Number
From Infocentral
A number is an abstract idea used in counting and measuring. A symbol which represents a number is called a numeral, but in common usage the word number is used for both the idea and the symbol. In addition to their use in counting and measuring, numerals are often used for labels (telephone numbers), for ordering (serial numbers), and for codes (ISBNs). In mathematics, the definition of a number has been extended to include new kinds of numbers, such as rational numbers, irrational numbers, zero, negative numbers, and complex numbers.
Arithmetic is the study of operations on numbers such as addition, subtraction, multiplication, and division.
Abstract algebra is the branch of mathematics that studies abstract number systems such as groups, rings and fields.
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Types of numbers
Numbers can be classified into sets, called number systems. (For different methods of expressing numbers with symbols, such as the Roman numerals, see numeral systems.)
Natural numbers
The most familiar numbers are the natural numbers or counting numbers: one, two, three, ... . Some people also include zero in the natural numbers, others do not.
Decimal numbers
These numbers have got decimals in them.
