The problem of giving an explicit formula for $A\_q(n,d)$ is sometimes referred to as "the main problem in coding theory." The value of $A\_q(n,d)$ is given by the maximum number of codewords in a q-ary code of length $n$ and distance $d$. More specifically, let the hamming weight of an element of $\mathbb{F}\_q^n$ be its $l\_0$-pseudonorm, the number of non-zero components, and the hamming distance between two elements $f,g$ the weight of their difference $d(f,g)$. Then $A\_q(n,d)$ is the largest set $S \subset F\_q^n$ s.t. for two elements $f,g \in S$, $d(f,g)\geq d$.


There are a number of famous upper bounds on $A\_q(n,d)$, including Hamming's sphere packing bound. The best are given by a linear programming approach (now improved to a semi-definite programming approach) given by Delsarte in the late 70s. I have recently been searching for an explicit formula for Delsarte's Linear Programming Upper Bound for $A\_q(n,3)$ in the literature, which correspond to single error correcting codes, and have not had much luck for non-binary codes. For binary codes this appears to be well known, and shown as early as 1977 by Best and Brouwer. 


Non-binary codes seem to be a completely different story. There is a paper called "Some upper bounds for codes derived from Delsartes inequalities for
Hamming schemes" by C. Roos and C. de Vroedt, which the authors claim deals with the q-ary case, but I have not been able to find a copy. There appears to have been a very large amount of work in this field so I would be shocked if no such formula exists (well, at least a formula for some special cases of n,q).


Is there a body of work in this area I am missing? Do such formulae exist? 


Note: I have also posted this question on the TCS SE here: <https://cstheory.stackexchange.com/questions/40238/explicit-formula-of-delsartes-linear-programming-upper-bound-for-a-qn-3>
The results on $A\_q(n,d)$ are often published in top combinatorics journals (Journal of Combinatorial Theory, Series A, for instance), and so I think it is appropriate and hopefully of interest to MO users as well.



Explicit Formula of Delsarte's Linear Programming Upper Bound for $A_q(n,3)$
The paper "Some upper bounds for codes derived from Delsarte's inequalities for Hamming schemes" by C. Roos and C. de Vroedt does give a formula for Delsarte's bound. Or at least according to Mathematical Reviews it provides such a formula. I too was unable to locate the paper, but I did find the MR which states the following formula for the bound denoted by $D(n,3,q)$.


$$D(n,3,q) = q^n\frac{\lambda n - a(q-a)}{(\lambda n + a)(\lambda n + a - q)}$$


Where $\lambda = q-1$, $n \equiv a \pmod q$, and $1 \leq a \leq q$. The formula is said to hold for $q > 2$ and $n$ sufficiently large.


