# Generate middle levels Gray codes

Generate all bitstrings of length $2n+1$ with Hamming weight $n$ or $n+1$ by flipping a single bit in each step.
 Parameter $n$ (length is $2n+1$) (max. 15) Algorithm no symmetry (2n+1)-fold symmetry Shift $s$ (between 1 and $2n$, coprime to $2n+1$) Initial bitstring Output format graphics (≤500 objects) text (≤10000 objects) file (≤100000 objects)
Output  numbering graphics

## Object info

The Hamming weight of a bitstring is the number of 1s in it. The middle levels conjecture asserts that for every $n\geq 1$, there is a cyclic listing of all bitstrings of length $2n+1$ with Hamming weight $n$ or $n+1$ that flips a single bit in each step. This conjecture was raised in the 1980s by Havel [Hav83] and by Buck and Wiedemann [BW84]. It attracted considerable attention by several researchers over the years, and has been resolved in Mütze's paper [Müt16]. A simplified proof was subsequently given by Gregor, Mütze and Nummenpalo [GMN18]. Both of these proofs are constructive, and they yield in fact double-exponentially many distinct such listings. A constant-time algorithm for generating one solution is described in the paper by Mütze and Nummenpalo [MN20], and this is the first algorithm running on this website (no symmetry). The following figure shows a wheel diagram of this solution for $n=3$, where 0=white and 1=black.

In Problem 56 in Section 7.2.1.3 of his book [Knu11], Knuth raised a stronger version of the middle levels conjecture. He asks for a solution in which the flip sequence, i.e., the sequence of bit positions flipped in each step, can be subdivided into $2n+1$ blocks $\alpha_0,\alpha_1,\ldots,\alpha_{2n}$ of the same length, and the $i$th block $\alpha_i$ is obtained from the initial block $\alpha_0$ by element-wise addition of $i$ modulo $2n+1$ for all $i=1,\ldots,2n$. This allows encoding the entire flip sequence by a factor $2n+1$ more compactly, which was a crucial ingredient in early attempts to tackle the middle levels conjecture experimentally with computer help (see the paper by Shields and Savage [SS99]). For example, such a solution with cyclic symmetry for $n=3$ is given by $\alpha_0=6253462135$. Starting at the bitstring $x=1110000$, applying the flip sequence $\alpha_0$ yields the sequence of strings $1110000,1110010,1010010,1010110,1000110,1001110,\ldots,0111000$, i.e., after 10 flips we reach a bitstring that differs from the initial string $x$ by cyclic right-rotation by one position. More generally, after applying $\alpha_0,\ldots,\alpha_{i-1}$ for all $i=1,\ldots,2n+1$, we reach a copy of $x$ that is cyclically shifted to the right by $i$ positions. Knuth's conjecture was proved by Merino, Mička and Mütze [MMM20] in a more general form, allowing for an arbitrary shift $s\in\{1,\ldots,2n\}$ that is coprime to $2n+1$, i.e., $\alpha_i$ is obtained from $\alpha_0$ by element-wise addition of $i\cdot s$ modulo $2n+1$ for all $i=1,\ldots,2n$. It is not hard to see that the condition on $s$ to be coprime to $2n+1$ is necessary and cannot be omitted. An implementation of this algorithm, using linear time for each generated bitstring, was also provided in the paper [MMM20], and this is the second algorithm running on this website ($(2n+1)$-fold symmetry). The cyclic symmetry can be seen nicely in the wheel diagrams below, which show solutions for $n=3$ and $s=1,\ldots,6$. The animation shifts the bits cyclically around on a torus. Click on each figure to start/stop the animation.

 .p{stroke-width:0.6}.m{stroke-width:1;stroke:black}.c{fill:none;stroke:black} .p{stroke-width:0.6}.m{stroke-width:1;stroke:black}.c{fill:none;stroke:black} $s=1$ $s=2$ .p{stroke-width:0.6}.m{stroke-width:1;stroke:black}.c{fill:none;stroke:black} .p{stroke-width:0.6}.m{stroke-width:1;stroke:black}.c{fill:none;stroke:black} $s=3$ $s=4$ .p{stroke-width:0.6}.m{stroke-width:1;stroke:black}.c{fill:none;stroke:black} .p{stroke-width:0.6}.m{stroke-width:1;stroke:black}.c{fill:none;stroke:black} $s=5$ $s=6$

There are several equivalent formulations of the middle levels conjecture. For instance, the conjecture is equivalent to asking for a listing of all bitstrings of length $2n+2$ with Hamming weight $n+1$ such that any two consecutive bitstrings differ in exchanging the first bit with some other bit (the first bit will alternate between 0 and 1). This is similar to Ehrlich's star transposition Gray code for permutations. The conjecture can also be phrased as follows: Imagine a group of $2n+1$ people, split into two groups of size $n$ and $n+1$ in two rooms that are separated by a revolving door. In each step, one person from the bigger group moves through the door to join the other group. The goal is to come up with a protocol so that each partition into two groups is encountered exactly once.

## Enumeration (OEIS)

The number of generated bitstrings is given by the central binomial coefficients $2\binom{2n+1}{n}=\binom{2n+2}{n+1}$, which is OEIS A000984.