# User:Richard Pinch/sandbox-5

## Contents

# Möbius inversion for arithmetic functions

The original form of Möbius inversion developed by A. Möbius for arithmetic functions.

Möbius considered also inversion formulas for finite sums running over the divisors of a natural number $n$:
$$
F(n) = \sum_{d | n} f(d) \ ,\ \ \ f(n) = \sum_{d | n} \mu(d) F(n/d) \ .
$$
The correspondence $f \mapsto F$ is the *Möbius transform*, and $F \mapsto f$ the *inverse Möbius transform*.

Another inversion formula: If $P(n)$ is a totally multiplicative function for which $P(1) = 1$, and $f(x)$ is a function defined for all real $x > 0$, then $$ g(x) = \sum_{n \le x} P(n) f(x/n) $$ implies $$ f(x) = \sum_{n \le x} \mu(n) P(n) g(x/n) \ . $$

All these (and many other) inversion formulas follow from the basic property of the Möbius function that it is the inverse of the unit arithmetic function $E(n) \equiv 1$ under Dirichlet convolution, cf. (the editorial comments to) Möbius function and Multiplicative arithmetic function.

#### References

[1] | A. Möbius, "Ueber eine besondere Art der Umkehrung der Reihen" J. Reine Angew. Math. , 9 (1832) pp. 105–123 DOI 10.1515/crll.1832.9.105 Zbl 009.0333cj |

[2] | I.M. Vinogradov, "Elements of number theory" , Dover, reprint (1954) (Translated from Russian) Zbl 0057.28201 |

[3] | K. Prachar, "Primzahlverteilung" , Springer (1957) Zbl 0080.25901 |

[4] | Hua Loo Keng, "Introduction to number theory" Springer-Verlag (1982) ISBN 3-540-10818-1 Zbl 0483.10001 |

[5] | Alan Baker, "A concise introduction to the theory of numbers" Cambridge University Press (1984) ISBN 0-521-28654-9 Zbl 0554.10001 |

[6] | G.H. Hardy, E.M. Wright, "An introduction to the theory of numbers" (6th edition, edited and revised by D. R. Heath-Brown and J. H. Silverman with a foreword by Andrew Wiles) Oxford University Press (2008) ISBN 978-0-19-921986-5 Zbl 1159.11001 |

# Puiseux series

*over a field $K$*

A formal power series or Laurent series with coefficients in $K$ and formal variable $Z^{1/k}$ for some natural number $k$. The Puiseaux series form a field $K\{\{Z\}\} = \bigcup_{k\ge1} K(( Z ^{1/k} ))$.

For complex analytic functions, the functions with branch points are represented by Puiseux series.

The Newton–Puiseux theorem states that if $K$ is an algebraically closed field of characteristic zero then the algebraic closure of the field of formal power series $K((Z))$ is the field of Puiseux series $K\{\{Z\}\}$. This is not the case for positive characteristic.

#### References

[1] | Kedlaya, Kiran S. "The algebraic closure of the power series field in positive characteristic" Proc. Am. Math. Soc. 129 (2001) DOI 10.1090/S0002-9939-01-06001-4 Zbl 1012.12007 |

# Metabelian

A term with several meanings.

A **metabelian group** is a solvable group of derived length two, i.e. a group whose commutator subgroup is Abelian, see Meta-Abelian group.

In the Russian mathematical literature, by a metabelian group one sometimes means a nilpotent group of nilpotency class 2.

A **metabelian algebra** satisfies the identity $[[x,y],z] = 0$ where $[x,y] = x \cdot y - y \cdot x$ is the commutator.

A **metabelian Lie algebra** is an extension of an abelian algebra by an abelian algebra.

#### References

[1] |

# Knuth equivalence

*Plactic equivalence*

An equivalence relation on words over a totally ordered alphabet $(A,{<})$ generated by the equivalences $$ a c b \leftrightarrow c a b $$ when $a \le b < c$ and $$ b a c \leftrightarrow b c a $$ when $a < b \le c$.

The set of equivalence classes under concatenation forms the *plactic monoid* over $A$.

#### References

[1] | D.E. Knuth, "Permutations, matrices and generalized Young tableaux" Pacific J. Math. , 34 (1970) pp. 709–727 |

# Mould calculus

The mould algebra over a set $X$ is the total monoid algebra $M^*(X,A)$ of the free monoid on $X$ over a complex unital commutative algebra $A$. A mould is a function $w \mapsto v^w$ from words $w$ over the alphabet $X$ to $A$, formally written as $\sum_{w \in X^*} v^w$. Addition is defined pointwise, $(u+v)^w = u^w + v^w$ and multiplication by convolution $$ u*v : w \mapsto \sum_{w = xy} u^x v^y $$ where the sum is over all finitely many factorisations of the word $w$ into non-trivial words $x,y$.

#### References

[1] | Ebrahimi-Fard, Kurusch, Fauvet, Frédéric (edd.) Faà di Bruno Hopf algebras, Dyson-Schwinger equations, and Lie-Butcher series. IRMA Lectures in Mathematics and Theoretical Physics 21 European Mathematical Society (2015) ISBN 978-3-03719-143-9 Zbl 1318.16001 | |

[2] | Sauzin, David; "Mould expansions for the saddle-node and resurgence monomials"; Renormalization and Galois theories Connes, Alain et al (edd.) IRMA Lectures in Mathematics and Theoretical Physics 15 European Mathematical Society (2009) ISBN 978-3-03719-073-9 pp.83-163 Zbl 1191.34104 |

**How to Cite This Entry:**

Richard Pinch/sandbox-5.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Richard_Pinch/sandbox-5&oldid=51489