# Subset of natural numbers such that any natural number except 1 can be expressed as sum of two elements

Let $X$ be the set of natural numbers $k_i$, $k_i \geq 1$, with the property that at least one of the equations $p_i =$6$k_i \pm 1$ gives the $i$-th prime number (disregarding $2$ and $3$), and define the set $Y$ to be $Y = \mathbb{N} \setminus \{1\}$. Is it true that each element of Y can be represented as $2 k_i$ or as the sum $k_i + k_j$, where $k_i$ and $k_j$ are both elements of set X?

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Crossposted to MO – t.b. Sep 9 '11 at 5:35
X is also infinite since there is infinite number of primes – pedja Sep 9 '11 at 5:53
anon,minimal element of X is k=1 so 2 can be represented as 2*1 – pedja Sep 9 '11 at 6:00
Dan,it states..."Let X be a set of natural numbers k_i....with the property..." – pedja Sep 9 '11 at 6:05
I've edited the question to clarify the meaning pedja had in mind. @Dan – anon Sep 9 '11 at 6:14

Consider the numbers ${6n-2, 6n, 6n+2}$. If at least one of these can be expressed as the sum of two primes (which are not 3), then $n$ is in $X+X$. The not-3 bit is tricky, I'll admit, but that's a vanishingly small proportion of $Y$. – Craig Sep 8 '11 at 17:49
To clarify, if Goldbach's Conjecture is true, then this statement is true, because $6n$ is the sum of two primes of this form for $n>1$. – Craig Sep 14 '11 at 16:28