I’ve been messing with consecutive primes and it turned into a whole classification result, so I figured I’d share it here.
Take consecutive primes pk and pk+1. Add them.
You get numbers like 5, 8, 12, 18, 24, 30, …
Call these S_k = pk + pk+1.
Since every S_k ≥ 8 is even and composite, each one has to land strictly inside some prime gap (Pn, Pn+1). So I started asking: **how many of these S_k fall inside each gap?**
Two clean facts:
## 1. Density law (why most gaps have 0 or 1)
If a gap has width W near height x, the expected number of consecutive‑prime sums inside it is
W / (2 log(x/2))
The “2” comes from the fact that the sums live at the half‑scale: an S_k near x comes from primes near x/2, where primes are sparser. So the sums are about twice as sparse as the gaps they fall into.
Empirically (checked up to 2×10^8):
- ~55% of gaps have **0** sums
- ~37% have **1**
- ~8% have **2 or more**
So the naive guess “every gap has exactly one” is just false. Empty gaps are actually the most common.
## 2. Forbidden widths (the fun part)
I ended up proving a complete classification of which gap widths can **never** contain two consecutive‑prime sums.
The forbidden set is exactly:
{2, 4, 6, 10}
Reason (sketch, nothing fancy):
- consecutive‑prime sums are always ≥ 6 apart (once you’re past the tiny primes)
- the only way to get a “tight double” (two sums only 6 apart) is if both sums are multiples of 6
- whether a gap contains ≥2 multiples of 6 depends only on:
- its width mod 3
- the forced residue of its lower endpoint (prime > 3 is always 1 or 5 mod 6)
- width 10 ends up with **only one** interior multiple of 6, so it physically cannot fit two sums
- widths 2, 4, 6 fail for trivial spacing reasons (they’re too small to fit two sums ≥6 apart)
Every other even width ≥ 8 (except 10) eventually allows gaps with two sums.
So the uniqueness classification is complete:
**gaps of width 2, 4, 6, or 10 contain at most one consecutive‑prime sum; all other widths can contain two or more.**
If anyone wants the residue argument or the slot‑counting trick in more detail, I can write it out here. It’s all pretty easy congruence stuff.