# Almost tight $$\ell$$-covering of $$\mathbb{Z}_n$$

Ke Shi

Chao Xu

Flash Memory Storage

state $$\in \{0,\ldots,\ell\}$$

cell stores a state

block $$\sim 10^6$$ cells

Operations

increase a state

= single cell update

set a state to 0

= clear entire block

Abstract memory cell (AMC): stores a value in $$\N$$

- Update: increase the cell by a number in $$\{0,\ldots,\ell\}$$

$$\Z_n=\{0,1,2,\ldots,n-1\}$$

### Store and rewrite a number in $$\Z_n$$

1 rewrite = 1 AMC update

Store a number in $$\Z_n$$ use AMCs

Store a number in $$\Z_n$$ use AMCs

store $$y\in \Z_n$$ by storing $$x\in \N^t$$

$$s\in \Z_n^t$$ a fixed vector

D(x) = s\cdot x \pmod n = y

$$2$$

$$4$$

$$3$$

$$1$$

$$D(x) = (1,2,3,4) ·(1,7,2,9)\pmod{12} =9$$

Example:

AMC storage

t=4
n=12
y=9
s=(1,7,2,9)
x

1 rewrite = 1 AMC update

$$D(x) = (1,2,3,4) ·(1,7,2,9)\pmod{12} =9$$

Example:

AMC storage

t=4
n=12
y=9
s=(1,7,2,9)
x

$$D(x) = (1,2,{\color{red}5},4) ·(1,7,2,9)\pmod{12} ={\color{red}1}$$

$$2$$

$$4$$

$$3$$

$$1$$

$$5$$

1 AMC update per rewrite is possible if and only if

\displaystyle \Z_n = \bigcup_{i\in s}\{ei\pmod{n} | e\leq \ell\}
x_j\gets x_j + e \Leftrightarrow y \gets y+e s_j\pmod{n}
y={\color{red}1}

1 rewrite = 1 AMC update

1 AMC update per rewrite is possible if and only if

\displaystyle \Z_n = \bigcup_{i\in s}\{ei\pmod{n} | e\leq \ell\}

$$t$$ should be as small as possible to save space

For a fixed $$\ell$$ and $$n$$, how large must $$t$$ be?

## $$\ell$$-covering

S_i^{\ell} = \{0i,1i,2i,\ldots, \ell i\}

$$A\subseteq \mathbb{Z}_n$$ is a $$\ell$$-covering of $$U$$,

If $$U\subseteq \bigcup_{i\in A} S_i^{\ell}$$.

$$U=\Z_n$$ most of the time.

We omit $$\ell$$ when it is clear

A segment of length $$\ell$$ with slope $$i$$.

Find a small $$\ell$$-covering $$A\subseteq \mathbb{Z}_n$$

$$n=7, \ell=3$$

$$S_1$$

$$S_3$$

$$S_4$$

$$\mathbb{Z}_n$$

$$S_2$$

$$S_5$$

$$S_6$$

$$1$$

$$4$$

$$6$$

$$0$$

$$1$$

$$3$$

$$5$$

$$0$$

$$1$$

$$4$$

$$5$$

$$0$$

$$3$$

$$2$$

$$6$$

$$0$$

$$2$$

$$4$$

$$6$$

$$0$$

$$1$$

$$3$$

$$2$$

$$0$$

$$1$$

$$3$$

$$2$$

$$4$$

$$5$$

$$6$$

$$0$$

$$S_0$$

$$0$$

$$n=7, \ell=3$$

$$\mathbb{Z}_n$$

$$1$$

$$4$$

$$5$$

$$0$$

$$3$$

$$2$$

$$6$$

$$0$$

$$1$$

$$3$$

$$2$$

$$4$$

$$5$$

$$6$$

$$0$$

$$A=\{3,4\}$$

$$A$$

$$S_3$$

$$S_4$$

Find a small $$\ell$$-covering $$A\subseteq \mathbb{Z}_n$$

$$n=7, \ell=3$$

$$\mathbb{Z}_n$$

$$1$$

$$4$$

$$5$$

$$0$$

$$3$$

$$2$$

$$6$$

$$0$$

$$1$$

$$3$$

$$2$$

$$4$$

$$5$$

$$6$$

$$0$$

$$f(n,\ell)$$ is the size of the smallest $$\ell$$-covering of $$\mathbb{Z}_n$$.

$$f(7,3)\leq |A| = 2$$

\left\lceil \frac{n-1}{\ell} \right\rceil \leq f(n,\ell) \leq n-1

$$2 = \left\lceil \frac{7-1}{3} \right\rceil \leq$$

$$A=\{3,4\}$$

Find a small $$\ell$$-covering $$A\subseteq \mathbb{Z}_n$$

## Main Questions

Combinatorial: What is the correct bound for $$f(n,\ell)$$?

