///|
let i0_arrA : ReadOnlyArray[Double] = [
  -4.41534164647933937950E-18, 3.33079451882223809783E-17, -2.43127984654795469359E-16,
  1.71539128555513303061E-15, -1.16853328779934516808E-14, 7.67618549860493561688E-14,
  -4.85644678311192946090E-13, 2.95505266312963983461E-12, -1.72682629144155570723E-11,
  9.67580903537323691224E-11, -5.18979560163526290666E-10, 2.65982372468238665035E-9,
  -1.30002500998624804212E-8, 6.04699502254191894932E-8, -2.67079385394061173391E-7,
  1.11738753912010371815E-6, -4.41673835845875056359E-6, 1.64484480707288970893E-5,
  -5.75419501008210370398E-5, 1.88502885095841655729E-4, -5.76375574538582365885E-4,
  1.63947561694133579842E-3, -4.32430999505057594430E-3, 1.05464603945949983183E-2,
  -2.37374148058994688156E-2, 4.93052842396707084878E-2, -9.49010970480476444210E-2,
  1.71620901522208775349E-1, -3.04682672343198398683E-1, 6.76795274409476084995E-1,
]

///|
let i0_arrB : ReadOnlyArray[Double] = [
  -7.23318048787475395456E-18, -4.83050448594418207126E-18, 4.46562142029675999901E-17,
  3.46122286769746109310E-17, -2.82762398051658348494E-16, -3.42548561967721913462E-16,
  1.77256013305652638360E-15, 3.81168066935262242075E-15, -9.55484669882830764870E-15,
  -4.15056934728722208663E-14, 1.54008621752140982691E-14, 3.85277838274214270114E-13,
  7.18012445138366623367E-13, -1.79417853150680611778E-12, -1.32158118404477131188E-11,
  -3.14991652796324136454E-11, 1.18891471078464383424E-11, 4.94060238822496958910E-10,
  3.39623202570838634515E-9, 2.26666899049817806459E-8, 2.04891858946906374183E-7,
  2.89137052083475648297E-6, 6.88975834691682398426E-5, 3.36911647825569408990E-3,
  8.04490411014108831608E-1,
]

///|
///Returns modified Bessel function of order zero of the argument.
pub fn bessel_i0(x : Double) -> Double {
  let x = x.abs()
  if x <= 8.0 {
    let y = x / 2.0 - 2.0
    return exp(x) * chbevl(y, i0_arrA)
  }
  return exp(x) * chbevl(32.0 / x - 2.0, i0_arrB) / x.sqrt()
}

///|
/// `i0` is the alias of `bessel_i0`.
pub let i0 : (Double) -> Double = bessel_i0