Mugen is Strong
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because ABABABs are products of AB and 10101, where 10101 are multiple of 7.
Northstar
Split the number into two pieces and check to see if they are divisible by 7.
ABABAB = (10000B + 100B + B) + (100000A + 1000A + 10A)
ABABAB = 10101B + 101010A
But 10101 / 7 = 1443
So we have:
10101B + 101010A = 1443B + 14430A which is an integer
So all numbers of the form ABABAB are evenly divisible by 7.
kkr
An example of 'number form' stated by you is...
131313, which is technically 13*(010101)
All numbers like this has said common factor '010101'
As 10101/7 is 7* 1443, said 7 is a factor of all numbers of the form ABABAB!
Regards.