Algorithmic: How to find a small $$\ell$$-covering?

$$f(n,\ell)=\left\lceil \frac{n-1}{\ell} \right\rceil$$?

No.

$$f(16,5)>3=\lceil \frac{16-1}{5}\rceil$$

Klove et. al. multiple papers in the 2010s: $$f(n,\ell)$$ for $$\ell=2,3,4$$

Chen et. al. 2013: $$f(n,\ell) \leq \frac{n^{1+o(1)}}{\sqrt{\ell}}$$

# Previous Work

$$f(n,\ell) = O(\frac{n}{\ell}\log n\log \log n)$$

if $$\ell$$ is neither too small nor too large

n^{\Omega(\frac{1}{\log \log n})} \leq \ell \leq n^{1-\Omega(\frac{1}{\log \log n})}

Koiliaris & Xu 2019

## Our results

$$f(n,\ell) = O(\frac{n}{\ell}\log n\log \log n)$$

A $$O(f(n,\ell)\log n)$$ size $$\ell$$-covering can be found in time

$$\tilde{O}(\frac{n}{\ell}) + n^{o(1)}$$

There exist examples where

$$f(n,\ell)=\Omega(\frac{n}{\ell}\frac{\log n}{\log \log n})$$

$$f(n,\ell) = O(\frac{n}{\ell}\log n\log \log n)$$

* $$\ell$$ is not too small and not too large.

*

A more refined analysis of $$\varphi(n,\ell)$$.

A more refined analysis of covering.

\displaystyle\varphi(n,\ell)=\sum_{\substack{ x\leq\ell \\ \gcd(x,n)=1}}1
\varphi(n)=\varphi(n,n)

## Set cover

Given $$\mathcal{S} \subseteq 2^U$$.

A collection of sets in $$\mathcal{S}$$ covers $$U$$ is a set cover.

Formally, if $$\mathcal{S}'\subseteq \mathcal{S}$$ such that $$U= \bigcup_{X\in \mathcal{S}'} X$$, then $$\mathcal{S}'$$ is a set cover of $$U$$.

$$\ell$$-covering as set cover

$$U=\Z_n$$

$$S_i=\{ij\pmod{n}|j\leq\ell\}$$

$$\mathcal{S}=\{S_i|i\in\Z_n\}$$

An algebraic special case

## $$O\left(\frac{|\mathcal{S}|}{b}\log \ell\right)$$ size set cover

(naively) apply to $$\ell$$-covering?

\begin{aligned} &\Z_{n,d}&&=&&\{x|\gcd(x,n)=d,x\in\Z_n\} \\ &\Z_n^*&&=&&\Z_{n,1} \end{aligned}

$$p$$ a large prime, $$n=2p, \ell=p-1$$. $$p$$ only covered by $$S_p$$.

# No

$$f^*(n,\ell)$$ the smallest $$\ell$$-covering of $$\Z^*_n$$ such that all segments has slope in $$\Z^*_n$$.

Covering $$\Z_n^*$$ with $$\Z_n^*$$ slope segments

Every elment in $$\Z^*_n$$ is covered by $$\varphi(n,\ell)$$ segments with slope in $$\Z^*_n$$.

## Previous bound for $$\varphi(n,\ell)$$

\varphi(n,\ell) = \begin{cases} \Omega(\frac{\ell}{n} \varphi(n))& \text{ if } \ell > n^{\Omega(\frac{1}{\log \log n})}\\ \Omega(\frac{\ell}{\log \ell}) & \text{ if } \ell > c \log n\\ \end{cases}

Constants $$c,u\geq 1$$

\varphi(n,\ell) = \begin{cases} \Omega(\frac{\ell}{n} \varphi(n))& \text{ if } \ell > n^{\Omega(\frac{1}{\log \log n})}\\ \Omega(\frac{\ell}{\log \ell}) & \text{ if } \ell > c \log n\\ \end{cases}
c \log^u n

# Sieve Theory

$$\pi(\ell)\geq c_1\frac{\ell}{\log \ell}$$ (Prime Number Theorem)

$$\omega(n)\leq c_2\frac{\log n}{\log \log n}$$.

$$\omega(n)\leq \frac{\pi(\ell)}{2}$$ if $$\ell \geq c\log n$$ for sufficiently large $$n$$.

