# ON THE MULTIPLICATIVE ORDER OF n MODULO n For every positive integer n and every integer a coprime...

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ON THE MULTIPLICATIVE ORDER OF an

MODULO n

Jonathan Chappelon

February 25, 2009

Abstract

Let n be a positive integer and αn be the arithmetic function which assigns the multiplicative order of an modulo n to every integer a coprime to n and vanishes elsewhere. Similarly, let βn assign the projective multiplicative order of a

n modulo

n to every integer a coprime to n and vanish elsewhere. In this paper, we present a study of these two arithmetic functions. In particular, we prove that for every positive

integers n1 and n2 with the same square-free part, there exists an exact relationship between the functions αn1 and αn2 and between the functions βn1 and βn2 . This allows us to reduce the determination of αn and βn to the case where n is square-free. These arithmetic functions recently appeared in the context of an old problem of Molluzzo,

and more precisely in the study of which arithmetic progressions yield a balanced

Steinhaus triangle in Z/nZ for n odd.

1 Introduction

We start by introducing some notation relating to the order of certain elements modulo n. For every positive integer n and every prime number p, we denote by vp(n) the p-adic valuation of n, i.e. the greatest exponent e > 0 for which pe divides n. The prime factorization of n may then be written as

n = ∏

p∈P

pvp(n),

where P denotes the set of all prime numbers. We denote by rad(n) the radical of n, i.e. the largest square-free divisor of n, namely

rad(n) = ∏

p∈P

p|n

p.

1

http://arxiv.org/abs/0902.4366v1

For every positive integer n and every integer a coprime to n, we denote by On(a) the multiplicative order of a modulo n, i.e. the smallest positive integer e such that ae ≡ 1 (mod n), namely

On(a) = min {e ∈ N ∗ | ae ≡ 1 (mod n)}

and we denote by Rn(a) the multiplicative remainder of a modulo n, i.e. the multiple of n defined by

Rn(a) = a On(a) − 1.

The multiplicative order of a modulo n also corresponds with the order of the element πn(a), where πn : Z −։ Z/nZ is the canonical surjective morphism, in the multiplicative group (Z/nZ)∗, the group of units of Z/nZ. Note that On(a) divides ϕ(n), ϕ being the Euler’s totient function.

For every positive integer n, we define and denote by αn the arithmetic function

αn : Z −→ N

a 7−→

{

On (a n) for all a ∧ n = 1,

0 otherwise,

where a ∧ n denotes the greatest common divisor of a and n, with the convention that 0 ∧ n = n. Observe that, for every a coprime to n, the integer αn(a) divides ϕ(n)/(ϕ(n) ∧ n). This follows from the previous remark on On(a) and the equality αn(a) = On(a

n) = On(a)/(On(a) ∧ n).

For every positive integer n and every integer a coprime to n, we denote by POn(a) the projective multiplicative order of a modulo n, i.e. the smallest positive integer e such that ae ≡ ±1 (mod n), namely

POn(a) = min {e ∈ N ∗ | ae ≡ ±1 (mod n)} .

The projective multiplicative order of a modulo n also corresponds with the order of the element πn(a) in the multiplicative quotient group (Z/nZ)

∗/{−1, 1}.

For every positive integer n, we define and denote by βn the arithmetic function

βn : Z −→ N

a 7−→

{

POn (a n) for all a ∧ n = 1,

0 otherwise.

Observe that we have the alternative αn = βn or αn = 2βn.

In this paper, we study in detail these two arithmetic functions. In particular, we prove that, for every positive integers n1 and n2 such that

{

rad(n1)|n2|n1 if v2(n1) 6 1, 2 rad(n1)|n2|n1 if v2(n1) > 2,

the integer αn1(a) (respectively βn1(a)) divides αn2(a) (resp. βn2(a)), for every integer a. More precisely, we determine the exact relationship between the functions αn1 and αn2 and

2

between βn1 and βn2 . We prove that we have

αn1(a) = αn2(a)

αn2(a) ∧ (

n1∧Rn2 (a)

n2

) for all a ∧ n1 = 1

in Theorem 2.5 of Section 2 and that we have

βn1(a) = βn2(a)

βn2(a) ∧ (

n1∧Rn2 (a)

n2

) for all a ∧ n1 = 1

in Theorem 3.3 of Section 3. Thus, for every integer a coprime to n, the determination of αn(a) is reduced to the computation of αrad(n)(a) andRrad(n)(a) if v2(n) 6 1 and of α2 rad(n)(a) and R2 rad(n)(a) if v2(n) > 2. These theorems on the functions αn and βn are derived from Theorem 2.6 of Section 2, which states that

On1(a) = On2(a) n1

n1 ∧ Rn2(a) ,

for all integers a coprime to n1 and n2. This result generalizes the following theorem of Nathanson which, in the above notation, states that for every odd prime number p and for every positive integer k, we have the equality

Opk(a) = Op(a) pk

pk ∧Rp(a)

for all integers a not divisible by p.

