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2. 20 (number) - Wikipedia

   en.wikipedia.org/wiki/20_(number)

   In combinatorics, 20 is the number of distinct combinations of 6 items taken 3 at a time. Equivalently, it is the central binomial coefficient for n=3 (sequence A000984 in the OEIS). In decimal, 20 is the smallest non-trivial neon number equal to the sum of its digits when raised to the thirteenth power (20 13 = 8192 × 10 13). [citation needed]

3. Tally marks - Wikipedia

   en.wikipedia.org/wiki/Tally_marks

   He also writes that "no attempt has been made to explain why a tally of something should exhibit multiples of two, prime numbers between 10 and 20, and some numbers that are almost multiples of 10." [2] Alexander Marshack examined the Ishango bone microscopically, and concluded that it may represent a six-month lunar calendar. [3]

4. List of types of numbers - Wikipedia

   en.wikipedia.org/wiki/List_of_types_of_numbers

   Such a number is algebraic and can be expressed as the sum of a rational number and the square root of a rational number. Constructible number: A number representing a length that can be constructed using a compass and straightedge. Constructible numbers form a subfield of the field of algebraic numbers, and include the quadratic surds.

5. Number - Wikipedia

   en.wikipedia.org/wiki/Number

   The natural numbers, starting with 1. The most familiar numbers are the natural numbers (sometimes called whole numbers or counting numbers): 1, 2, 3, and so on. Traditionally, the sequence of natural numbers started with 1 (0 was not even considered a number for the Ancient Greeks.)

6. List of Mersenne primes and perfect numbers - Wikipedia

   en.wikipedia.org/wiki/List_of_Mersenne_primes...

   Since this is also a multiple of 4 for k > 0, 2 4k ±1 ≡ ±12 (mod 20). Thus, all Mersenne numbers M 4k +1 are congruent to 11 modulo 20 and end in 11, 31, 51, 71 or 91, while Mersenne numbers M 4k −1 ≡ 7 (mod 20) and end in 07, 27, 47, 67, or 87. For the perfect numbers, define P n = 2 n−1 M n be the value which is perfect if M n is prime.

7. Number theory - Wikipedia

   en.wikipedia.org/wiki/Number_theory

   Aside from a few fragments, the mathematics of Classical Greece is known to us either through the reports of contemporary non-mathematicians or through mathematical works from the early Hellenistic period. [20] In the case of number theory, this means, by and large, Plato and Euclid, respectively.