n-ary relations and their applications

**robocop_911** · June 7th 2008, 02:14 PM

Can anybody do the following problem?
I don't know how to go about doing it...

Give an example to show that if R and S are both n-ary relations, then

$<br /> P_{i_1,i_2,...,i_m}(R \cap S)$ may be different from $<br /> P_{i_1,i_2,...,i_m}(R) \cap P_{i_1,i_2,...,i_m}(S)$

**Plato** · June 7th 2008, 02:31 PM

Originally Posted by robocop_911

Give an example to show that if R and S are both n-ary relations, then $<br /> P_{i_1,i_2,...,i_m}(R \cap S)$ may be different from $<br /> P_{i_1,i_2,...,i_m}(R) \cap P_{i_1,i_2,...,i_m}(S)$

How does your text define $P_{i_1,i_2,...,i_m}(S)$ ?
Please be complete in your answer.

**robocop_911** · June 7th 2008, 02:46 PM

Originally Posted by Plato

How does your text define $P_{i_1,i_2,...,i_m}(S)$ ?
Please be complete in your answer.

The projection $P_{i_1,i_2,...,i_m}$ where $i_1 < i_2 < ... < i_m$ , maps the n-tuple $(a_1, a_2,..., a_n)$ to the m-tuple $(a_{i_1}, a_{i_2}, ..., a_{i_m})$ , where m <= n.

In other words, the projection P_i1, i_2,..., i_m deletes $n-m$ of the components of an n-tuple, levaing the i1th, i2th and imth components.

**robocop_911** · June 7th 2008, 02:57 PM

Originally Posted by robocop_911

The projection $P_{i_1,i_2,...,i_m}$ where $i_1 < i_2 < ... < i_m$ , maps the n-tuple $(a_1, a_2,..., a_n)$ to the m-tuple $(a_{i_1}, a_{i_2}, ..., a_{i_m})$ , where m <= n.

In other words, the projection P_i1, i_2,..., i_m deletes $n-m$ of the components of an n-tuple, levaing the i1th, i2th and imth components.

For example:
If $P_{1,3}$ is applied to a 4 tuple (2,3,0,4) gives (2,0)

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