Find all positive real numbers λ such that for all integers n≥2 and all positive real numbers a1,a2,…,an with a1+a2+⋯+an=n, the following inequality holds:
i=1∑nai1−λi=1∏nai1≤n−λ.
To find the values of λ, we first show that λ<e fails. Rewrite the inequality by multiplying both sides by a1a2⋯an:
i=1∑na1a2⋯ai−1ai+1⋯an−λ≤(n−λ)⋅a1a2⋯an.
Set an=0 and a1=a2=⋯=an−1=n−1n. The inequality reduces to:
(n−1n)n−1−λ≤0⟹λ≥(n−1n)n−1.
Taking the limit as n→∞, we get λ≥e.
Next, we show that λ≥e satisfies the inequality. Using Lagrange multipliers, the left-hand side of the original inequality is maximized when:
−ai21+aiλ⋅a1a2⋯an1=μ⟹a1a2⋯anλ=μai+ai1 for i=1,2,…,n.
Assume ai=aj for some i,j. Then:
μai+ai1=μaj+aj1⟹μ=aiaj1.
Plugging this in for μ and using AM-GM, we get:
a1a2⋯anλ=aj1+ai1⟹λ=aiajai+aj⋅a1a2⋯an≤(n−1a1+a2+⋯+an)n−1=(n−1n)n−1<e,
which is a contradiction. Hence, ai=aj for all i,j.
The inequality is easy to verify when all ai are equal. For boundary cases, set an=0 and consider the rewritten form of the inequality. It suffices to show that a1a2⋯an−1≤λ. By AM-GM:
a1a2⋯an−1≤(n−1n)n−1≤e≤λ.
Thus, the values of λ that satisfy the inequality are:
λ≥e.
The answer is: λ≥e.