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“4137X˙CH04˙Akerkar” — 2007/9/17 — 11:02 — page 151 — #33
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4.3 Classification 151
Let us assume that the database T contains the following transactions: {a, b, c}, {a, b, d},
{a, c, d}, {a, c, e}, {a, d, e}, and {a, d, f}. Let us further assume that the items in the transaction
are ordered.
We define an equivalence relation between transactions as having two common prefixes. In
other words, two transactions are equivalent if their first two elements are the same. Now let us
define the lower and upper approximations for X, which contains transactions {a, b, c}, {a, b, d},
and {a, c, d}. The lower approximation of X will be given by {{a, b, c}, {a, ...