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Is the following number Prime or Composite?   9999997777 7755555533 3333111111 . Notice the descending order of the odd numbers grouped in strings of fives!. Thanks for any help.

Guest Apr 15, 2017
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 #1
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Very interesting and clever, whoever came up with it!!.

However, using a primality test shows that it is a composite number and factors as follows:

999999 777777 555555 333333 111111 = 3 x 7 x 11 x 13 x 37 x 9000007000005000003000001.

 

P.S. Notice that it is grouped in "sixes" not "fives". Also, notice that all its factors are odd!.

Guest Apr 15, 2017
 #2
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Calculate the digital root:  

6*(9+7+5+3+1) = 150

1+5+0 = 6  

6 is divisible by 3 hence the original number is divisible by 3, so it is composite, not prime.

.  

Alan  Apr 16, 2017

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