The starting point of this question is the observation that the smallest positive integers $a,b,c$ satisfying

$$\frac{a}{b+c} + \frac{b}{a+c} + \frac{c}{a+b} = 4$$

are absurdly high, namely _{$$(154476802108746166441951315019919837485664325669565431700026634898253202035277999,$$ $$36875131794129999827197811565225474825492979968971970996283137471637224634055579
,$$ $$
4373612677928697257861252602371390152816537558161613618621437993378423467772036)
.$$} This leads to the following general question: Is the set $C\subseteq {\mathbb N}$ defined by $$ C = \{n\in\mathbb{N}\setminus\{0\}: (\exists a,b,c \in\mathbb{N}\setminus\{0\}):\frac{a}{b+c} + \frac{b}{a+c} + \frac{c}{a+b} = n\}$$
computable? (As user Watson points out in the comment section below, $C$ contains no odd numbers. It would also be great to see an even number $\geq 6$ not contained in $C$.)

providedШ is finite, which is conjecturally always the case. So I think conjecturally your set is indeed computable. $\endgroup$ – Gro-Tsen Aug 14 '17 at 19:28congruent numbers(i.e. the area of a right-angled triangle with rational sides), which comes down to asking whether the elliptic curve $E_n \colon y^2 = x^3-n^2x$ has positive rank over $\mathbb Q$. In this case, there is a conjectural answer (Tunnell's Theorem, conditional on the BSD conjecture), which relies on the fact that the $E_n$ are all quadratic twists of a fixed curve. The question asked here is likely to be harder, since the resulting curves are not twists, and there is the positivity condition. --> $\endgroup$ – Michael Stoll Oct 26 '17 at 15:29