When optimizing a digital system, if a given input cannot occur, we "don't care" what the output is for that input.

For the purposes of simplification, we can make the "don't care" output be whatever makes the system simple; it will not affect the proper operation of the system.

On a Karnaugh map, "don't care" is usually represented by a 'D' or 'X', and can be counted as either a 0 or a 1, depending on which makes the system simpler.

See also Quine-McClusky method.

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