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$a_1=1$, $a_{n+1} = 3 a_n^2$.

Prove for all positive integers, $a_n\leq{3^{2^n}}$ using induction.

My work so far:

Base case is true (1 < 9)

Induction Hypothesis: $a_k\leq{3^{2^k}}$

IS: prove that n = k+1 is true

I'm stuck because I just can't seem to prove the induction step. Any help is appreciated.

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    $\begingroup$ square and multiply by 3 on both sides:$$a_{k+1}=3a_k^2\leq 3^{2^{k+1}+1}$$ $\endgroup$ – Roddy MacPhee Oct 2 at 19:55
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    $\begingroup$ @RoddyMacPhee but the induction step is prove $a_{k+1}\leq3^{2^{k+1}}$ $\endgroup$ – Harold Smith Oct 2 at 20:00
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We need to show the stronger condition

$$a_n\leq{3^{2^n-1}}(\leq{3^{2^n}})$$

and therefore assuming as Induction Hypothesis $a_k\leq{3^{2^k-1}}$ we have

$$a_{k+1}=3a_k^2\stackrel{Ind. Hyp.}\leq 3\cdot (3^{2^{k}-1})^2={3^{2^{k+1}-1}}$$

Refer also to the related

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    $\begingroup$ Because $2^{k+1} >2^k+1$ for $k>0$ $\endgroup$ – Mason Oct 2 at 19:56
  • $\begingroup$ @Mason Yes of course, that's why in short. $\endgroup$ – user Oct 2 at 19:57
  • $\begingroup$ I don't understand how you got $3(a_k)^2\leq3\cdot{3^{2^k}}$ using the IH. What happened to the squared on $3(a_k)^2$? $\endgroup$ – Harold Smith Oct 2 at 20:19
  • $\begingroup$ @HarrySmith Ops sorry, you ar eright, I fox that. $\endgroup$ – user Oct 2 at 20:22
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Induction proves $a_n=3^{2^n-1}$, which implies the inequality.

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