Let $a_1,a_2,a_3,a_4$ be four positive numbers and let:
$$S_1 = a_1 + a_2 + a_3 + a_4$$
$$S_2 = \sum_{i \neq j} a_ia_j$$
$$S_3 = a_1a_2a_3 + a_1a_2a_4 + a_1a_3a_4 + a_2a_3a_4$$
$$S_4 = a_1a_2a_3a_4$$
Given that $$| \frac{S_1-S_3}{1-S_2+S_4} | < 1$$ show that there are two distinct $a_i,a_j$ such that : $$|a_i-a_j| < (\sqrt{2}-1)(1+a_ia_j)$$
Solution 1
If any two $a_i$ are the same then we are done. WLOG, we may now assume that $a_1 < a_2 < a_3 < a_4$.
Suppose that for each two $a_i > a_j$ we always have:
$$a_i - a_j > (\sqrt{2}-1)(1+a_ia_j)$$
Consider $a_3$ and $a_4$:
$$(a_4 - a_3) > (\sqrt{2}-1)(1+a_4a_3)$$
$$(a_4 - a_3)(\sqrt{2}+1) > (1+a_4a_3)$$
$$a_4(\sqrt{2}+1 - a_3) > 1+a_3(\sqrt{2}+1)$$
Because the RHS is positive, in order for the LHS to be positive, we must have $a_3 < \sqrt{2} + 1$.
Thus now we have $a_1 < a_2 < a_3 < \sqrt{2}+1$.
Next, note that the function $f(x) = \frac{x + \sqrt{2} - 1}{1 - (\sqrt{2}-1)x}$ defined for $0 < x < \sqrt{2}+1$ is definite positive and strictly increasing.
The assumption $$a_i - a_j > (\sqrt{2}-1)(1+a_ia_j)$$ for $j=1,2,3$ becomes:
$$a_i(1 - (\sqrt{2}-1)a_j) > a_j + \sqrt{2}-1$$
And because $(1 - (\sqrt{2}-1)a_j) > 0 \iff a_j < \sqrt{2}+1$ we can divide both sides to obtain:
$$a_i > \frac{a_i + \sqrt{2}-1}{1 - (\sqrt{2}-1)a_j} =f(a_j)$$
So now we have:
$$a_4 > f(a_3), a_3 > f(a_2), a_2 > f(a_1)$$
We note that $f(0) = \sqrt{2}-1,f(\sqrt{2}-1) = 1, f(1) = \sqrt{2}+1$.
Furthermore, because $f$ is strictly increasing,
$$a_2 > f(a_1) > f(0) = \sqrt{2}-1$$
$$a_3 > f(a_2) > 1$$
$$a_4 > f(a_3) > \sqrt{2}+1$$
Also, if $a_2 \geq 1$ then we'd have $a_3 > f(a_2) > f(1) = \sqrt{2}+1$ a contradiction, so we must have $a_2 < 1$.
By the same token, if $a_1 \geq \sqrt{2}-1$ we'd have $a_2 > f(a_1) > f(\sqrt{2}-1) = 1$ a contradiction, so we must have $a_1 < \sqrt{2} -1 $.
Now we've established that:
$$0 < a_1 < \sqrt{2} - 1 < a_2 < 1 < a_3 < \sqrt{2} + 1 < a_4$$
From there it's clear that $a_1a_3 < 1$ and $a_2a_4 > 1$.
Next, we assert the following:
$$a_1 + a_3 + a_1a_3 > 1$$
which is very easy to see from the bounds above. Also because $a_4 > 0$ and $a_2 < 1$,
$$\frac{1}{a_2a_4} + \frac{1}{a_2} + \frac{1}{a_4} > 1$$
$$1 + a_2 + a_4 > a_2a_4$$
Therefore:
$$(1-a_1a_3)(1-a_2a_4) > (a_1+a_3)(a_2+a_4)$$
