k=100 (2s)
  k=200 (10s)
  k=300 (30s)
  k=400 (70s)
  k=500 (137s)
  k=600 (244s)
  k=700 (399s)
sweep [78,734]: 0 non-positive
  k=78: crit_lower = 0.91
  k=734: crit_lower = 33754.05