Real-rooted atlas

A121574: Riordan array (1/(1-2*x), x*(1+x)/(1-2*x)).

A121574 nonnegative coefficients

Proved for all n: all roots real, in (−∞, 0) · coefficients of one sign · interlacing · Hadamard kernel basis map preserves real roots what this means

Recurrence from OEIS A121574
\(P_{n}(x) = \left(2 + x\right)\,P_{n-1}(x) + \left(x\right)\,P_{n-2}(x)\)
\(P_{0}(x) = 1\)
\(P_{1}(x) = 2 + x\)

Rows ?

nx0x1x2x3x4x5x6x7Pn(1)
011
1213
245110
38168133
4164437111109
53211213467141360
6642724243051061711189
7128640123211685841542013927

Show all 15 stored rows

Generated b-files: A020704 (row sums), A121574 ?

Computed in your browser ?

Technical details
Node id
n-d5c14b9fec875ecc · revision 1 · JSON
Definition source
oeis_catalog:A121574
Fingerprint
f6-c8ccd205be05e7c0

Observed on the stored rows ?

  • Degrees: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 (affine: slope 1, intercept 0)
  • Palindromic rows: no
  • γ-positive (gamma_positive): no
  • Nonnegative coefficients (nonn): yes
  • Only rational roots (rr_by_factors: every nonzero row is a product of linear factors over ℚ): no

OEIS matches ?

A generated b-file is computed from the node's definition along the row alignment of its OEIS link and starts at the entry's offset. It is an identification on a finite prefix, not a proof that the node and the OEIS entry agree for every n.

OEIS submission text ?

Fields to paste into an OEIS submission: the example %e, the formula %F and programs %t (Mathematica) and %o (PARI/GP) that list the same rows. For an OEIS link the row numbers, the offset and the orientation are those of the entry, as matched; the matched terms were compared with the published ones, which identifies the node with the entry on a finite prefix, not for every n. The PARI/GP program is not machine-checked here.

The atlas's own rows: n = 0, …, ascending powers of x
%e Example
Triangle begins:
  0:   1;
  1:   2,    1;
  2:   4,    5,    1;
  3:   8,   16,    8,     1;
  4:  16,   44,   37,    11,     1;
  5:  32,  112,  134,    67,    14,    1;
  6:  64,  272,  424,   305,   106,   17,    1;
  7: 128,  640, 1232,  1168,   584,  154,   20,   1;
  8: 256, 1472, 3376,  3992,  2641,  998,  211,  23,  1;
  9: 512, 3328, 8864, 12592, 10442, 5221, 1574, 277, 26, 1;
%F Formula
The row polynomials satisfy P(n,x) = (2+x)*P(n-1,x) + x*P(n-2,x) for n >= 2, with P(0,x) = 1, P(1,x) = 2 + x.
%t Mathematica
P[0, x_] := 1;
P[1, x_] := 2 + x;
P[n_, x_] := P[n, x] = Expand[(2 + x)*P[n - 1, x] + x*P[n - 2, x]];
Table[CoefficientList[P[n, x], x], {n, 0, 9}] // Flatten
%o PARI/GP
P(N) = {my(v = vector(N + 1)); v[1] = 1; v[2] = 2 + x; for(n = 2, N, my(i = n + 1); v[i] = (2 + x)*v[i - 1] + x*v[i - 2]); v};
row(n, p) = {my(c = Vecrev(p)); c};
T(N) = {my(v = P(N)); concat(vector(#v, i, row(i - 1, v[i])))};
T(9)

Generated with polytool c48629374597.

