Nilsequence Explained

In mathematics, a nilsequence is a type of numerical sequence playing a role in ergodic theory and additive combinatorics.^[1] The concept is related to nilpotent Lie groups and almost periodicity. The name arises from the part played in the theory by compact nilmanifolds of the type

G/ \Gamma where G is a nilpotent Lie group and \Gamma a lattice in it.

The idea of a basic nilsequence defined by an element

g of G and continuous function f on G/ \Gamma is to take b(n) , for n an integer, as f(g ^{n} \Gamma) . General nilsequences are then uniform limits of basic nilsequences.^[2] For the statement of conjectures and theorems, technical side conditions and quantifications of complexity are introduced. Much of the combinatorial importance of nilsequences reflects their close connection with the Gowers norm.^[3] As explained by Host and Kra, nilsequences originate in evaluating functions on orbits in a "nilsystem"; and nilsystems are "characteristic for multiple correlations".^[4]

Case of the circle group

The circle group arises as the special case of the real line and its subgroup of the integers. It has nilpotency class equal to 1, being abelian, and the requirements of the general theory are to generalise to nilpotency class

s > 1 . The semi-open unit interval is a fundamental domain, and for that reason the fractional part function is involved in the theory. Functions involving the fractional part \{\{x\}\} of the variable in the circle group occur, under the name "bracket polynomials". Since the theory is in the setting of Lipschitz functions, which are a fortiori continuous, the discontinuity of the fractional part at 0 has to be managed.

That said, the sequences

\{\{\alpha n\}\} , where \alpha is a given irrational real number, and n an integer, and studied in diophantine approximation, are simple examples for the theory. Their construction can be thought of in terms of the skew product construction in ergodic theory, adding one dimension.^{[5] [6]}

Polynomial sequences

The imaginary exponential function

e(x) maps the real numbers to the circle group (see Euler's formula#Topological interpretation). A numerical sequence e(P(n)) where P is a polynomial function with real coefficients, and n is an integer variable, is a type of trigonometric polynomial, called a "polynomial sequence" for the purposes of the nilsequence theory. The generalisation to nilpotent groups that are not abelian relies on the Hall–Petresco identity from group theory for a workable theory of polynomials.^[7] In particular the polynomial sequence comes with a definite degree.

Möbius function and nilsequences

A family of conjectures

MN(s) was made by Ben Green and Terence Tao, concerning the Möbius function of prime number theory and s -step nilsequences. Here the underlying Lie group G is assumed simply connected and nilpotent with length at most s . The nilsequences considered are of type f(g ^{n} x\Gamma) with some fixed x in G , and the function f continuous and taking values in .^[8] The form of the conjecture, which requires a stated metric on the nilmanifold and Lipschitz bound in the implied constant, is that the average of \mu (n) f(g ^{n} x\Gamma) up to N is smaller asymptotically than any fixed inverse power of log N. ^[9] As a subsequent paper published in 2012 proving the conjectures put it, The Möbius function is strongly orthogonal to nilsequences.^[10]

Subsequently Green, Tao and Tamar Ziegler also proved a family

IG(s) of inverse theorems for the Gowers norm, stated in terms of nilsequences. This completed a program of proving asymptotics for simultaneous prime values of linear forms.^[11]

Tao has commented in his book Higher Order Fourier Analysis on the role of nilsequences in the inverse theorem proof. The issue being to extend IG results from the finite field case to general finite cyclic groups, the "classical phases"—essentially the exponentials of polynomials natural for the circle group—had proved inadequate. There were options other than nilsequences, in particular direct use of bracket polynomials. But Tao writes that he prefers nilsequences for the underlying Lie theory structure.^[12]

