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Old 2020-05-17, 19:54   #6
ATH
Einyen
 
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Dec 2003
Denmark

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The 5 chains are only length 2.

The next steps 2p+2 - 7 are composite and the first 4 of them are square numbers:
p=2: 2p+2 - 7 = 32
p=3: 2p+2 - 7 = 52
p=5: 2p+2 - 7 = 112
p=13: 2p+2 - 7 = 1812
p=19: 2p+2 - 7 = 5*419429
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