A121574: triangle, offset 0, from the row P0
%e Example
Triangle begins:
  0:   1;
  1:   2,    1;
  2:   4,    5,    1;
  3:   8,   16,    8,     1;
  4:  16,   44,   37,    11,     1;
  5:  32,  112,  134,    67,    14,    1;
  6:  64,  272,  424,   305,   106,   17,    1;
  7: 128,  640, 1232,  1168,   584,  154,   20,   1;
  8: 256, 1472, 3376,  3992,  2641,  998,  211,  23,  1;
  9: 512, 3328, 8864, 12592, 10442, 5221, 1574, 277, 26, 1;
%F Formula
The row polynomials satisfy P(n,x) = (2+x)*P(n-1,x) + x*P(n-2,x) for n >= 2, with P(0,x) = 1, P(1,x) = 2 + x.
%t Mathematica
P[0, x_] := 1;
P[1, x_] := 2 + x;
P[n_, x_] := P[n, x] = Expand[(2 + x)*P[n - 1, x] + x*P[n - 2, x]];
Table[PadRight[CoefficientList[P[n, x], x], n + 1], {n, 0, 9}] // Flatten
%o PARI/GP
P(N) = {my(v = vector(N + 1)); v[1] = 1; v[2] = 2 + x; for(n = 2, N, my(i = n + 1); v[i] = (2 + x)*v[i - 1] + x*v[i - 2]); v};
row(n, p) = {my(c = Vecrev(p)); c = concat(c, vector(n + 1 - #c)); c};
T(N) = {my(v = P(N)); concat(vector(#v, i, row(i - 1, v[i])))};
T(9)

Generated with polytool c48629374597.

Proved properties

  • Real-rooted. Every row P_n, n >= 0, is zero or real-rooted with all roots in (-inf, 0)
    Every polynomial of the sequence has only real roots, inside the stated interval.
    by the cited steps: e-81aa6611c032ec70
  • Coefficients of one sign. Every nonzero row, n >= 0: the coefficients of P_n have one sign
    All coefficients of each polynomial have the same sign (after x -> -x when alternating).
    derived from the cited steps by a stated theorem: e-81aa6611c032ec70
  • Hadamard kernel (Malo). Rows P_n, n >= 0, are Hadamard kernels (preserve real-rootedness)
    Multiplying the coefficients of any real-rooted polynomial termwise by those of a row (Hadamard product) gives a real-rooted polynomial again.
    derived from the cited steps by a stated theorem: e-81aa6611c032ec70
  • Interlacing of consecutive rows. Rows P_(m-1), P_m interlace (strict, previous interlaces next) for every m >= 1
    The roots of consecutive polynomials alternate on the real line.
    by the cited steps: e-81aa6611c032ec70
  • Basis map preserves real-rootedness. Basis map u^k -> P_k sends roots in [-1 - sqrt(3), -1 + sqrt(3)] to roots in (-inf, +inf): every real polynomial with all roots in E = [-1 - sqrt(3), -1 + sqrt(3)] has a real-rooted image
    The linear map sending x^k to the k-th polynomial of the sequence turns polynomials with roots in one interval into real-rooted ones.
    by the cited steps: e-c9d1880c7ed7baa2
  • Basis map preserves real-rootedness. Basis map u^k -> P_k sends roots in [-1 - sqrt(3), -1 + sqrt(3)] to roots in [-4 - 2 sqrt(3), -4 + 2 sqrt(3)]: every real polynomial with all roots in E = [-1 - sqrt(3), -1 + sqrt(3)] has an image with all roots in I = [-4 - 2 sqrt(3), -4 + 2 sqrt(3)]
    The linear map sending x^k to the k-th polynomial of the sequence turns polynomials with roots in one interval into real-rooted ones.
    by the cited steps: e-c9d1880c7ed7baa2
  • Basis map preserves real-rootedness. Basis map u^k -> P_k sends roots in [-1 - sqrt(3), 0] to roots in [-4 - 2 sqrt(3), -4 + 2 sqrt(3)]: every PF input with all roots in [-1 - sqrt(3), 0] has a PF image; no larger [-R, 0] works
    The linear map sending x^k to the k-th polynomial of the sequence turns polynomials with roots in one interval into real-rooted ones.
    by the cited steps: e-c9d1880c7ed7baa2

Proof chains

2 incoming proof steps. Each step is expanded back to the roots of the proof DAG, the theorem checks of a node's own rule. JSON

Used in 14 proof steps

Same fingerprint (1)

Equal up to content, sign, reversal and a small index shift.

References

  1. [LW07] Lily L. Liu and Yi Wang. A unified approach to polynomial sequences with only real zeros. Advances in Applied Mathematics, 38(4):542–560, May 2007. doi:10.1016/j.aam.2006.02.003