A problem Q can be **reduced** to another problem Q' if any instance of Q
can be ``easily rephrased'' as an instance of Q', whose solution provides a
solution to the instance of Q.

**Example:** Solving
*ax* + *b* = 0 reduces to solving
.

A language *L*_{1} is **poly-time reducible** to language *L*_{2}, written
, if there exists a poly-time computable function
such that for all x {0,1}*:

iff

where f is the **reduction function**.

This is a one-way function. Q' will not always reduce to Q.