Equivalent form for averaged Chowla and Sarnak conjectures

Tao has proved that a conjecture on nilsequences is an equivalent of an averaged form of a noted conjecture of Sarvadaman Chowla involving only the Möbius function, and the way it self-correlates. Peter Sarnak made a conjecture on the non-correlation of the Möbius function with more general sequences from ergodic theory, which is a consequence of Chowla's conjecture. Tao's result on averaged forms showed all three conjectures are equivalent.^[13] The 2018 paper The logarithmic Sarnak conjecture for ergodic weights by Frantzikinakis and Host used this approach to prove unconditional results on the Liouville function.^[14]

Notes and References

1. Book: Granville . Andrew . Nathanson . Melvyn Bernard . Solymosi . Jozsef . Additive Combinatorics . American Mathematical Soc. . 978-0-8218-7039-6 . 138 . 2007 . en.
2. Book: Ferenczi . Sébastien . Kułaga-Przymus . Joanna . Lemańczyk . Mariusz . Ergodic Theory and Dynamical Systems in their Interactions with Arithmetics and Combinatorics: CIRM Jean-Morlet Chair, Fall 2016 . 15 June 2018 . Springer . 978-3-319-74908-2 . 194 note 40 . en.
3. Book: Elsholtz . Christian . Grabner . Peter . Number Theory – Diophantine Problems, Uniform Distribution and Applications: Festschrift in Honour of Robert F. Tichy's 60th Birthday . 26 May 2017 . Springer . 978-3-319-55357-3 . 395 . en.
4. Book: Host . Bernard . Kra . Bryna . Nilpotent Structures in Ergodic Theory . 12 December 2018 . American Mathematical Soc. . 978-1-4704-4780-9 . 7 . en.
5. Book: Bunimovich . L. A. . Dani . S. G. . Dobrushin . R. L. . Jakobson . M. V. . Kornfeld . I. P. . Maslova . N. B. . Pesin . Ya B. . Smillie . J. . Sukhov . Yu M. . Vershik . A. M. . Dynamical Systems, Ergodic Theory and Applications . 5 April 2000 . Springer Science & Business Media . 978-3-540-66316-4 . 24 . en.
6. Book: Tao . Terence . Poincare's Legacies, Part I: Pages from Year Two of a Mathematical Blog . 2009 . American Mathematical Soc. . 978-0-8218-4883-8 . 258 . en.
7. Book: Tao . Terence . Higher Order Fourier Analysis . 30 December 2012 . American Mathematical Soc. . 978-1-4704-5998-7 . 98 . en.
8. Green . Benjamin . Tao . Terence . Linear equations in primes . Annals of Mathematics . 2010 . 171 . 3 . 1783 . 20752252 . 0003-486X.
9. Green . Benjamin . Tao . Terence . Linear equations in primes . Annals of Mathematics . 2010 . 171 . 3 . 1788 . 20752252 . 0003-486X.
10. Green . Ben . Tao . Terence . The Möbius function is strongly orthogonal to nilsequences . Annals of Mathematics . 2012 . 175 . 2 . 541–566 . 23234622 . 0003-486X.
11. Book: Grechuk . Bogdan . Landscape of 21st Century Mathematics: Selected Advances, 2001–2020 . 21 September 2021 . Springer Nature . 978-3-030-80627-9 . 10–11 . en.
12. Book: Tao . Terence . Higher Order Fourier Analysis . 4 October 2012 . American Mathematical Soc. . 978-0-8218-8986-2 . 92 . en.
13. Book: Ferenczi . Sébastien . Kułaga-Przymus . Joanna . Lemańczyk . Mariusz . Ergodic Theory and Dynamical Systems in their Interactions with Arithmetics and Combinatorics: CIRM Jean-Morlet Chair, Fall 2016 . 15 June 2018 . Springer . 978-3-319-74908-2 . 188 . en.
14. Frantzikinakis . Nikos . Host . Bernard . The logarithmic Sarnak conjecture for ergodic weights . Annals of Mathematics . 1 May 2018 . 187 . 3 . 10.4007/annals.2018.187.3.6. 1708.00677 .