# Question about theta of $T(n)=4T(n/5)+n$

I have this recurrence relation $T(n)=4T(\frac{n}{5})+n$ with the base case $T(x)=1$ when $x\leq5$. I want to solve it and find it's $\theta$. I think i have solved it correctly but I can't get the theta because of this term $\frac{5}{5^{log_{4}n}}$ . Any help?

$T(n)=4(4T(\frac{n}{5^{2}})+\frac{n}{5})+n$

$=4^{2}(4T(\frac{n}{5^{3}})+\frac{n}{5^{2}})+4\frac{n}{5}+n$

$=...$

$=4^{k}T(\frac{n}{5^{k}})+4^{k-1}\frac{n}{5^{k-1}}+...+4\frac{n}{5}+n$

$=...$

$=4^{m}T(\frac{n}{5^{m}})+4^{m-1}\frac{n}{5^{m-1}}+...+4\frac{n}{5}+n$ Assuming $n=4^{m}$

$=4^{m}T(\lceil(\frac{4}{5})^{m}\rceil)+((\frac{4}{5})^{m-1}+...+1)n$

$=n+\frac{1-(\frac{4}{5})^{m}}{1-\frac{4}{5}}n=n+5n-n^{2}\frac{5}{5^{log_{4}n}}$

$=6n-n^{2}\frac{5}{5^{log_{4}n}}$

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A lot of Master theorem type recurrences can be solved exactly. While this computes more information than what is required for the asymptotics I believe it does add to the understanding of what exactly is going on with these recurrences. There is one example here and another one here and here. – Marko Riedel Jan 23 '13 at 21:48

Hint :$\frac {\log{n}}{\log4}=\log_4{n}$

So $5^{\log_4{n}}=n^{\frac {1}{\log_5{4}}}$

Use this.

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An alternative approach is to prove that $T(n)\leqslant5n$ for every $n$. This holds for every $n\leqslant5$ and, if $T(n/5)\leqslant5(n/5)=n$, then $T(n)\leqslant4n+n=5n$. By induction, the claim holds.

On the other hand, $T(n)\geqslant n$ for every $n\gt5$, hence $T(n)=\Theta(n)$.

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hmm, however it's different from what I get with the solved recurrence $T(n)=6n-5n^{2}n^{\frac{1}{log_{5}4}}=6n-5n^{2}n^{1.1}$ which should be almost $\theta(n^{3})$ – J Mint Jan 23 '13 at 8:17
Indeed, since you make me say it, your "solution" is wrong. A fact which should make you suspicious is that, according to your formula, T(n) is negative when n is large enough... something absurd. – Did Jan 23 '13 at 11:08