## Case 2: $$\ell > c \log n$$

\varphi(n,\ell)\geq \pi(\ell)-\omega(n)

$$\pi(n)$$: number of primes no larger than $$n$$

$$\omega(n)$$: number of distinct prime factors of $$n$$

\varphi(n,\ell)=\Omega\left(\frac{\ell}{\log \ell}\right)

## Bound for $$f^*(n,\ell)$$

f^*(n,\ell) = O\left(\frac{n}{\ell}\log n\right)

Using the fact about $$\varphi(n,\ell)$$.

## Case 1: $$\ell > c \log^u n$$

\begin{aligned} f^*(n,\ell) &= O\left(\frac{\varphi(n)\log \ell}{\frac{\ell}{n}\varphi(n)}\right)\\ &= O\left(\frac{n}{\ell}\log n\right) \end{aligned}
\text{each element covered by } b \text{ sets} \Rightarrow O(\frac{|S|\log \ell}{b})
\varphi(n,\ell) = \Omega(\frac{\ell}{n}\varphi(n))
\begin{aligned} f^*(n,\ell) &= O\left(\frac{\varphi(n)\log \ell}{\frac{\ell}{\log \ell}}\right)\\ &= O\left(\frac{\varphi(n)}{\ell}(\log \ell)^2\right) \\ &= O\left(\frac{n}{\ell}(\log \log n)^2\right)\\ &= O\left(\frac{n}{\ell}\log n\right) \end{aligned}

## Case 2: $$c\log n<\ell < c \log^u n$$

O(\frac{|S|\log \ell}{b})
\varphi(n,\ell) = \Omega(\frac{\ell}{\log \ell})
\ell = O(\log^u n)
\begin{aligned} f^*(n,\ell)&\leq \varphi(n) \\ &= O\left(\varphi(n)\frac{\log n}{\ell}\right) \\ &= O\left(\frac{n}{\ell}\log n\right) \end{aligned}

## Case 3: $$\ell < c \log n$$

We can cover $$\mathbb{Z}_{n,d}$$ with $$f^*(\frac{n}{d},\ell)$$ segments in $$\mathbb{Z}_{n,d}$$.

We can cover $$\mathbb{Z}_n^*$$ with $$f^*(n, \ell)$$ segments in $$\mathbb{Z}_n^*$$.

$$\mathbb{Z}_n$$

We can cover $$\mathbb{Z}_{n,d}$$ with $$f^*(\frac{n}{d},\ell)$$ segments in $$\mathbb{Z}_{n,d}$$.

$$\mathbb{Z}_n$$

$$d \mid x$$

We can cover $$\mathbb{Z}_n^*$$ with $$f^*(n, \ell)$$ segments in $$\mathbb{Z}_n^*$$.

We can cover $$\mathbb{Z}_{n,d}$$ with $$f^*(\frac{n}{d},\ell)$$ segments in $$\mathbb{Z}_{n,d}$$.

$$\mathbb{Z}_n$$

$$d \mid x$$

$$\mathbb{Z}_{n,d}$$

We can cover $$\mathbb{Z}_n^*$$ with $$f^*(n, \ell)$$ segments in $$\mathbb{Z}_n^*$$.

We can cover $$\mathbb{Z}_{n,d}$$ with $$f^*(\frac{n}{d},\ell)$$ segments in $$\mathbb{Z}_{n,d}$$.

$$\mathbb{Z}_n$$

$$d \mid x$$

$$\mathbb{Z}_{n,d}$$

$$\mathbb{Z}^{*}_{\frac{n}{d}}$$

We can cover $$\mathbb{Z}_n^*$$ with $$f^*(n, \ell)$$ segments in $$\mathbb{Z}_n^*$$.

