# American Institute of Mathematical Sciences

July  2013, 7(3): 461-488. doi: 10.3934/jmd.2013.7.461

## Ergodic properties of $k$-free integers in number fields

 1 Department of Mathematics, Altgeld Hall, 1409 W Green Street, Urbana, IL 61801, United States 2 School of Mathematics, University Walk, Bristol, BS8 1TW, United Kingdom

Received  March 2013 Revised  September 2013 Published  December 2013

Let $K/\mathbf{Q}$ be a degree-$d$ extension. Inside the ring of integers $\mathscr O_K$ we define the set of $k$-free integers $\mathscr F_k$ and a natural $\mathscr O_K$-action on the space of binary $\mathscr O_K$-indexed sequences, equipped with an $\mathscr O_K$-invariant probability measure associated to $\mathscr F_k$. We prove that this action is ergodic, has pure point spectrum, and is isomorphic to a $\mathbf Z^d$-action on a compact abelian group. In particular, it is not weakly mixing and has zero measure-theoretical entropy. This work generalizes the work of Cellarosi and Sinai [J. Eur. Math. Soc. (JEMS) 15 (2013), no. 4, 1343--1374] that considered the case $K=\mathbf{Q}$ and $k=2$.
Citation: Francesco Cellarosi, Ilya Vinogradov. Ergodic properties of $k$-free integers in number fields. Journal of Modern Dynamics, 2013, 7 (3) : 461-488. doi: 10.3934/jmd.2013.7.461
##### References:
 [1] M. Baake, R. V. Moody and P. A. B. Pleasants, Diffraction from visible lattice points and $k$th power free integers,, Discrete Math., 221 (2000), 3. doi: 10.1016/S0012-365X(99)00384-2. Google Scholar [2] V. Bergelson and A. Gorodnik, Weakly mixing group actions: A brief survey and an example,, in Modern Dynamical Systems and Applications, (2004), 3. Google Scholar [3] F. Cellarosi and Ya. G. Sinai, Ergodic properties of square-free numbers,, J. Eur. Math. Soc. (JEMS), 15 (2013), 1343. doi: 10.4171/JEMS/394. Google Scholar [4] R. R. Hall, The distribution of squarefree numbers,, J. Reine Angew. Math., 394 (1989), 107. doi: 10.1515/crll.1989.394.107. Google Scholar [5] P. R. Halmos and J. von Neumann, Operator methods in classical mechanics. II,, Ann. of Math. (2), 43 (1942), 332. doi: 10.2307/1968872. Google Scholar [6] D. R. Heath-Brown, The square sieve and consecutive square-free numbers,, Math. Ann., 266 (1984), 251. doi: 10.1007/BF01475576. Google Scholar [7] E. Hewitt and K. A. Ross, Abstract Harmonic Analysis. Vol. I. Structure of Topological Groups, Integration Theory, Group Representations,, Second edition, (1979). Google Scholar [8] I. Karatzas and S. E. Shreve, Brownian Motion and Stochastic Calculus,, Second edition, (1991). Google Scholar [9] L. B. Koralov and Y. G. Sinai, Theory of probability and random processes,, Second edition, (2007). Google Scholar [10] J. Liu and P. Sarnak, The Möbius function and distal flows,, preprint., (). Google Scholar [11] G. W. Mackey, Ergodic transformation groups with a pure point spectrum,, Illinois J. Math., 8 (1964), 593. Google Scholar [12] L. Mirsky, Arithmetical pattern problems relating to divisibility by $r$th powers,, Proc. London Math. Soc. (2), 50 (1949), 497. Google Scholar [13] W. Narkiewicz, Elementary and Analytic Theory of Algebraic Numbers,, Third edition, (2004). Google Scholar [14] J. Neukirch, Algebraic Number Theory,, Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences], (1999). Google Scholar [15] R. Peckner, Uniqueness of the measure of maximal entropy for the squarefree flow,, preprint, (2012). Google Scholar [16] P. A. B. Pleasants and C. Huck, Entropy and diffraction of the $k$-free points in $n$-dimensional lattices,, Discrete Comput. Geom., 50 (2013), 39. doi: 10.1007/s00454-013-9516-y. Google Scholar [17] V. A. Rokhlin, On the problem of the classification of automorphisms of Lebesgue spaces,, Doklady Akad. Nauk SSSR (N. S.), 58 (1947), 189. Google Scholar [18] V. A. Rokhlin, Unitary rings,, Doklady Akad. Nauk SSSR (N. S.), 59 (1948), 643. Google Scholar [19] P. Sarnak, Three lectures on the Möbius function randomness and dynamics (Lecture 1)., Available at: , (). Google Scholar [20] K. Schmidt, Dynamical Systems of Algebraic Origin,, [2011 reprint of the 1995 original] [MR1345152], (1995). Google Scholar [21] K. M. Tsang, The distribution of $r$-tuples of squarefree numbers,, Mathematika, 32 (1985), 265. doi: 10.1112/S0025579300011049. Google Scholar [22] J. von Neumann, Zur Operatorenmethode in der klassischen Mechanik,, Ann. of Math. (2), 33 (1932), 587. doi: 10.2307/1968537. Google Scholar [23] R. J. Zimmer, Ergodic actions with generalized discrete spectrum,, Illinois J. Math., 20 (1976), 555. Google Scholar

