Code:
UCLN[a_, b_] := If[a == b,
Return[a]
,
If[a > b,
Return[UCLN[a - b, b]];
,
Return[UCLN[a, b - a]];
];
];
UC[A_, n_] := If[n == 1,
Return[A[[1]]];
,
Return[UCLN[A[[n]], UC[A, n - 1]]];
];