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Tuesday, September 6, 2011

Perrin numbers

Let $P(0)=3, P(1)=0, P(2)=2$ and $$P(k) = P(k-2) + P(k-3.)$$ These numbers are called the Perrin numbers. They have the interesting property that $\mod{[P(k), k]} = 0$ in almost all cases when k is prime. ( Otherwise we would have found a true prime generator! ). In any case the property holds until $k=271441.$ Interesting, isn't it? See the table below for the first 40 Perrin numbers.

k P[k] Mod[P[k],k] PrimeQ
2 2 0 True
3 3 0 True
4 2 2 False
5 5 0 True
6 5 5 False
7 7 0 True
8 10 2 False
9 12 3 False
10 17 7 False
11 22 0 True
12 29 5 False
13 39 0 True
14 51 9 False
15 68 8 False
16 90 10 False
17 119 0 True
18 158 14 False
19 209 0 True
20 277 17 False
21 367 10 False
22 486 2 False
23 644 0 True
24 853 13 False
25 1130 5 False
26 1497 15 False
27 1983 12 False
28 2627 23 False
29 3480 0 True
30 4610 20 False
31 6107 0 True
32 8090 26 False
33 10717 25 False
34 14197 19 False
35 18807 12 False
36 24914 2 False
37 33004 0 True
38 43721 21 False
39 57918 3 False
40 76725 5 False

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