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In number theory, Jordan's totient function, denoted as $J_{k}(n)$ , where $k$ is a positive integer, is a function of a positive integer, $n$ , that equals the number of $k$ -tuples of positive integers that are less than or equal to $n$ and that together with $n$ form a coprime set of $k+1$ integers

Jordan's totient function is a generalization of Euler's totient function, which is the same as $J_{1}(n)$ . The function is named after Camille Jordan.

Definition

For each positive integer $k$ , Jordan's totient function $J_{k}$ is multiplicative and may be evaluated as

J_{k}(n)=n^{k}\prod _{p|n}\left(1-{\frac {1}{p^{k}}}\right)\,

, where

p

ranges through the prime divisors of

n

.

Properties

$\sum _{d|n}J_{k}(d)=n^{k}.\,$

which may be written in the language of Dirichlet convolutions as^[1]

J_{k}(n)\star 1=n^{k}\,

and via Möbius inversion as

J_{k}(n)=\mu (n)\star n^{k}

.

Since the Dirichlet generating function of

\mu

is

1/\zeta (s)

and the Dirichlet generating function of

n^{k}

is

\zeta (s-k)

, the series for

J_{k}

becomes

\sum _{n\geq 1}{\frac {J_{k}(n)}{n^{s}}}={\frac {\zeta (s-k)}{\zeta (s)}}

.

An average order of $J_{k}(n)$ is

J_{k}(n)\sim {\frac {n^{k}}{\zeta (k+1)}}

.

The Dedekind psi function is

\psi (n)={\frac {J_{2}(n)}{J_{1}(n)}}

,

and by inspection of the definition (recognizing that each factor in the product over the primes is a cyclotomic polynomial of

p^{-k}

), the arithmetic functions defined by

{\frac {J_{k}(n)}{J_{1}(n)}}

or

{\frac {J_{2k}(n)}{J_{k}(n)}}

can also be shown to be integer-valued multiplicative functions.

$\sum _{\delta \mid n}\delta ^{s}J_{r}(\delta )J_{s}\left({\frac {n}{\delta }}\right)=J_{r+s}(n)$ .^[2]

Order of matrix groups

The general linear group of matrices of order $m$ over $\mathbf {Z} /n$ has order^[3]

|\operatorname {GL} (m,\mathbf {Z} /n)|=n^{\frac {m(m-1)}{2}}\prod _{k=1}^{m}J_{k}(n).

The special linear group of matrices of order $m$ over $\mathbf {Z} /n$ has order

|\operatorname {SL} (m,\mathbf {Z} /n)|=n^{\frac {m(m-1)}{2}}\prod _{k=2}^{m}J_{k}(n).

The symplectic group of matrices of order $m$ over $\mathbf {Z} /n$ has order

|\operatorname {Sp} (2m,\mathbf {Z} /n)|=n^{m^{2}}\prod _{k=1}^{m}J_{2k}(n).

The first two formulas were discovered by Jordan.

Examples

Explicit lists in the OEIS are J₂ in OEIS: A007434, J₃ in OEIS: A059376, J₄ in OEIS: A059377, J₅ in OEIS: A059378, J₆ up to J₁₀ in OEIS: A069091 up to OEIS: A069095.
Multiplicative functions defined by ratios are J₂(n)/J₁(n) in OEIS: A001615, J₃(n)/J₁(n) in OEIS: A160889, J₄(n)/J₁(n) in OEIS: A160891, J₅(n)/J₁(n) in OEIS: A160893, J₆(n)/J₁(n) in OEIS: A160895, J₇(n)/J₁(n) in OEIS: A160897, J₈(n)/J₁(n) in OEIS: A160908, J₉(n)/J₁(n) in OEIS: A160953, J₁₀(n)/J₁(n) in OEIS: A160957, J₁₁(n)/J₁(n) in OEIS: A160960.
Examples of the ratios J_2k(n)/J_k(n) are J₄(n)/J₂(n) in OEIS: A065958, J₆(n)/J₃(n) in OEIS: A065959, and J₈(n)/J₄(n) in OEIS: A065960.

Notes

^ Sándor & Crstici (2004) p.106
^ Holden et al in external links. The formula is Gegenbauer's.
^ All of these formulas are from Andrica and Piticari in #External links.

References

L. E. Dickson (1971) [1919]. History of the Theory of Numbers, Vol. I. Chelsea Publishing. p. 147. ISBN 0-8284-0086-5. JFM 47.0100.04.
M. Ram Murty (2001). Problems in Analytic Number Theory. Graduate Texts in Mathematics. Vol. 206. Springer-Verlag. p. 11. ISBN 0-387-95143-1. Zbl 0971.11001.
Sándor, Jozsef; Crstici, Borislav (2004). Handbook of number theory II. Dordrecht: Kluwer Academic. pp. 32–36. ISBN 1-4020-2546-7. Zbl 1079.11001.

External links

Andrica, Dorin; Piticari, Mihai (2004). "On some extensions of Jordan's arithmetic functions". Acta Universitatis Apulensis. 7: 13–22. MR 2157944.
Holden, Matthew; Orrison, Michael; Vrable, Michael. "Yet Another Generalization of Euler's Totient Function" (PDF). Archived from the original (PDF) on 2016-03-05. Retrieved 2011-12-21.

[1] Sándor & Crstici (2004) p.106

[2] Holden et al in external links. The formula is Gegenbauer's.

[3] All of these formulas are from Andrica and Piticari in #External links.

[1]

[2]

[3]

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