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Let `p` subset **N** x **N** such that
`p`(`x`, `y`) <=> `x`+`y`
is even. Then we have

[ 0]_{p}= {0, 2, 4, 6, 8, 10, ...}, [ 1]_{p}= {1, 3, 5, 7, 9, 11, ...}, [ 2]_{p}= {0, 2, 4, 6, 8, 10, ...}, [ 3]_{p}= {1, 3, 5, 7, 9, 11, ...}, [ 4]_{p}= {0, 2, 4, 6, 8, 10, ...}, ...

We see that [`0`]_{p} union [`1`]_{p} =
**N** and [`0`]_{p} intersection [`1`]_{p} =
**0**.

Author: Wolfgang Schreiner

Last Modification: January 12, 2000