Cover of $$\Z^*_\frac{n}{d}$$ using $$\Z^*_{\frac{n}{d}}$$ lifts to a cover of $$\Z_{n,d}$$ using $$\Z_{n,d}$$

\mathbb{Z}^*_{12}
\mathbb{Z}_{12,2}
\mathbb{Z}_{12,3}
\mathbb{Z}_{12,4}
\mathbb{Z}_{12,6}
\mathbb{Z}_{12,12}
f^*(12,4)
\ell=4
\mathbb{Z}^*_{12}
\mathbb{Z}_{12,2}
\mathbb{Z}_{12,3}
\mathbb{Z}_{12,4}
\mathbb{Z}_{12,6}
\mathbb{Z}_{12,12}
f^*(12,4)
f^*(6,4)
\ell=4
\mathbb{Z}^*_{12}
\mathbb{Z}_{12,2}
\mathbb{Z}_{12,3}
\mathbb{Z}_{12,4}
\mathbb{Z}_{12,6}
\mathbb{Z}_{12,12}
f^*(12,4)
f^*(6,4)
f^*(3,4)
f^*(4,4)
f^*(2,4)
f^*(1,4)
f(12,4)
+
+
+
+
+
\leq
\ell=4

## Bound for $$f(n,\ell)$$

$$d(n) = n^{O\left(\frac{1}{\log \log n}\right)}$$

$$\sigma(n) = O(n\log \log n)$$

f(n,\ell) \leq \sum_{d|n} f^*(\frac{n}{d},\ell)
\leq \sum_{d|n} \max(d \frac{\log n}{\ell},1)
\leq \sigma(n)\frac{\log n}{\ell} + d(n)
= \underbrace{ O\left(\frac{n}{\ell}\log n\log \log n\right)}_{\textbf{main term}} + {\color{red}\underbrace{n^{O\left(\frac{1}{\log \log n}\right)}}_{\textbf{error term}}}

Desired bound unless $$\ell$$ is too large.

f^*(n,\ell) = O(\frac{n}{\ell}\log n)
\mathbb{Z}^*_{12}
\mathbb{Z}_{12,2}
\mathbb{Z}_{12,3}
\mathbb{Z}_{12,4}
\mathbb{Z}_{12,6}
\mathbb{Z}_{12,12}

Larger Cover

$$\ell = 4$$

Theorem:

If $$\ell$$ is large, $$d\leq \ell'$$, then each element in $$\Z_{n,d}$$ is covered $$\Omega(\frac{\ell}{n}\varphi(n))$$ times by segments with slope in $$\Z^*_n$$.

\ell' := \frac{\ell}{c \log^u \frac{n}{\ell}}

Theorem:

If $$\ell$$ is large, each element in $$\Z^*_n$$ is covered $$\Omega(\frac{\ell}{n}\varphi(n))$$ times by segments with slope in $$\Z^*_n$$.

$$f^\dagger(n,\ell)$$ the smallest $$\ell$$-covering of $$\bigcup_{d|n,d\leq \ell'} \Z_{n,d}$$ such that all segments has slope in $$\Z^*_n$$.

f^\dagger(n,\ell) = O(\frac{n}{\ell}\log n)
f^*(n,\ell) = O(\frac{n}{\ell}\log n)

What's the relation between $$f(n,\ell)$$ and $$f^\dagger(n,\ell)$$?

\displaystyle f(n,\ell) \leq \sum_{d|m} f^\dagger(\frac{n}{d},\ell)
\displaystyle n = \prod_{i=1}^k p_i
\displaystyle m = \prod_{i=1}^j p_i \geq \frac{n}{\ell'}

If $$d|n$$, then $$d=d_1d_2$$, where $$d_1|m$$, and $$d_2\leq \ell'$$.

How large is $$d(m)$$?

\begin{aligned} &\leq \sum_{d|m}O\left(\frac{n}{d}\frac{\log n}{\ell} \right)+1\\ &\leq O\left(\sigma(n)\frac{\log n}{\ell}\right) + d(m) \end{aligned}
d(n) = n^{O(\frac{1}{\log \log n})}
d(m)
\leq 2d(m')
=m'^{O(\frac{1}{\log \log m'})}
= \left(\frac{n}{\ell}\log^u \frac{n}{\ell}\right)^{O\left(\frac{1}{\log \log \frac{n}{\ell}}\right)}
= \frac{n}{\ell} c\log^u \frac{n}{\ell}
< \frac{n}{\ell'}
m':=\frac{m}{p_j}
\begin{aligned} f(n,\ell) &\leq \sum_{d|m} f^\dagger(\frac{n}{d},\ell)\\ &\leq \sum_{d|m}O\left(\frac{n}{d}\frac{\log n}{\ell} \right)+1\\ &\leq O\left(\sigma(n)\frac{\log n}{\ell}\right) + d(m)\\ &= \underbrace{ O\left(\frac{n}{\ell}\log n\log \log n\right)}_{\textbf{main term}} + {\color{red} \underbrace{\left(\frac{n}{\ell}\log^u \frac{n}{\ell}\right)^{O\left(\frac{1}{\log \log \frac{n}{\ell}}\right)}}_{\textbf{error term}}}\\ &= O\left(\frac{n}{\ell}\log n\log \log n\right) \end{aligned}

