Function quickTwoSum

src/compute-engine/numerics/special-functions.ts:2191–2194

Exact: a + b = s + e, assuming |a| ≥ |b| (fast two-sum)

(a: number, b: number)

/** Power series (DLMF 10.25.2): I_n(x) = (x/2)^n Σ (x²/4)^k / (k!(n+k)!).
 *  All terms are positive — no cancellation at any x. Kahan-compensated:
 *  near x = 30 the ~60 accumulations otherwise cost ~5 ulp. */
function besselISeries(n: number, x: number): number {
  const halfX = x / 2;
  const quarter = (x * x) / 4;
  let term = 1;
  for (let i = 1; i <= n; i++) term /= i;