Function trinomialMatchQ

src/compute-engine/rubi/rubi-utils.ts:2201–2208

(u: Expression, x: string)

    if (classes.length !== 2) return null;
    const [c1, c2] = classes;
    if (c1.exp.isSame(0) || c2.exp.isSame(0)) return null;
    // n is the "larger" exponent (PosQ[n−q])
    const nFirst = posQ(safeSimplify(c1.exp.sub(c2.exp)));
    const [qc, nc] = nFirst ? [c2, c1] : [c1, c2];
    return { a: qc.coef, b: nc.coef, n: nc.exp, q: qc.exp };
  }
  // unexpanded product s·x^m·v with v (generalized) binomial
  const split = splitMonoFactor(u, x);
  if (split === null) return null;
  const { scale, exp: mExp, rest } = split;
  if (rest === null) return null;