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##### References:
 [1] M. Baake, R. V. Moody and P. A. B. Pleasants, Diffraction from visible lattice points and $k$th power free integers,, Discrete Math., 221 (2000), 3. doi: 10.1016/S0012-365X(99)00384-2. Google Scholar [2] V. Bergelson and A. Gorodnik, Weakly mixing group actions: A brief survey and an example,, in Modern Dynamical Systems and Applications, (2004), 3. Google Scholar [3] F. Cellarosi and Ya. G. Sinai, Ergodic properties of square-free numbers,, J. Eur. Math. Soc. (JEMS), 15 (2013), 1343. doi: 10.4171/JEMS/394. Google Scholar [4] R. R. Hall, The distribution of squarefree numbers,, J. Reine Angew. Math., 394 (1989), 107. doi: 10.1515/crll.1989.394.107. Google Scholar [5] P. R. Halmos and J. von Neumann, Operator methods in classical mechanics. II,, Ann. of Math. (2), 43 (1942), 332. doi: 10.2307/1968872. Google Scholar [6] D. R. Heath-Brown, The square sieve and consecutive square-free numbers,, Math. Ann., 266 (1984), 251. doi: 10.1007/BF01475576. Google Scholar [7] E. Hewitt and K. A. Ross, Abstract Harmonic Analysis. Vol. I. Structure of Topological Groups, Integration Theory, Group Representations,, Second edition, (1979). Google Scholar [8] I. Karatzas and S. E. Shreve, Brownian Motion and Stochastic Calculus,, Second edition, (1991). Google Scholar [9] L. B. Koralov and Y. G. Sinai, Theory of probability and random processes,, Second edition, (2007). Google Scholar [10] J. Liu and P. Sarnak, The Möbius function and distal flows,, preprint., (). Google Scholar [11] G. W. Mackey, Ergodic transformation groups with a pure point spectrum,, Illinois J. Math., 8 (1964), 593. Google Scholar [12] L. Mirsky, Arithmetical pattern problems relating to divisibility by $r$th powers,, Proc. London Math. Soc. (2), 50 (1949), 497. Google Scholar [13] W. Narkiewicz, Elementary and Analytic Theory of Algebraic Numbers,, Third edition, (2004). Google Scholar [14] J. Neukirch, Algebraic Number Theory,, Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences], (1999). Google Scholar [15] R. Peckner, Uniqueness of the measure of maximal entropy for the squarefree flow,, preprint, (2012). Google Scholar [16] P. A. B. Pleasants and C. Huck, Entropy and diffraction of the $k$-free points in $n$-dimensional lattices,, Discrete Comput. Geom., 50 (2013), 39. doi: 10.1007/s00454-013-9516-y. Google Scholar [17] V. A. Rokhlin, On the problem of the classification of automorphisms of Lebesgue spaces,, Doklady Akad. Nauk SSSR (N. S.), 58 (1947), 189. Google Scholar [18] V. A. Rokhlin, Unitary rings,, Doklady Akad. Nauk SSSR (N. S.), 59 (1948), 643. Google Scholar [19] P. Sarnak, Three lectures on the Möbius function randomness and dynamics (Lecture 1)., Available at: , (). Google Scholar [20] K. Schmidt, Dynamical Systems of Algebraic Origin,, [2011 reprint of the 1995 original] [MR1345152], (1995). Google Scholar [21] K. M. Tsang, The distribution of $r$-tuples of squarefree numbers,, Mathematika, 32 (1985), 265. doi: 10.1112/S0025579300011049. Google Scholar [22] J. von Neumann, Zur Operatorenmethode in der klassischen Mechanik,, Ann. of Math. (2), 33 (1932), 587. doi: 10.2307/1968537. Google Scholar [23] R. J. Zimmer, Ergodic actions with generalized discrete spectrum,, Illinois J. Math., 20 (1976), 555. Google Scholar
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