…пусть каждый наш “персонаж” хочет выбрать себе не одну шляпу, а ДВЕ (на всякий случай оговорим, что количество экземпляров “шляп” у нас очень велико, и их всем хватает). Далее, будем считать, что каждый надевает на себя сначала одну “шляпу”, а потом другую, и в каждом случае делает “снимок” себя с надетой “шляпой”, который выглядит как некоторое дерево. Причём на снимке — это здесь самое главное — уже не видно, где кончается “шляпа”, и где начинается надевший её “персонаж”: всё сливается в единое целое…


Falcao on the amenability of Thompson group.

Por la liberacion de Alf Onshuus, Ana Aldana y secuestrados en Colombia

Thorny Forkings – Thorny Days (Alf’s blog)

Una oración y muchas velas por Alf,Ana y todos los secuestrasdos

Blogging and Berlin

September 9, 2007

While I was sort of stuck looking for a stable accommodation here in Berlin, Dima Sustretov had written two nice posts for our joint newly-started blog Forking, forcing and back-and-forcing. They both deal with, I would say, exotic areas of logic. The first one (also probably first in a series) is a brief overview of basic definitions and ideas of modal logic (I constantly keep trying to force Dima go into general-style philosophical handwaving providing, at least illusional, grasp of the field. Or to exhibit particular mathematically interesting examples. So far it lacks both to my mind, and the latter seems to have some objective reasons). The second post is about using existence of winning strategies in certain games as a way to evaluate truth of formulas. Many things can be represented in this fashion. Among others playing games over finite structures is a widely used method to prove inexpressibility in different fragments of second-order logic (actually, it seems to be the only method available for finite context in combinatorially-nontrivial situations. Yet, our ability to play games leaves much more to be a desire). Hopefully I will contribute to the blog soon concerning this matter and its tight connection with questions in computational complexity theory.