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BGU Probability and Ergodic Theory (PET) seminar

On pointwise periodicity and expansiveness

מרץ 20, 11:00—12:00, 2018, 201

מרצה

Tom Meyerovitch (BGU)

תקציר

Following Kaul, a discrete (topological) group G of transformations of set X is pointwise periodic if the stabilizer of every point is of finite index (co-compact) in G. Equivalently, all G-orbits are finite (compact). Generalizing a result of Montgomery, Kaul showed in the early 70‘s that a pointwise periodic transformation group is always compact when the group acts (faithfully) on a connected manifold without boundary. I will discuss implications of expansiveness and pointwise periodicity of certain groups and semigroups of transformations. In particular I‘ll state implications for cellular automata and for planner tilings. Based on joint work with Ville Salo.

אשנב למתמטיקה

מזעריות-סדר: גן העדן בו אין אינטגרלים, כל הפונקציות גזירות, וגבולות תמיד קיימים

מרץ 20, 18:15—19:45, 2018, אולם 101-

מרצה

אסף חסון

תקציר

The Big Rock Candy Mountains

One evening as the sun went down And the jungle fires were burning, Down the track came a hobo hiking, And he said, ”Boys, I‘m not turning I‘m headed for a land that‘s far away Besides the crystal fountains So come with me, we‘ll go and see The Big Rock Candy Mountains

In the Big Rock Candy Mountains, There‘s a land that‘s fair and bright, Where the handouts grow on bushes And you sleep out every night. Where the boxcars all are empty And the sun shines every day And the birds and the bees And the cigarette trees The lemonade springs Where the bluebird sings In the Big Rock Candy Mountains.

In the Big Rock Candy Mountains All the cops have wooden legs And the bulldogs all have rubber teeth And the hens lay soft-boiled eggs The farmers‘ trees are full of fruit And the barns are full of hay Oh I‘m bound to go Where there ain‘t no snow Where the rain don‘t fall The winds don‘t blow In the Big Rock Candy Mountains.

In the Big Rock Candy Mountains You never change your socks And the little streams of alcohol Come trickling down the rocks The brakemen have to tip their hats And the railway bulls are blind There‘s a lake of stew And of whiskey too You can paddle all around it In a big canoe In the Big Rock Candy Mountains

In the Big Rock Candy Mountains, The jails are made of tin. And you can walk right out again, As soon as you are in. There ain‘t no short-handled shovels, No axes, saws nor picks, I‘m bound to stay Where you sleep all day, Where they hung the jerk That invented work In the Big Rock Candy Mountains.

Combinatorics Seminar

How to guess an n-digit number

מרץ 26, 14:10—15:10, 2018, -101

מרצה

Zilin Jiang (Technion)

תקציר

In a deductive game for two players, SF and PGOM, SF conceals an n-digit number $x = x_1, ..., x_n,$ and PGOM, who knows n, tries to identify x by asking a number of questions, which are answered by SF. Each question is an n-digit number $y = y_1, ..., y_n$; each answer is a number a(x, y), the number of subscripts i such that $x_i = y_i$. Moreover we require the questions from PGOM are predetermined.

In this talk, I will show that the minimum number of questions required to determine x is (2+o(1))n / log n. A more general problem is to determine the asymptotic formula of the metric dimension of Cartesian powers of a graph.

I will state the class of graphs for which the formula can be determined, and the smallest graphs for which we did not manage to settle.

Joint work with Nikita Polyanskii.


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