I am very confused and I thank you in advance for your help about a very stupid question. I would write a summation sequence of non consecutive primes $a,b,c,d...$ greater than $3$ with a distance of $d\geq{3}$ between each others. So, what would be the correct mathematical writing?
$$\cfrac{b}{a_{i-2}+c_{i-1}+\ldots+d_{i}}=x$$
or
$$\cfrac{b}{\sum_{i\geq3}^{n}a_{i}a_{1}+\ldots+a_{n}}=x$$
Actually your question is not clear. As I understand, if $\{p_i\}$ is a sequence of non consecutive primes greater than $3$ with at least a distance of $3$ between each others, then the summation is $\displaystyle \sum_{p_i>3 \;\text{and}\; p_i-p_j\geq 3;\;\text{for}\;i\neq j}p_i$