Carmichael function

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In number theory, the Carmichael function associates to every positive integer n a positive integer , defined as the smallest positive integer m such that

for every integer a between 1 and n that is coprime to n.

(Dropping the phrase "between 1 and n" leads to an equivalent definition.) In algebraic terms, equals the exponent of the multiplicative group of integers modulo n.

The Carmichael function is named after the American mathematician Robert Carmichael and is also known as the reduced totient function or the least universal exponent function.

The first 36 values of (sequence A002322 in the OEIS) compared to Euler's totient function . (in bold if they are different, the ns such that they are different are listed in OEISA033949)

n 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
1 1 2 2 4 2 6 2 6 4 10 2 12 6 4 4 16 6 18 4 6 10 22 2 20 12 18 6 28 4 30 8 10 16 12 6
1 1 2 2 4 2 6 4 6 4 10 4 12 6 8 8 16 6 18 8 12 10 22 8 20 12 18 12 28 8 30 16 20 16 24 12

Numerical example[edit]

Carmichael's function at 8 is 2, i.e. , because for any number a co-prime to 8 it holds that a2 ≡ 1 (mod 8). Namely, 12 = 1 (mod 8), 32 = 9 ≡ 1 (mod 8), 52 = 25 ≡ 1 (mod 8) and 72 = 49 ≡ 1 (mod 8). Euler's totient function at 8 is 4, i.e. , because there are 4 numbers lesser than and coprime to 8 (1, 3, 5, and 7). Euler's theorem assures that a4 ≡ 1 (mod 8) for all a coprime to 8, but 4 is not the smallest such exponent.

Computing with Carmichael's theorem[edit]

By the fundamental theorem of arithmetic any n > 1 can be written in a unique way as

where p1 < p2 < ... < pk are primes and r1 , ..., rk are positive integers. It is then the case that λ(n) is the least common multiple of the λ of each of its prime power factors:

This can be proved using the Chinese Remainder Theorem.

Carmichael's theorem explains how to compute λ of a prime power: for a power of an odd prime and for 2 and 4, λ(n) is equal to the Euler totient φ(n); for powers of 2 greater than 4 it is equal to half of the Euler totient.

Euler's function for prime powers is given by

Properties of the Carmichael function[edit]

λ(n) divides φ(n)[edit]

This follows from elementary group theory, because the exponent of any finite abelian group must divide the order of the group. λ(n) is the exponent of the multiplicative group of integers modulo n while φ(n) is the order of that group.

We can thus view Carmichael's theorem as a sharpening of Euler's theorem.

Minimality[edit]

Suppose for all numbers a coprime with n. Then .

Proof. If with , then for all numbers a coprime with n. It follows , since and the minimal positive such number.

Divisibility[edit]

Proof. The result follows from the formula

mentioned above.

Composition[edit]

For all positive integers and it holds

.

This is an immediate consequence of the recursive definition of the Carmichael function.

Primitive m-th roots of unity[edit]

Let and be coprime and let be the smallest exponent with , then it holds

.

That is, the orders of primitive roots of unity in the ring of integers modulo are divisors of .

Exponential cycle length[edit]

If has maximum prime exponent under prime factorization, then for all (including those not co-prime to ) and all ,

In particular, for squarefree (), for all

Average and typical value[edit]

For any x > 16:

.[1][2]

Where B is a constant,

For all numbers N and all except o(N) positive integers n ≤ N:

where A is a constant,[2][3]

Lower bounds[edit]

For any sufficiently large number N and for any , there are at most

positive integers such that .[4]

For any sequence of positive integers, any constant , and any sufficiently large i:

.[5][6]

Small values[edit]

For a constant c and any sufficiently large positive A, there exists an integer such that .[6] Moreover, n is of the form

for some square-free integer .[5]

Image of the function[edit]

The set of values of the Carmichael function has counting function

where ….[7]

See also[edit]

Notes[edit]

  1. ^ Theorem 3 in Erdős (1991)
  2. ^ a b Sándor & Crstici (2004) p.194
  3. ^ Theorem 2 in Erdős (1991)
  4. ^ Theorem 5 in Friedlander (2001)
  5. ^ a b Theorem 1 in Erdős 1991
  6. ^ a b Sándor & Crstici (2004) p.193
  7. ^ Ford, Kevin; Luca, Florian; Pomerance, Carl (27 August 2014). "The image of Carmichael's λ-function". Algebra & Number Theory. 8: 2009–2026. arXiv:1408.6506Freely accessible [math.NT]. doi:10.2140/ant.2014.8.2009. 

References[edit]