How to prove 1111......11 (91 digits) is a prime or composite number? 
How to prove $1111......11$ ($91$ digits) is a prime or composite number?

My Approach:
$1111......11$ can be expressed as $10^{0}+10^{1}+10^{2}+...…..+10^{90}$
Using summation of a geometric progression formula,
$$10^{0}+10^{1}+10^{2}+...…..+10^{90} = 1\frac{(10^{91}-1)}{(10-1)}=\frac{(10^{91}-1)}{(9)}$$
I do not know how to proceed after this step. Kindly guide me how to solve this problem.

Note that$$91=13(7)$$  so you may factor $$1111111$$ and your number is  $$1111111(10^{12\times {7}} + 10^{11\times 7}+10^{10\times 7}+...1)$$
So it is composite.