$$f(n,\ell) = O(\frac{n}{\ell}\log n\log \log n)$$

$$f(n,\ell) = O(\frac{n}{\ell}\log n\log \log n)$$

## Conclusion and Future Work

Better bound for $$f(n,\ell)$$?

What about $$\ell$$-covering of arbitrary set $$X\subseteq \Z_n$$?

Other interesting groups, e.g. $$\Z_n\times \Z_m$$.

## IP

\begin{aligned} & \min_{x} & & \sum_{i} x_i & \\ & \text{s.t.} & & \sum_{i:j\in i[\ell]} x_i\geq 1 & j\in \Z_n \\ & & & x_i \in \{0,1 \} & i\in \Z_n \\ \end{aligned}

Integer programming form of set cover

## LP (Relaxation)

\begin{aligned} & \min_{x} & & \sum_{i} x_i & \\ & \text{s.t.} & & \sum_{i:j\in i[\ell]} x_i\geq 1 & j\in \Z_n \\ & & & 0 \leq x_i & i\in \Z_n \\ \end{aligned}

## Symmetric LP

A linear program is symmetric if there exists a permutation of variables, so the linear program does not change it's value.

\begin{bmatrix} x_1 \\ x_2 \\.\\.\\.\\x_n \end{bmatrix} \xRightarrow[\textbf{permutation group G}]{\forall \textbf{permutation } \pi \text{ in}} \begin{bmatrix} x_{\pi(1)} \\ x_{\pi(2)} \\.\\.\\.\\x_{\pi(n)} \end{bmatrix}

$$\textbf{fesible}$$  $$\xRightarrow[\textbf{ }]{}$$  $${}\textbf{fesible}$$

$$\textbf{c}\cdot\textbf{x}$$      $$\xRightarrow[\textbf{ }]{\textbf{same value}}$$   $$\textbf{c}\cdot\pi\textbf{ x}$$

We call such a LP $$G$$-symmetric.

## Reduce variables

Let $$G$$ be a permutation group that consists of all permutations that swaps $$i$$ and $$j$$ if $$\gcd(i,n)=\gcd(j,n)$$. LP2  is $$G$$-symmetric.

\begin{aligned} \{x_i:i\in \Z_n\}&\longrightarrow &&\{y_d:d|n\}\\ \text{number:}\quad n&\longrightarrow &&d(n) \end{aligned}

We let $$y_d$$ to represent all variables in  $$\{x_i|\gcd(i,n)=d\}$$.

## Reduce  constraints

If $$\gcd(a,n) = \gcd(b,n)$$, then the constraints that covers $$a$$ and $$b$$ have same format since all variables $$y_d$$ have same coffcients in inequalities $$a$$ and $$b$$.

\begin{aligned} \{\mathrm{constraint}_i:i\in \Z_n\}&\longrightarrow &&\{\mathrm{constraint}_d:d|n\}\\ n&\longrightarrow &&d(n) \end{aligned}

## LP (compact)

\begin{aligned} & \min_{y} & & \sum_{d:d|n} \varphi(\frac{n}{d}) y_d & \\ & \text{s.t.} & & \sum_{d:d|c} \frac{\varphi(n/c, d \ell / c)\varphi(n/d)}{\varphi(n/c)} y_d\geq 1 & c|n \\ & & & 0\leq y_d & d|n \\ \end{aligned}

We have only $$d(n)$$ variables and inequalities now!

\begin{aligned} & \min_{y} & & \sum_{d:d|n} \color{red}{\varphi(\frac{n}{d}) y_d} & \\ & \text{s.t.} & & \sum_{d:d|c} \frac{\varphi(n/c, d \ell / c)\color{red}{\varphi(n/d)}}{\varphi(n/c)} {\color{red}y_d} \geq 1 & c|n \\ & & & 0\leq y_d & d|n \\ \end{aligned}

## LP*

\begin{aligned} & \min_{d} & & \sum_{d:d|n} u_d & \\ & \text{s.t.} & & \sum_{d:d|c} \frac{\varphi(n/c, d \ell / c)}{\varphi(n/c)} u_d\geq 1 & c|n \\ & & & 0\leq u_d & d|n \end{aligned}

Let $$u_d=\varphi(n/d)y_d$$, We have the following concise form.

