thumb|upright|The elements of the set {A, B} can combine with the elements of the set {1, 2, 3} in six different ways.

In combinatorics, the rule of product or multiplication principle is a basic counting principle (a.k.a. the fundamental principle of counting). Stated simply, it is the intuitive idea that if there are ways of doing something and ways of doing another thing, then there are ways of performing both actions.

Examples

:<math>

\begin{matrix}

& \underbrace{ \left\{A,B,C\right\} }

& & \underbrace{ \left\{ X,Y\right\} } \\

\mathrm{To}\ \mathrm{choose}\ \mathrm{one}\ \mathrm{of} & \mathrm{these} &

\mathrm{AND}\ \mathrm{one}\ \mathrm{of} & \mathrm{these}

\end{matrix}

</math>

:<math>

\begin{matrix}

\mathrm{is}\ \mathrm{to}\ \mathrm{choose}\ \mathrm{one}\ \mathrm{of} & \mathrm{these}. \\

& \overbrace{ \left\{ AX, AY, BX, BY, CX, CY \right\} }

\end{matrix}</math>

In this example, the rule says: multiply 3 by 2, getting 6.

The sets {A, B, C} and {X, Y} in this example are disjoint sets, but that is not necessary. The number of ways to choose a member of {A, B, C}, and then to do so again, in effect choosing an ordered pair each of whose components are in {A, B, C}, is 3&nbsp;&times;&nbsp;3&nbsp;=&nbsp;9.

As another example, when you decide to order pizza, you must first choose the type of crust: thin or deep dish (2 choices). Next, you choose one topping: cheese, pepperoni, or sausage (3&nbsp;choices).

Using the rule of product, you know that there are 2&nbsp;&times;&nbsp;3&nbsp;=&nbsp;6 possible combinations of ordering a pizza.

Applications

In set theory, this multiplication principle is often taken to be the definition of the product of cardinal numbers.

See also

  • Combinatorial principles

References

fi:Todennäköisyysteoria#Tuloperiaate ja summaperiaate