Function sinc

src/compute-engine/numerics/special-functions.ts:2607–2609

(x: number)

  let dtermG: DD = [1, 0]; // g′ term at k = 0: (3·0+1)·b_0 = 1
  let fp: DD = dtermF;
  let gp: DD = dtermG;
  for (let k = 1; k <= 200; k++) {
    const k3 = 3 * k;
    // advance f′ term from index k to k+1, g′ term from index k−1 to k
    dtermF = ddDivD(ddMul(dtermF, x3), k3 * (k3 + 2));
    dtermG = ddDivD(ddMul(dtermG, x3), (k3 - 2) * k3);