## Randomized algorithm

$$\underline{\textbf{Cover}(n,\ell)}$$

compute the prime factorization of $$n$$

compute $$\varphi(n/c,d\ell/c)$$ and $$\varphi(n/c)$$ for all $$d|c|n$$

compute the optimum for $$\mathrm{LP4}$$

$$C\leftarrow \empty$$

repeat $$4\log{n}$$ times

for $$d|n$$:

if $$u_d\geq1$$:

$$C\leftarrow C\cup (S\sim {\Z_{n,d}\choose \lceil u_d \rceil})$$

else if $$u_d > (x\sim[0,1])$$:

$$C\leftarrow C\cup (S\sim {\Z_{n,d}\choose 1})$$

return $$C$$

taking $$O(f(n,\ell)\mathrm{ploylog}(n))$$ time

taking $$n^{o(1)}$$ time

taking $$n^{o(1)}$$ time

taking $$n^{o(1)}$$ time

Theorem:

Algorithm before takes $$O(f(n,\ell)\mathrm{polylog} n + n^{o(1)})$$ time and computes a expected size $$O(f(n,\ell)\log n)$$ $$\ell$$-covering set in expectation with constant probability.

Theorem: There is an algorithm which successfully returns $$X\sim {\Z^*_n \choose k}$$ in $$O(k \mathrm{polylog}(n))$$ time with probability at least $$1-\frac{1}{n}$$.

## Running time analysis

Time of constructing the LP.

Step 1. Finding the prime factorization of $$n$$ takes $$n^{o(1)}$$ time through general number field sieve.

Step 2. Computing all $$\varphi(n/c),c|n$$ through $$\varphi(n)=n\prod_{p|n}(1-1/p)$$ takes $$n^{o(1)}$$ time.

## Sampling in $$\Z_n^*$$

Otherwise, $$k<\frac{\varphi(n)}{2}$$. We will sample a random element $$x\sim \Z_n$$, if $$x\in\Z_n^*$$ and we hasn't get enough elements, pick it. Repeat it specific times.

We can show the runing time of the algorithm is $$O(k \mathrm{polylog}(n))$$ and Chernoff bound can show the success probability at least $$1-\frac{1}{n}$$.

Consider two cases. If $$k\geq \frac{\varphi(n)}{2}$$, then find all elements in $$\Z_n^*$$ by brute force and pick $$k$$ from it.

## Running time analysis

Step 3. Computing all $$\varphi(n/c,d\ell/c),d|c|n$$ through the inclusion-exclusion principle takes $$n^{o(1)}$$ time.

\varphi(n,\ell) = \sum_{i=0}^{\omega(n)} (-1)^{i} \sum_{S\subset P, |S|=i} \left\lfloor\frac{\ell}{\prod_{p\in S} p} \right\rfloor

The running time of computing $$\varphi(n,\ell)$$ is $$\tilde{O}(2^{\omega(n)})=n^{O(\frac{1}{\log\log n})}=n^{o(1)}$$.  The number of all $$(c,d)$$ is also $$n^{o(1)}$$.

## Running time analysis

The total time of sampling is :

$$O(\sum_{d|n}u_d \mathrm{polylog}n)=O(f(n,\ell)\mathrm{ploylog}(n))$$.

Combining all above, the algorithm takes $$(O(f(n,\ell)\mathrm{ploylog}(n)+n^{o(1)})$$ time.

Thank you

Limited-magnitude errors:

Jiang A., Langberg M., Schwartz M., Bruck J.: Trajectory codes for flash memory. IEEE Trans. Inf. Theory 59(7), 4530–4541 (2013).

Sieve theory:

Alina Carmen Cojocaru, M Ram Murty, et al. An introduction to sieve methods and their applications, volume 66. Cambridge University Press, 2006.

References