i) ∧
        Summable (fun i => α i + β i) ∧ ∃ ε > 0, ∀ x : ℝ, |x| < ε →
          HasSum (fun n => a n * x ^ n)
            (a 0 * Real.exp (γ * x) * (∏' i, (1 + α i * x)) / ∏' i, (1 - β i * x))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Lemma 16 (Schoenberg–Edrei Theorem, Karlin p. 154).
Uses 1

TheoremPólya–Schur transcendental characterization of multiplier sequences#●ready to prove

Let \(t_0, t_1, \dots\) be non-negative real numbers with \(t_0 \ne 0\). Say a real polynomial splits over \(\mathbb{R}\) if it is a constant times a product of linear factors \(x - r\) with \(r\) real (the zero polynomial splits). Then the following are equivalent: for every real polynomial \(\sum_k a_k x^k\) that splits over \(\mathbb{R}\), the polynomial \(\sum_k a_k t_k x^k\) splits over \(\mathbb{R}\); and there are a real \(\gamma \ge 0\) and non-negative reals \(\alpha_0, \alpha_1, \dots\) with \(\sum_i \alpha_i < \infty\) such that for every real \(x\) the series \(\sum_{n \ge 0} t_n x^n / n!\) converges to \(t_0 e^{\gamma x} \prod_{i \ge 0} (1 + \alpha_i x)\).

Convergence at every real \(x\) encodes that the exponential generating function is entire.

public theorem_wanted multiplier_sequence_iff_hasSum_exp_mul_tprod (t : ℕ → ℝ)
    (ht : ∀ k, 0 ≤ t k) (ht0 : t 0 ≠ 0) :
    (∀ p : ℝ[X], p.Splits →
      (∑ k ∈ Finset.range (p.natDegree + 1), C (p.coeff k * t k) * X ^ k).Splits) ↔
      ∃ (γ : ℝ) (α : ℕ → ℝ), 0 ≤ γ ∧ (∀ i, 0 ≤ α i) ∧ Summable α ∧ ∀ x : ℝ,
        HasSum (fun n => t n / n.factorial * x ^ n)
          (t 0 * Real.exp (γ * x) * ∏' i, (1 + α i * x))
Sources
  • Ding and Zeng, Some New Results on the Minuscule Polynomials of Type A, Journal of Integer Sequences 28 (2025), Article 25.1.2, Lemma 17 (Pólya and Schur, PS14 p. 93).
4 fully proved15 ready to prove

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