证明大O求和规则? [英] Proving Big-O Sum Rule?
问题描述
我不确定如何正式证明大O规则的款项,即:
I am unsure how to formally prove the Big O Rule of Sums, i.e.:
f1(n) + f2(n) is O(max(g1(n)),g2(n))
到目前为止,我应该在我的努力下:
So far, I have supposed the following in my effort:
让我们有两个常量 C1
和 C2
,使得 C2> C1
。通过大O的定义:
Let there be two constants c1
and c2
such that c2 > c1
. By Big O definition:
f1(n) <= c1g1(n) and f2(n) <= c2g2(n)
谁能指教我应该如何进行?是否合理引进的变量数值替换在这一步,证明关系?不知道先按g
或 F
,这是我能想到接近的唯一途径。
Can anyone advise how I should proceed? Is it reasonable to introduce numerical substitutions for the variables at this step to prove the relationship? Not knowing g
or f
, that is the only way I can think to approach.
任何帮助将是很大的AP preciated。
Any help would be greatly appreciated.
推荐答案
让
gmax = max(g1, g2), and gmin = min(g1, g2).
GMIN是O(GMAX)。现在,使用的定义:
gmin is O(gmax). Now, using the definition:
gmin(n) <= c*gmax(n) for n > some k
添加GMAX(N),以每面为:
Adding gmax(n) to each side gives:
gmin(n) + gmax(n) <= c*gmax(n) + gmax(n) for n > some k
gmin(n) + gmax(n) <= (c+1)*gmax(n) for n > some k
g1(n) + g2(n) <= c'*gmax(n) for n > some k
因此,我们有G1 + G2为O(最大(G1,G2))。
So we have g1+g2 is O(max(g1, g2)).
由于F1 + F2是O(G1 + G2),大O的传递特性为我们提供了F1 + F2是O(MAX(G1,G2))。 QED。
Since f1+f2 is O(g1+g2), the transitive property of big-O gives us f1+f2 is O(max(g1, g2)). QED.
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