Determine gcd (66, 165, 253).
The factors of 66 are $\{ 1, 2, 3, 6, 11, 22, 33,66 \}$.
The factors of 165 are $\{ 1, 3, 5, 11, 15, 33, 55, 165 \}$.
The factors of 253 are $\{ 1, 11, 23, 253 \}$.
The greatest common divisor is 11.