What is called the quantity which can be measured ?federal ​

A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement. For example, a length is a physical quantity.

a sequence is a function whose domain is the set of natural numbers.

natural numbers are all the negative and positive integers