GAP Project repository
Compute an integral basis and the discriminant of a ring of algebraic
numbers.
Round2 and Round4 are the obvious choices
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H. Cohen, {\it A course in computational algebraic number theory},
Springer, Berlin, 1993; MR 94i:11105
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M. Pohst\ and\ H. Zassenhaus, {\it Algorithmic algebraic number theory},
Cambridge Univ. Press, Cambridge, 1989; MR 92b:11074
The `polycyclic' package uses an interface to KANT for this purpose.