Which side do I apply AM-GM on?

n!nn\sqrt[n]{n!} \geq \sqrt{n}

Prove the above for all positive integers nn.


  • Try to give a non-inductive, non-calculus proof

Note by Sharky Kesa
3 years, 7 months ago

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Try to give a non-inductive, non-calculus proof

So I will give an inductive, calculus-requiring proof.

Lemma 1: (1+1n)ne\left( 1 + \frac{1}{n} \right)^n \le e for all positive integer nn.

One way to prove it is to observe that 1+1n1+22n+14n2=(1+12n)21 + \frac{1}{n} \le 1 + \frac{2}{2n} + \frac{1}{4n^2} = \left( 1 + \frac{1}{2n} \right)^2, thus by induction,

(1+1n)n(1+12n)2n(1+14n)4nlimk(1+12kn)2kn=limn(1+1n)n=e\displaystyle\begin{aligned} \left(1 + \frac{1}{n} \right)^n &\le \left(1 + \frac{1}{2n} \right)^{2n} \\ &\le \left(1 + \frac{1}{4n} \right)^{4n} \\ &\le \ldots \\ &\le \lim_{k \to \infty} \left(1 + \frac{1}{2^k n} \right)^{2^k n} \\ &= \lim_{n \to \infty} \left(1 + \frac{1}{n} \right)^n \\ &= e \end{aligned}

Lemma 2: e<3e < 3

e=10!+11!+12!+13!+14!+15!+16!+<1+11+12+14+18+116+132+=1+2=3\displaystyle\begin{aligned} e &= \frac{1}{0!} + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \frac{1}{4!} + \frac{1}{5!} + \frac{1}{6!} + \ldots \\ &< 1 + \frac{1}{1} + \frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \frac{1}{16} + \frac{1}{32} + \ldots \\ &= 1 + 2 \\ &= 3 \end{aligned}

Now, the claim n!nn\sqrt[n]{n!} \ge \sqrt{n} is equivalent to (n!)2nn(n!)^2 \ge n^n. We will prove this by induction. The base cases are n=1,2,3n = 1, 2, 3 in which it is true. Now suppose that it's true for some n3n \ge 3, and we'll prove the claim for nn+1n \gets n+1.

((n+1)!)2=(n!)2(n+1)2nn(n+1)2nn(n+1)eby Lemma 2, because $n \ge3 > e$nn(n+1)(1+1n)nby Lemma 1=(n+nn)n(n+1)=(n+1)n(n+1)=(n+1)n+1\displaystyle\begin{aligned} ((n+1)!)^2 &= (n!)^2 \cdot (n+1)^2 \\ &\ge n^n \cdot (n+1)^2 \\ &\ge n^n \cdot (n+1) \cdot e & \text{by Lemma 2, because \$n \ge 3 > e\$} \\ &\ge n^n \cdot (n+1) \cdot \left( 1 + \frac{1}{n} \right)^n & \text{by Lemma 1} \\ &= \left(n + \frac{n}{n} \right)^n \cdot (n+1) \\ &= (n+1)^n \cdot (n+1) \\ &= (n+1)^{n+1} \end{aligned}

Ivan Koswara - 3 years, 7 months ago

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We have to prove (n!)2nn(n!)^2 \ge n^n. Instead of proving it using Sterling's approximation or induction here is a solution on AM-GM. I found a nice form of (n!)2(n!)^2 recently. (n!)2=[1n][2(n1)][n1](n!)^2=[1 \cdot n][2 \cdot (n-1)]\ldots[n \cdot1](there are nn terms). Now, for some variable mm, such that 1mn1 \le m \le n we have m(nm+1)nm(n-m+1) \ge n. The result follows.

A Former Brilliant Member - 3 years, 7 months ago

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Wise solution!

Nihar Mahajan - 3 years, 7 months ago

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Nice solution

Anirudh Chandramouli - 3 years, 7 months ago

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Why is it that for a variable mm
1mn1\leq m\leq n is m(nm+1)n m(n-m+1)\geq n ?

Upamanyu Mukharji - 3 years, 7 months ago

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Take n=m+kn=m+k where k0k \ge 0. The inequality simplifies to m(k+1)m+km(k+1) \ge m+k or mk+mm+kmk+m \ge m+k or m1m \ge 1 which is true.

A Former Brilliant Member - 3 years, 7 months ago

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@A Former Brilliant Member OK.. Thanks!

Upamanyu Mukharji - 3 years, 7 months ago

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n!nn\sqrt[n]{n!} \geq \sqrt{n}

n!nn{n!} \geq {\sqrt{n}}^n

(n)(n1)(n2)(1)nn(n) \cdot (n-1) \cdot (n-2) \cdot \dots \cdot (1) \geq {\sqrt{n}}^n

(n)(1)(n1)(1+1)(n2)(1+2)(n+12)(n12)nn(n)(1) \cdot (n-1)(1+1) \cdot (n-2)(1+2) \cdot \dots \cdot (\frac{n+1}{2})(\frac{n-1}{2}) \geq {\sqrt{n}}^n or (n)(1)(n1)(1+1)(n2)(1+2)(n2)nn(n)(1) \cdot (n-1)(1+1) \cdot (n-2)(1+2) \cdot \dots \cdot (\frac{n}{2}) \geq {\sqrt{n}}^n

We note that (n-k)(1+k) > n for all -1<k<n+1. So the result follows. By the way, i was sniped by Svatejas Shivakumar.

Aloysius Ng - 3 years, 7 months ago

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Ninja'ed haha tfs

Nihar Mahajan - 3 years, 7 months ago

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Well what about n-1?

Oscar Oquelí - 3 years, 7 months ago

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