Possible bug in the intersection array ((7, 6, 6), (1, 1, 3))

[a234]

The code is
p = drg.DRGParameters([7,6,6],[1,1,3])
p.check_feasible()

and the error message is

ZeroDivisionError Traceback (most recent call last)
<ipython-input-30-0665a7dbaa42> in <module>
----> 1 p.check_feasible()

~/drg/assoc_scheme.py in check_feasible(self, checked, skip, derived, levels, queue, part)
801 for name, check in lvl:
802 if name not in skip:
--> 803 check(self)
804 if i > 1:
805 skip.add(name)

~/drg/drg.py in check_terwilliger(self)
1329 elif th is not None and self._.a[1] != 2 and \
1330 -1 - self._.b[1]/(th+1) < self._.a[1]:
-> 1331 s = ceil(self._.b[0] / self._.a[1])
1332 else:
1333 s = ceil(self._.b[0] / (self._.a[1] + 1))

/opt/sagemath-9.3/local/lib/python3.7/site-packages/sage/rings/integer.pyx in sage.rings.integer.Integer.__truediv__ (build/cythonized/sage/rings/integer.c:14337)()
2038 if type(left) is type(right):
2039 if mpz_sgn((<Integer>right).value) == 0:
-> 2040 raise ZeroDivisionError("rational division by zero")
2041 x = <Rational> Rational.__new__(Rational)
2042 mpq_div_zz(x.value, (<Integer>left).value, (<Integer>right).value)

ZeroDivisionError: rational division by zero

There is an easier way to prove non-existence of such a distance-regular graph: it would have non-integer multiplicities, so it cannot exist.

[jaanos]

Indeed it is a bug in the current release. This is fixed in the working version, which introduces number fields for cases like this, making the actual computation of multiplicities efficient. However, it is not quite ready, so I haven't pushed the new commits yet.