# Dynamical Systems and Ergodic Theory by Mark Pollicott By Mark Pollicott

This e-book is an advent to topological dynamics and ergodic thought. it really is divided right into a variety of really brief chapters to ensure that every one can be utilized as an element of a lecture path adapted to the actual viewers. The authors offer a few functions, largely to quantity conception and mathematics progressions (through Van der Waerden's theorem and Szemerdi's theorem). this article is acceptable for complex undergraduate and starting graduate scholars.

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Additional info for Dynamical Systems and Ergodic Theory

Example text

It is perhaps worth mentioning that, as Hecke also proved, the converse of Proposition 13 holds as well: if an (n ≥ 1) are complex numbers of polynomial growth and the function L∗ (f, s) deﬁned by (45) continues analytically to the whole complex plane and satisﬁes the functional equation (46), then f (z) = ∞ 2πinz is a cusp form of weight k on Γ1 . 3 Modular Forms and Algebraic Number Theory In §3, we used the theta series θ(z)2 to determine the number of representations of any integer n as a sum of two squares.

No non-modular proof of the asymptotic formula (38) is known. Before continuing with the theory, we look at some examples, starting in rank 8. Deﬁne the lattice Λ8 ⊂ R8 to be the set of vectors belonging to either Z8 or (Z+ 12 )8 for which the sum of the coordinates is even. This is unimodular because the lattice Z8 ∪ (Z + 12 )8 contains both it and Z8 with the same index 2, and is even because x2i ≡ xi (mod 2) for xi ∈ Z and x2i ≡ 14 (mod 2) for xi ∈ Z + 12 . The lattice Λ8 is sometimes denoted E8 because, if we choose the Z-basis ui = ei − ei+1 (1 ≤ i ≤ 6), u7 = e6 + e7 , u8 = − 21 (e1 + · · · + e8 ) of Λ8 , then every ui has length 2 and (ui , uj ) for i = j equals −1 or 0 according whether the ith and jth vertices (in a standard numbering) of the “E8 ” Dynkin diagram in the theory of Lie algebras are adjacent or not.

Corollary (Theorem of Lagrange). Every positive integer is a sum of four squares. ♥ For another simple application of the q-expansion principle, we introduce two variants θM (z) and θF (z) (“M” and “F” for “male” and “female” or “minus sign” and “fermionic”) of the function θ(z) by inserting signs or by shifting the indices by 1/2 in its deﬁnition: θM (z) = 2 (−1)n q n = 1 − 2q + 2q 4 − 2q 9 + · · · , n∈Z 2 qn θF (z) = n∈Z+1/2 = 2q 1/4 + 2q 9/4 + 2q 25/4 + · · · . 28 D. Zagier These are again modular forms of weight 1/2 on Γ0 (4).