Theorem 3.6 of [3]. Let p be an odd prime, and let a 6= ±1 be an integer not divisible by p. Let d be the order of a modulo p. Let k0 be the largest integer such that a

d ≡ 1 (mod pk0). Then the order of a modulo pk is d for k = 1, . . . , k0 and dp

k−k0 for k > k0.

For every finite sequence S = (a1, . . . , am) of length m > 1 in Z/nZ, we denote by ∆S the Steinhaus triangle of S, that is the finite multiset of cardinal

(

m+1 2

)

in Z/nZ defined by

∆S =

{

i ∑

k=0

(

i

k

)

aj+k

∣

∣

∣

∣

∣

0 6 i 6 m− 1 , 1 6 j 6 m− i

}

.

A finite sequence S in Z/nZ is said to be balanced if each element of Z/nZ occurs in its Steinhaus triangle ∆S with the same multiplicity. For instance, the sequence (2, 2, 3, 3) of length 4 is balanced in Z/5Z. Indeed, as depicted in Figure 1, its Steinhaus triangle is composed by each element of Z/5Z occuring twice.

Note that, for a sequence S of length m > 1 in Z/nZ, a necessary condition on m for S to be balanced is that the integer n divides the binomial coefficient

(

m+1 2

)

. In 1976, John C. Molluzzo [2] posed the problem to determine whether this necessary condition on m is also sufficient to guarantee the existence of a balanced sequence. In [1], it was proved that, for each odd number n, there exists a balanced sequence of length m for every m ≡ 0 or −1 (mod αn(2)n) and for every m ≡ 0 or −1 (mod βn(2)n). This was achieved by analyzing the Steinhaus triangles generated by arithmetic progressions. In particular, since β3k(2) = 1 for all k > 1, the above result implies a complete and positive solution of Molluzzo’s Problem in Z/nZ for all n = 3k.

3

0 4 1

4 0 1 2 2 3 3

Figure 1: The Steinhaus triangle of a balanced sequence in Z/5Z

2 The arithmetic function αn

The table and the graphic depicted in Figure 2 give us the first values of αn(a) for every positive integer n ∈ J1, 20K and every integer a ∈ J−20, 20K.

The positive integer αn(a) seems to be difficult to determine. Indeed, there is no general formula known to compute the multiplicative order of an integer modulo n but, however, we get the following helpful propositions.

Lemma 2.1. Let n1 and n2 be two positive integers such that rad(n1) = rad(n2). Then, an integer a is coprime to n1 if, and only if, it is also coprime to n2.

Proof. This follows from the definition of the greatest common divisor of two integers and from the definition of the radical of an integer.

Proposition 2.2. Let n1 and n2 be two positive integers such that rad(n1) = rad(n2). Then, for every integer a, the integer αn1(a) divides αn2(a).

Proof. If a is not coprime to n1 and n2, then, by definition of the functions αn1 and αn2 and by Lemma 2.1, we have

αn1(a) = αn2(a) = 0.

Suppose now that a is coprime to n1 and n2. Let p be a prime factor of n1 such that vp(n1) > 2. We shall show that αn1(a) divides αn1/p(a). By definition of αn1/p(a), there exists an integer u such that

aαn1/p(a) n1 p = 1 + u

n1 p .

Therefore, by the binomial theorem, we have

aαn1/p(a)n1 = (

aαn1/p(a) n1 p

)p

=

(

1 + u n1 p

)p

= 1 + un1 +

p ∑

k=2

(

p

k

)

uk (

n1 p

)k

.

Since vp(n1) > 2, it follows that (n1/p) k is divisible by n1 for every integer k > 2 and so

aαn1/p(a)n1 ≡ 1 (mod n1).

Hence αn1(a) divides αn1/p(a). This completes the proof.

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