For each of the following numbers a and n decide whether or

For each of the following numbers a and n, decide whether or not a is invertible mod n

(a) a=7, n=91

(b) a=8, n=2^11 + 1

(c) a=242, n=888

(d) a=175, n=888

Solution

it is not invertible mod n beacuse we can not find inverse b such that n divide ab-1

it is inverible mod n and inverse is 1793

it is not invertible

it is invertible