91 digits number with 1 as all digits , example 111....111, is a Prime  number?

Is ther any systmatic approach to find a primenumber

Now 1111...91 times is not a prime number ...this is so becuase ..consider 1111111(7 times 1) ...

Now 111..91 times is nothing but 1111111,1111111, ...like this into 13 such groups ...that is it is in other words 1111...91 times is divisible by 1111111(7 times) ...

Hence it is not a prime number ,,,

 

In other words 111...(n times) cannot be a prime number if n is itself non prime (91 = 13*7) ..but n being a prime may not always be a sufficient condition to know if 1111..(that many times) is prime or not ..because 111(3 times) is not a prime number(obviously div. by 3) although 3 is prime ....