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Old 2006-07-28, 03:34   #28
geoff's Avatar
Mar 2003
New Zealand

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Default Factors of 4*5^n-1

For the sequence 4*5^n-1, can anyone explain why all the factors end in 1 or 9 when n is odd?

I haven't tested many examples, but I think all the odd factors of k*5^n-1 end in 1 or 9 when n is odd and k is a square. Can anyone explain this?
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