Is the 76th power of 2 minus 1 a prime?

Is the 76th power of 2 minus 1 a prime?

Consider the last bit of the number
The last bit of 2 ^ n increases with n
Cycle every four bits
That is, if n = 4m, the last bit is 6
If n = 4m + 1, the last position is 2
If n = 4m + 2, the last position is 4
If n = 4m + 3, the last position is 8
Because 76 = 4 * 19
So the mantissa of 2 ^ 76 is 6
2 ^ 76-1 mantissa is 5
So 5|2 ^ 76-1
2 ^ 76-1 is a composite number

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