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Basic step

Show that $P(2)$ is true:

$$2! < (2)^2$$

$$1*2 < 2*2$$

$$2 < 4 \quad \text{(which is true)}$$

Inductive Step

Assume $P(k) \implies (k)! < (k)^k $ is true

then, I'm not sure how to continue

Martin Sleziak
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unknown unknown
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  • You've started the induction proof nicely. You have your base case, and you have your induction hypothesis. Now, using your $P(k)$, think about what it is that you actually want to prove? – Arthur Nov 29 '19 at 13:59
  • And also https://math.stackexchange.com/questions/1260233/inductive-proof-that-kkk-for-k-geq-2 – Arnaud D. Nov 29 '19 at 14:15

4 Answers4

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If $k!<k^k$ then $(k+1)!<k^k\cdot(k+1)<(k+1)^k(k+1)=(k+1)^{k+1}$.

J.G.
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When $n=2$,

$n!=2<n^n=4$

Assume when $n=k$,$k!<k^k$ is true,i.e.$\frac{k^k}{k!}>1$

When$\ n=k+1$

We consider $$\frac{k+1^{k+1}}{(k+1)!}$$ $$=\frac{k+1^{k}}{k!}$$ $$\frac{k+1^{k}}{k!}>\frac{k^k}{k!}>1$$ So, when $n=k+1$,$n!<n^n$ is right.

Box Mars
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You did the base case now assume that $n! < n^n$. Multiply by $n+1$ on both sides $$(n+1)n! < (n+1)n^n$$ that is $$(n+1)! < (n+1)n^n$$ But $(n+1)n^n < (n+1)(n+1)^n = (n+1)^{n+1}$ so $$(n+1)! < (n+1)^{n+1}$$

Ahmad Bazzi
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For the inductive step, you need to show that for $n>1$,

$$n!<n^n\implies(n+1)!<(n+1)^{n+1}.$$

This can be rewritten $$n!<n^n\implies n!<(n+1)^{n},$$ which is obvious as $n^n<(n+1)^n$.