V. Guess the meaning of the following words:

collection (n) [ kq'lekSqn] algebra (n) [ 'xlGIbrq]

object (n) [ 'ObGIkt] phrase (n) [ 'freIz]

element (n) [ elImqnt] symbol (n) [ 'sImbql]

theory (n) [ 'TiqrI] standard (a) [ 'stxndqd]

concrete (a) [ 'kOnkrJt] season (n) [ 'sJzqn]

real (a) [ 'rIql] variation (n) [ vFrI'eISqn]

interest (n) [ 'IntrIst] idea (n) [ aI'dIq]

abstract (a) [ 'xbstrqkt] vertical (a) [ 'vWtIkql]

sphere (n) [ 'sfIq] fact (n) [ 'fxkt]

special (a) [ 'speSql] mathematics (n) [ mxTI'mxtIks]

VI. Read and learn these words:

set (n) [set] множество

belong (v) [bI'LoN] принадлежать

introduce (v) [Intrq'GHs] вводить

capital (a) ['kxpItql] заглавный

convention (n) [kqn'venSqn] договоренность, условие

keep (to) (v) [kJp] придерживаться, держаться (ч-л)

rigidly (adv) ['rJGIlI] строго

occur (v) [q'kW] встречаться

currently (adv) ['kArqntlI] в настоящее время

find out (v) ['faInd'aut] узнать

specify (v) ['spesISaI] точно определять

list (v) ['lIst] перечислять

notation (n) [nou'teISqn] обозначение

enclose (v) [IN'klouz] заключать

curly (a) ['kWlI] фигурный

mention (v) ['menSqn] упоминать, ссылаться

denote (v) [dI'nout] обозначать

allow (v) [q'lau] позволять

property (n) ['prOpqtI] свойство

precisely (adv) ['prIsaIslI] точно, определенно

exact (a) [Ig'zxkt] точный

infinite (a) ['InfInIt] бесконечный

infinitely (adv) ['InfInItlI] бесконечно

finite (a) ['faInaIt] конечный

notion (n) ['nouSqn] понятие

turn out (v) [tWn'Qut] оказываться

confuse (v) ['kqnfjHz] путать, смешивать

empty (a) ['emptI] пустой

distinguish (v) [dIs'tNgwIS] различать

establish (v) [Is'txblIS] установить

concisely (adv) [kqn'saIslI] кратко, сжато

S E T S

A set is a collection of objects. The objects belonging to the set are the elements or members of the set. Although in introducing set theory it is helpful to work with concrete sets, whose members are real objects, the sets of interest in mathematics always have members which are abstract mathematical objects: the set of all circles in the plane, the set of points on a sphere, the set of all numbers.

As in ordinary algebra we shall use letters to represent sets and elements, small letters being used for elements and capital letters for sets. But it is impossible to keep rigidly to this convention because sets can themselves be elements of other sets. The phrase “is a member of” occurs so often that it is convenient to have a symbol, the one currently in use is Є. So x Є S means “x is a member of S”.

A set is considered to be known if we know what its elements are – or at any rate if in theory we can find out. There are many ways of specifying a set, of which the simplest is to list all the members. The standard notation for this is to enclose the list in curly brackets. So {1, 2, 3, 4} is the set whose members are 1, 2, 3, 4 and only these, while {spring, summer, autumn, winter} is the set of seasons.

Two sets are equal if they have the same elements. We can easily write things like {1, 2, 3, 4, 4, 4}. Despite being mentioned several times, there is only one 4 in the set, which being thus equal to {1, 2, 3, 4}. When using the curly bracket notation, elements listed more than once are though as occurring once in the set. The order inside the brackets makes no difference. The set {1, 2, 3, 4} has the same elements as {1, 2, 3, 4}, so is the same set.

More generally, a symbol such as {all epic poems} denotes the set of all epic poems. A variation of this idea allows us to write {x/x is an epic poem} for the same set. The vertical bar may be read as “such that”, and the set of all x such that x is n epic poem is the same as the set of all epic poems. The set {n/n is an integer and 1 ≤ n ≤ 4} is the same as the set {1, 2, 3, 4}.

Instead of a list, we give a property which specified precisely the elements we wish to be included in the set. If we are careful with our definitions, making sure that we specify the exact property we want, this is as good as a list, and is usually more convenient. For sets with infinitely many members, such as {all whole numbers}, it is in any case impossible to give a complete list. The same is true for sets with a sufficiently large finite set of elements.

The mathematical notion of a set allows sets with only one member – or even no members at all. If you specify a set by some property it may turn out later that there is only one object with that property, or none at all. Sets with one element must not be confused with the element itself. It is not true that x and {x} are equal; {x} has just one member, namely x, while x may have any number of members depending on whether or not it is a set, and if it is, which set.

For exactly the same reasons that we allow sets with just one element, we have to allow sets with no elements at all. A set with no elements is called an empty set. A fact now emerges which many people find surprising: there is only one empty set. All empty sets are equal. Any two empty sets are equal because, in the absence of any members to distinguish them by, there is no way to tell them apart. Having established that there is just one empty set we can give it a symbol, the current one being Ø (which is a special symbol). The empty set is not “nothing” …, nor does it fail to exist. It is just as much in existence as any other set. It is its members that do not exist. It must not be confused with the number 0: for 0 is a number, whereas Ø is a set. Ø is one of the most useful sets in mathematics. One of its uses is to express concisely that something does not happen.