1. Redshift-distance coefficients, H0 dL = z + Q z^2 + C z^3 + O(z^4)
  (a) flat LambdaCDM, Om=0.3, OL=0.7: E^2 = Om(1+z)^3 + OL
     linear 1, Q = 31/40 = 0.7750, C = -219/800 = -0.27375; q0 from Q = (1-q0)/2: -11/20 = -0.550
  (b) STV 2017 eq. (3.39): E^2 = OL(1+z)^2 + Om(1+z)^3, flat integral
     linear 1, Q = 17/40 = 0.4250, C = -433/2400 = -0.18042; q0 from Q = (1-q0)/2: 3/20 = 0.150
  (c) open dust, Om=0.3, Ok=0.7, no Lambda (sinh)
     linear 1, Q = 17/40 = 0.4250, C = -51/800 = -0.06375; q0 from Q = (1-q0)/2: 3/20 = 0.150
  (d) Einstein-de Sitter, Om=1
     linear 1, Q = 1/4 = 0.2500, C = -1/8 = -0.12500; q0 from Q = (1-q0)/2: 1/2 = 0.500
  Paper values (STV 2017 eq. 3.42; ATV 2026 pp. 22-23): Q = 17/40 = 0.425, C = -433/2400 = -0.1804
  General Om, flat LambdaCDM: Q = 1 - 3 Om/4, C = -(5/4) Om + (9/8) Om^2  (q0 = 3 Om/2 - 1)
     Om = 1.000: flat LCDM Q = 0.2500, C = -0.12500;   STV-2017 form Q = 0.2500, C = -0.12500
     Om = 0.300: flat LCDM Q = 0.7750, C = -0.27375;   STV-2017 form Q = 0.4250, C = -0.18042
     Om = 0.315: flat LCDM Q = 0.7638, C = -0.28212;   STV-2017 form Q = 0.4213, C = -0.18051
     Om = 0.000: flat LCDM Q = 1.0000, C = 0.00000;   STV-2017 form Q = 0.5000, C = -0.16667
  So the STV-2017 form gives 0.25 <= Q <= 0.5 (q0 from 1/2 to 0: decelerating to coasting),
  while flat LambdaCDM gives 0.25 <= Q <= 1 (q0 from 1/2 to -1); they agree only at Om = 1.
  Cross-check by Visser (2004): a component's density scales as (1+z)^(3(1+w)) (eq. 48), so (1+z)^2 means
  w = -1/3 and a constant density w = -1; w0 is the density-weighted mean of the w's (eq. 49);
  q0 = (1 + 3 w0)/2 for a flat universe (eq. 20); Q = (1 - q0)/2 (eq. 46).
     Om = 0.3, dark energy as a cosmological constant, w = -1: w0 = -7/10 = -0.7000, q0 = -11/20 = -0.550, Q = 31/40 = 0.7750
     Om = 0.3, dark energy as the 2017 form's (1+z)^2 term, w = -1/3: w0 = -7/30 = -0.2333, q0 = 3/20 = 0.150, Q = 17/40 = 0.4250

  Numerical luminosity distances (H0 dL, Simpson's rule) and distance-modulus differences:
   z = 0.1: LCDM 0.1075 (series 0.1075); STV-2017 form 0.1041; open dust 0.1042; EdS 0.1024
          mu(LCDM) - mu(STV-2017 form) = +0.070 mag; mu(LCDM) - mu(open dust) = +0.068 mag; mu(LCDM) - mu(EdS) = +0.105 mag
   z = 0.5: LCDM 0.6615 (series 0.6595); STV-2017 form 0.5883; open dust 0.5990; EdS 0.5505
          mu(LCDM) - mu(STV-2017 form) = +0.254 mag; mu(LCDM) - mu(open dust) = +0.216 mag; mu(LCDM) - mu(EdS) = +0.399 mag
   z = 1.0: LCDM 1.5429 (series 1.5012); STV-2017 form 1.3053; open dust 1.3712; EdS 1.1716
          mu(LCDM) - mu(STV-2017 form) = +0.363 mag; mu(LCDM) - mu(open dust) = +0.256 mag; mu(LCDM) - mu(EdS) = +0.598 mag

2. STV ODE of order 2 (eqs. 2.48-2.51): rest points
   F(SM) = ['0', '0', '0', '0']   F(M) = ['0', '0', '0', '0']
   at SM: n=1 block eigenvalues 0.666667, -1.000000 (trace -1/3, det -2/3, disc 25/9); n=2 block 1.333333, -0.333333 (trace 1, det -4/9, disc 25/9)
   at M: n=1 block eigenvalues -1.000000, -1.000000 (trace -2, det 1, disc 0); n=2 block -1.000000, -1.000000 (trace -2, det 1, disc 0)
   Paper: at SM (eqs. 2.29-2.31), lambda_A1 = 2/3, lambda_B1 = -1, lambda_A2 = 4/3, lambda_B2 = -1/3; at M (eq. 2.28), lambda = -1 (degenerate)

3. Eigenvalues (2.29) and the density contrast of a perturbation of the xi^(2n) coefficient
   exponent at a fixed areal radius: lambda - 2n + 2 (times r^(2n-2)); at a fixed comoving radius (r ~ t^(2/3)): lambda - (2n-2)/3
   n = 1: lambda_A = 2/3, lambda_B = -1; comoving exponents 2/3 and -1
   n = 2: lambda_A = 4/3, lambda_B = -1/3; comoving exponents 2/3 and -1
   n = 3: lambda_A = 2, lambda_B = 1/3; comoving exponents 2/3 and -1
   n = 4: lambda_A = 8/3, lambda_B = 1; comoving exponents 2/3 and -1
   n = 5: lambda_A = 10/3, lambda_B = 5/3; comoving exponents 2/3 and -1
   n = 6: lambda_A = 4, lambda_B = 7/3; comoving exponents 2/3 and -1
   Textbook linear perturbations of dust in Einstein-de Sitter: growing mode t^(2/3), decaying mode t^(-1).