Why is Eight Six(es) Afraid of a Single Seven? Because There's No Nine in this joke!

What is the remainder when the following number is divided by 7 7 ?

6 6 6 6 6 6 6 6 \LARGE 6^{6^{6^{6^{6^{6^{6^6}}}}}}

1 3 46 2 9 36 5

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1 solution

Noel Lo
Jul 14, 2015

6 6 6 . . . . 6^{6^6....} is a perfect sixth power. a 6 1 = ( a 3 1 ) ( a 3 + 1 ) = ( a 1 ) ( a 2 + a + 1 ) ( a + 1 ) ( a 2 a + 1 ) a^6 - 1 = (a^3 - 1)(a^3 + 1)= (a-1)(a^2+a+1)(a+1)(a^2-a+1) . In this case, we see that 6 6 6 . . . . 1 6^{6^6....} -1 can be factorised to give a factor of 6 + 1 = 7 6+1 =7 .

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