= Function oracle
= Function-evaluation oracle
{synonym}
A <function oracle> supplies access to an unknown <function> $f$: given an allowed input $x$, one query returns $f(x)$. <Query complexity> counts these accesses separately from known local computation. A classical query receives one definite answer; a <quantum oracle> specifies coherent access through a <unitary operator>, allowing inputs in <quantum superposition>. The distinction is part of the access model, not a change to the mathematical function.
A <Boolean quantum oracle> preserves $x$ and adds a one-<bit> value $f(x)$ into an answer <qubit> by <exclusive or>. On known <computational-basis states>, its output determines the same value as a classical <function oracle> query.
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