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## Acta Arithmetica

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## The density of primes dividing a particular non-linear recurrence sequence

### Volume 175 / 2016

Acta Arithmetica 175 (2016), 71-100 MSC: Primary 11G05; Secondary 11F80. DOI: 10.4064/aa8265-4-2016 Published online: 3 August 2016

#### Abstract

Define the ECHO sequence $\{b_n\}$ recursively by $(b_0,b_1,b_2,b_3)=(1,1,2,1)$ and for $n\geq 4$, $$b_n=\begin{cases} \dfrac{b_{n-1}b_{n-3}-b_{n-2}^2}{b_{n-4}} & \text{if }n\not\equiv 0\pmod 3,\\ \dfrac{b_{n-1}b_{n-3}-3b_{n-2}^2}{b_{n-4}} &\text{if }n\equiv 0\pmod 3.\end{cases}$$ We relate $\{b_n\}$ to the coordinates of points on the elliptic curve $E:y^2+y=x^3-3x+4$. We use Galois representations attached to $E$ to prove that the density of primes dividing a term in this sequence is equal to $\frac{179}{336}$. Furthermore, we describe an infinite family of elliptic curves whose Galois images match those of $E$.

#### Authors

• Alexi Block GormanDepartment of Mathematics
Wellesley College
Wellesley, MA 02481, U.S.A.
e-mail
• Tyler GenaoDepartment of Mathematical Sciences
Florida Atlantic University
Boca Raton, FL 33431, U.S.A.
e-mail
• Heesu HwangDepartment of Mathematics
Princeton University
Princeton, NJ 08544, U.S.A.
e-mail
• Noam KantorDepartment of Mathematics
Emory University
Atlanta, GA 30322, U.S.A.
e-mail
• Sarah ParsonsDepartment of Mathematics and Statistics
Wake Forest University
Winston-Salem, NC 27109, U.S.A.
e-mail
• Jeremy RouseDepartment of Mathematics and Statistics
Wake Forest University
Winston-Salem, NC 27109, U.S.A.
e-mail

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