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Quasi-sociable numbers or reduced sociable numbers are numbers whose aliquot sums minus one form a cyclic sequence that begins and ends with the same number. They are generalizations of the concepts of betrothed numbers and quasiperfect numbers. The first quasi-sociable sequences, or quasi-sociable chains, were discovered by Mitchell Dickerman ...
In a sociable sequence, each number is the sum of the proper divisors of the preceding number, i.e., the sum excludes the preceding number itself. For the sequence to be sociable, the sequence must be cyclic and return to its starting point. The period of the sequence, or order of the set of sociable numbers, is the number of numbers in this cycle.
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For a perfect number n the sum of all its divisors is equal to 2n. For an almost perfect number n the sum of all its divisors is equal to 2n - 1. Numbers n exist whose sum of factors = 2n + 2. They are of form 2^(n - 1) * (2^n - 3) where 2^n - 3 is a prime. The only exception known till yet is 650 = 2 * 5^2 * 13.
Not all number systems can represent the same set of numbers; for example, Roman numerals cannot represent the number zero. Ideally, a numeral system will: Represent a useful set of numbers (e.g. all integers, or rational numbers) Give every number represented a unique representation (or at least a standard representation)
The order of operations, that is, the order in which the operations in an expression are usually performed, results from a convention adopted throughout mathematics, science, technology and many computer programming languages. It is summarized as: [2] [5] Parentheses; Exponentiation; Multiplication and division; Addition and subtraction
The number of iterations needed for , to reach a fixed point is the Dudeney function's persistence of , and undefined if it never reaches a fixed point. It can be shown that given a number base b {\displaystyle b} and power p {\displaystyle p} , the maximum Dudeney root has to satisfy this bound:
The number of alternative assignments for a given number of workers, taking into account the choices of how many stages to use and how to assign workers to each stage, is an ordered Bell number. [29] As another example, in the computer simulation of origami , the ordered Bell numbers give the number of orderings in which the creases of a crease ...