Assuming the sequence is unsorted.
Try a rudimentary proposition, do a bubble sort, that gives O(n^2) for worst and average case. It is a polynomial algorithm.
We can also do a quick sort, with worst case O(n^2) and average case O(nlogn), which is already better.
Do we need to sort everything? Not really.
What about a single pass, and store the minimum found, exchange as required, such as:
small=A(0)
for i:1, n {
if A(i)<small small: A(i)
}
return small
This is a linear algorithm, best case n, worst case 2n so O(n)