function eigenvalues2x2 (a, b, c, d) {
    // lambda_+- = 1/2 trA +- 1/2 sqrt( tr^2 A - 4 detA )
    const trA = addScalar(a, d)
    const detA = subtract(multiplyScalar(a, d), multiplyScalar(b, c))
    const x = multiplyScalar(trA, 0.5)
    const y = multiplyScalar(sqrt(subtract(multiplyScalar(trA, trA), multiplyScalar(4, detA))), 0.5)

    return [addScalar(x, y), subtract(x, y)]
  }
Compute the eigenvalues of an 2x2 matrix
@return {[number,number]}