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Algebra Difficulty 8.4 Shortlist Find the answer

Find all positive real numbers λ\lambda such that for all integers n2n\geq 2 and all positive real numbers a1,a2,,ana_1,a_2,\cdots,a_n with a1+a2++an=na_1+a_2+\cdots+a_n=n, the following inequality holds:
i=1n1aiλi=1n1ainλ\sum_{i=1}^n\frac{1}{a_i}-\lambda\prod_{i=1}^{n}\frac{1}{a_i}\leq n-\lambda.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Find all positive real numbers λ\lambda such that for all integers n2n \geq 2 and all positive real numbers a1,a2,,ana_1, a_2, \ldots, a_n with a1+a2++an=na_1 + a_2 + \cdots + a_n = n, the following inequality holds:
i=1n1aiλi=1n1ainλ. \sum_{i=1}^n \frac{1}{a_i} - \lambda \prod_{i=1}^{n} \frac{1}{a_i} \leq n - \lambda.

To find the values of λ\lambda, we first show that λ<e\lambda < e fails. Rewrite the inequality by multiplying both sides by a1a2ana_1 a_2 \cdots a_n:
i=1na1a2ai1ai+1anλ(nλ)a1a2an. \sum_{i=1}^n a_1 a_2 \cdots a_{i-1} a_{i+1} \cdots a_n - \lambda \leq (n - \lambda) \cdot a_1 a_2 \cdots a_n.

Set an=0a_n = 0 and a1=a2==an1=nn1a_1 = a_2 = \cdots = a_{n-1} = \frac{n}{n-1}. The inequality reduces to:
(nn1)n1λ0    λ(nn1)n1. \left(\frac{n}{n-1}\right)^{n-1} - \lambda \leq 0 \implies \lambda \geq \left(\frac{n}{n-1}\right)^{n-1}.

Taking the limit as nn \rightarrow \infty, we get λe\lambda \geq e.

Next, we show that λe\lambda \geq e satisfies the inequality. Using Lagrange multipliers, the left-hand side of the original inequality is maximized when:
1ai2+λai1a1a2an=μ    λa1a2an=μai+1ai for i=1,2,,n. -\frac{1}{a_i^2} + \frac{\lambda}{a_i} \cdot \frac{1}{a_1 a_2 \cdots a_n} = \mu \implies \frac{\lambda}{a_1 a_2 \cdots a_n} = \mu a_i + \frac{1}{a_i} \text{ for } i = 1, 2, \ldots, n.

Assume aiaja_i \neq a_j for some i,ji, j. Then:
μai+1ai=μaj+1aj    μ=1aiaj. \mu a_i + \frac{1}{a_i} = \mu a_j + \frac{1}{a_j} \implies \mu = \frac{1}{a_i a_j}.

Plugging this in for μ\mu and using AM-GM, we get:
λa1a2an=1aj+1ai    λ=ai+ajaiaja1a2an(a1+a2++ann1)n1=(nn1)n1<e, \frac{\lambda}{a_1 a_2 \cdots a_n} = \frac{1}{a_j} + \frac{1}{a_i} \implies \lambda = \frac{a_i + a_j}{a_i a_j} \cdot a_1 a_2 \cdots a_n \leq \left(\frac{a_1 + a_2 + \cdots + a_n}{n-1}\right)^{n-1} = \left(\frac{n}{n-1}\right)^{n-1} < e,
which is a contradiction. Hence, ai=aja_i = a_j for all i,ji, j.

The inequality is easy to verify when all aia_i are equal. For boundary cases, set an=0a_n = 0 and consider the rewritten form of the inequality. It suffices to show that a1a2an1λa_1 a_2 \cdots a_{n-1} \leq \lambda. By AM-GM:
a1a2an1(nn1)n1eλ. a_1 a_2 \cdots a_{n-1} \leq \left(\frac{n}{n-1}\right)^{n-1} \leq e \leq \lambda.

Thus, the values of λ\lambda that satisfy the inequality are:
λe. \lambda \geq e.

The answer is: λe\boxed{\lambda \geq e}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.