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.121221222... is not quadratic.

Florian Luca — 2005

Revista Matemática Complutense

In this note, we show that if b > 1 is an integer, f(X) ∈ Q[X] is an integer valued quadratic polynomial and K > 0 is any constant, then the b-adic number ∑ (a / b), where a ∈ Z and 1 ≤ |a| ≤ K for all n ≥ 0, is neither rational nor quadratic.

Arithmetic properties of positive integers with fixed digit sum.

Florian Luca — 2006

Revista Matemática Iberoamericana

In this paper, we look at various arithmetic properties of the set of those positive integers n whose sum of digits in a fixed base b > 1 is a fixed positive integer s. For example, we prove that such integers can have many prime factors, that they are not very smooth, and that most such integers have a large prime factor dividing the value of their Euler φ function.

On the Euler function of repdigits

Florian Luca — 2008

Czechoslovak Mathematical Journal

For a positive integer n we write φ ( n ) for the Euler function of n . In this note, we show that if b > 1 is a fixed positive integer, then the equation φ x b n - 1 b - 1 = y b m - 1 b - 1 , where x , y { 1 , ... , b - 1 } , has only finitely many positive integer solutions ( x , y , m , n ) .

On the equation ϕ ( | x m - y m | ) = 2 n

Florian Luca — 2000

Mathematica Bohemica

In this paper we investigate the solutions of the equation in the title, where φ is the Euler function. We first show that it suffices to find the solutions of the above equation when m = 4 and x and y are coprime positive integers. For this last equation, we show that aside from a few small solutions, all the others are in a one-to-one correspondence with the Fermat primes.

Multiplicative relations on binary recurrences

Florian LucaVolker Ziegler — 2013

Acta Arithmetica

Given a binary recurrence u n n 0 , we consider the Diophantine equation u n 1 x 1 u n L x L = 1 with nonnegative integer unknowns n 1 , . . . , n L , where n i n j for 1 ≤ i < j ≤ L, m a x | x i | : 1 i L K , and K is a fixed parameter. We show that the above equation has only finitely many solutions and the largest one can be explicitly bounded. We demonstrate the strength of our method by completely solving a particular Diophantine equation of the above form.

On the range of Carmichael's universal-exponent function

Florian LucaCarl Pomerance — 2014

Acta Arithmetica

Let λ denote Carmichael’s function, so λ(n) is the universal exponent for the multiplicative group modulo n. It is closely related to Euler’s φ-function, but we show here that the image of λ is much denser than the image of φ. In particular the number of λ-values to x exceeds x / ( l o g x ) . 36 for all large x, while for φ it is equal to x / ( l o g x ) 1 + o ( 1 ) , an old result of Erdős. We also improve on an earlier result of the first author and Friedlander giving an upper bound for the distribution of λ-values.

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