xi2 = 55/136
   Gauss-Legendre n=16: I1=1.718745e-07  I2=6.433356e-06  c1=6.605231e-06
   Gauss-Legendre n=24: I1=1.715848e-07  I2=6.433056e-06  c1=6.604640e-06
   Gauss-Legendre n=32: I1=1.715006e-07  I2=6.432826e-06  c1=6.604327e-06
   Monte Carlo 2e8 points: I1=1.7111e-07 +- 2.8e-10  I2=6.4569e-06 +- 2.0e-08  c1=6.6280e-06 +- 2.0e-08
xi2 = 0.40481 (Stadlmann 2023, published bound c1 <= 2e-5)
   Gauss-Legendre n=16: I1=2.428644e-07  I2=9.092741e-06  c1=9.335605e-06
   Gauss-Legendre n=24: I1=2.424549e-07  I2=9.092316e-06  c1=9.334771e-06
   Gauss-Legendre n=32: I1=2.423361e-07  I2=9.091992e-06  c1=9.334328e-06
   Monte Carlo 2e8 points: I1=2.4178e-07 +- 4.0e-10  I2=9.1260e-06 +- 2.8e-08  c1=9.3678e-06 +- 2.8e-08