Repunit prime

In the United States, today is abbreviated 10/31. Define the nth repunit number as

R_n = \frac{10^n-1}{9} = 1111\dots1,

a base-10 number consisting of n consecutive 1s. For example,

R_1 = 1

R_2 = 11

R_3 = 111

R_4 = 1,111,

and so on.

It turns out that R_{1031} is the largest known prime repunit number.

Source: http://mathworld.wolfram.com/Repunit.html

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