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Want facts and want them fast? Our Maths in a minute series explores key mathematical concepts in just a few words.
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The fact that the difference of the numbers in consecutive rows forms the arithmetic progression (3, 5, 7, 9, 11,...) is a side note - those are not the numbers of primary interest.
The numbers of interest are the ones in the 2-d array:
4, 7, 10, 13, ...
7, 12, 17, 22, ...
10, 17, 24, 31, ...
For any number N in the 2-d array, 2N + 1 is composite. Similarly for any number N not in the array, 2N + 1 is prime.