# the topology of arithmetic progressions of primes

The primary motivation for this question is the following: I would like to extract some topological statistics which capture how arithmetic progressions of prime numbers "fit together" in a manner that will be made precise below.

## Setup

Consider a nested family of simplicial complexes $K(p)$ indexed by prime $p \in \mathbb N$ defined as follows:

1. the vertices are all primes less than or equal to $p$, and
2. insert a $d$-simplex ($d \geq 2$) spanning $d+1$ vertices if and only if they constitute an arithmetic progression. Of course, one must also insert all faces, and faces-of-faces etc. so that the defining property of a simplicial complex is preserved.

For instance, $K(7)$ has the vertices $2,3,5,7$ and a single $2$-simplex $(3,5,7)$ along with all its faces. $K(11)$ has all this, plus the vertex $11$ and the simplex $(3,7,11)$. The edge $(3,7)$ already exists so only the other two need to be added. Thus, the fact that $(3,7)$ occurs in two arithmetic progressions bounded by $11$ is encoded by placing the corresponding edge in the boundary of two simplices.

## Question

Has someone already defined and studied this complex? What I am mostly interested in is

How does the homology of $K(p)$ change with $p$?

It is easy to check that the only interesting homology is in dimensions $\leq 1$. If it helps, here are -- according to home-brew computations -- the statistics for the first few primes (Betti 0 and 1 over $\mathbb{Z}_2$). I've already confirmed that the sequence of Betti-1's is not in Sloane's online encyclopedia of integer sequences. If an intermediate K[p] is missing in the list, that means that the homology is the same as that for the previous prime.

## Update 1##

Zack pointed out an error in the previous computations, so here are the betti numbers with that error fixed. I have also removed "2" from the vertex set since the only contribution of that vertex to the homology is incrementing all the $0$ dimensional betti numbers by +1.

K [3]: 1 0
K [5]: 2 0
K [7]: 1 0
K [13]: 2 0
K [17]: 2 1
K [19]: 1 2
K [23]: 1 4
K [31]: 1 6
K [37]: 2 6
K [43]: 1 7
K [53]: 1 8
K [59]: 1 9
K [61]: 1 10
K [67]: 1 12
K [71]: 1 17
K [73]: 1 20
K [79]: 1 23
K [83]: 1 26
K [89]: 1 31
K [97]: 1 32
K [101]: 1 35
K [103]: 1 41
K [107]: 1 43
K [109]: 1 47
K [113]: 1 53
K [127]: 1 58
K [131]: 1 62
K [137]: 1 67
K [139]: 1 73
K [149]: 1 78

## Update 2

I've just finished running the homology computations (over $\mathbb{Z_2}$) for all primes less than $30,000$, and this text file contains the resulting Betti numbers. Once such data is available for control experiments, such as Cramer numbers or primes which are $1$ mod 4, I will put up those text files as well. Also, I am no longer confident that higher homology will not appear, so here is an auxiliary question:

Is it true that the higher homology groups of $K(p)$ are trivial?

-
"The primary motivation for this question...." Was that a pun? –  Gerry Myerson Aug 5 '12 at 22:44
Gerry: sadly, yes. I fear that my sense of humor is progressively deteriorating... –  Vidit Nanda Aug 5 '12 at 22:49
@David: you mean like this? arxiv.org/abs/1101.5704 –  Vidit Nanda Aug 5 '12 at 23:01
Are the entries with $b_0 = 4$ errors? The vertex 17 in K[17] is connected via 5-11-17, and 41 in K[41] is connected via 17-29-41. –  Zack Wolske Aug 6 '12 at 7:14
Regarding connectedness of the inf. complex: if I did not overlook something stupid, it is 'clear' (in the sense that 'everybody' believes it but nobody can prove it) that for any tow odd primes q1,q2 there will be a prime p (indeed infinitely many) such that q1,p and q2,p can each be completed to a 3-AP of primes. In other words for q1,q2 fixed there will be a prime p such that 2p -q1 and 2p -q2 are both prime. More generally any system of linear equations is believed to be solvable 'in the primes', except if there is a 'local' obstruction (congr. or size), and for some this can be shown. –  quid Aug 6 '12 at 11:32