Dear all,

The following code raises a ValueError: [*some big element of L*] is not an 
S-unit.

L.<theta_L> = NumberField(x^6 - 68463*x^4 - 5120808*x^3 + 1250774892*x^2 + 
192368273328*x + 7520491439712)
OLSstar = UnitGroup(L,proof=False,S=tuple(L.primes_above(2*5*7*13)))
u = OLSstar.gen(11)
print(u)              # yields u11
print(L(u))           # yields some very large element of L
print(OLSstar(L(u)))  # raises a ValueError

The last output should of course be u11 again. The above is the simplest 
example that I could extract. I have other examples, where the composed 
coersion from OLSstar to L and back to OLSstar is not the identity 
(according to sage, sometimes a minus is falsely introduced), although it 
should. I do not know where the error comes from.

Sometimes UnitGroup() also raises a PariError, which comes from bnfsunit() 
within UnitGroup.__init__(), which can be resolved by increasing pari's 
precision. The above error however persists after increasing pari's 
precision generously.

This was run on Sage 9.0, Linux Mint 19.3. Any help is appreciated.

Thanks,
Benjamin

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