[英]How To Solve Recurrence Relation with a quadratic term
I'm trying to solve this recurrence relation. 我试图解决这个递推关系。 Here's what I've attempted so far, but I think I'm wrong.
下面是我到目前为止尝试,但我想我错了。 I would really appreciate some guidance.
我真的很感谢一些指导。
This is the recurrence relation I am trying to solve:- 2T(n^1/2) + C 这是递归关系我试图解决: - 2T(N ^ 1/2)+ C
T(n) = 2T(n^1/2) + C
2((2T(n^1/4)+C) + C
>> 4T(n^1/16) + 3C
>> 8T(n^1/256) + 6C
So I can formulate it into this algebraic expression:- 因此我可以将其表达为以下代数表达式:
(2^k)T(n^(1/2^k)) + 2k
So to solve the recurrence relation, I simply say 所以要解决的递推关系,我简单说
n^(1/(2^k)) = 1
Therefore:- 2k = log (base n) 1
But this makes k = 0....
I don't think this is correct. 我认为这是不正确的。 Please advise me, I'd be delighted to get some assistance!
请告诉我,我很高兴获得一些帮助!
My try. 我的尝试。 (warn: I'm not sure substitution is the right thing to do here. Let's give a shot.)
(警告:我不确定替代在这里是否正确。让我们试一下。)
Let's say that T(x) = 1
for x < 2 假设对于x <2,
T(x) = 1
with T(1) is a no-go (see my comment), so maybe we can try to compute T(2) 与T(1)是一个不走的(见我的意见),所以也许我们可以尝试计算T(2)
T(2) = 2 * T(sqrt(2)) + C = 2 + C
now we search for k such that n^(1/2^k) = 2 + C
现在我们搜索k使得
n^(1/2^k) = 2 + C
1/(2^k) = log_n(2 + C) [base n log]
1/log_n(2 + C) = 2^k
log_(2 + C) n = 2^k [1 / log_a b = log_b a]
lg n / lg (2 + C) = 2^k [change-of-base]
c2 lg n = 2^k [since lg (2 + C) is fixed we put c2 = 1 / lg (2 + C)]
k = lg (c2 * lg n) = (lg c2) + lg lg n
k = c3 + lg lg n [since lg c2 is fixed]
now we substitute back k into T(N) = 2^k + 2k
and we find 现在我们将k代入
T(N) = 2^k + 2k
,我们发现
T(n) = 2^c3 * lg n + 2*c3 + lg lg n
now if we put togheter 现在,如果我们把togheter
T(n) = c1 lg n + c2 lg lg n
where c1 and c2 are fixed and different from the ones we used above. 其中c1和c2是固定的,从我们上面使用的不同。
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