7n - 3 = O(n)       : witnessed by c = 7, n0 = 1
3n^2+5n+7 = O(n^2)  : witnessed by c = 4, n0 = 7
2n+10 = O(n)        : witnessed by c = 3, n0 = 10
n^2 = O(n)          : NO witness with c <= 20 on n <= 100000 -- the claim looks false
2^n = O(n^10)       : NO witness with c <= 20 on n <= 200 -- the claim looks false
