我被提出了这个问题:
f(n) are asymptotically positive functions. Prove f(n) = Θ(g(n)) iff g(n) = Θ(f(n)).
我找到的所有信息都表明这个陈述是无效的。例如,我遇到的一个答案说:
f(n) = O(g(n)) implies g(n) = O(f(n))
f(n) = O(g(n)) means g(n) grows faster than f(n). It cannot imply that f(n) grows
faster than g(n). Hence not true.
另一种说法:
If f(n) = O(g(n)) then O(f(n)). This is false. If f(n) = 1 and g(n) = n
for all natural numbers n, then f(n) <= g(n) for all natural numbers n, so
f(n) = O(g(n)). However, suppose g(n) = O(f(n)). Then there are natural
numbers n0 and a constant c > 0 such that n=g(n) <= cf(n) = c for all n >=
n0 which is impossible.
我知道我的问题与我找到的例子之间存在细微差别,但我只能想出不能证明它的解决方案。请问我是否正确认为无法证明,还是我忽略了一些细节?
f(n) = O(g(n)) implies g(n) = O(f(n))
,这可能是正确的,但只适用于某些特定情况)。 - ROMANIA_engineer