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Recamán's sequence: 0, 1, 3, 6, 2, 7, 13, 20, 12, 21, 11, 22, 10, 23, 9, 24, 8, 25, 43, 62, ... "subtract if possible, otherwise add": a(0) = 0; for n > 0, a(n) = a(n − 1) − n if that number is positive and not already in the sequence, otherwise a(n) = a(n − 1) + n, whether or not that number is already in the sequence. A005132: Look-and ...
P(n) is the number of ways of writing n + 2 as an ordered sum in which each term is either 2 or 3 (i.e. the number of compositions of n + 2 in which each term is either 2 or 3). For example, P (6) = 4, and there are 4 ways to write 8 as an ordered sum of 2s and 3s:
The aliquot sequence starting with a positive integer k can be defined formally in terms of the sum-of-divisors function σ 1 or the aliquot sum function s in the following way: [1] = = = > = = = If the s n-1 = 0 condition is added, then the terms after 0 are all 0, and all aliquot sequences would be infinite, and we can conjecture that all aliquot sequences are convergent, the limit of these ...
and the sequences A053873, "Numbers n such that OEIS sequence A n contains n", and A053169, "n is in this sequence if and only if n is not in sequence A n". Thus, the composite number 2808 is in A053873 because A002808 is the sequence of composite numbers, while the non-prime 40 is in A053169 because it is not in A000040, the prime numbers
A Sudoku starts with some cells containing numbers (clues), and the goal is to solve the remaining cells. Proper Sudokus have one solution. [1] Players and investigators use a wide range of computer algorithms to solve Sudokus, study their properties, and make new puzzles, including Sudokus with interesting symmetries and other properties.
Equivalently, the nth number of this sequence is the number n written in binary representation, inverted, and written after the decimal point. This is true for any base. As an example, to find the sixth element of the above sequence, we'd write 6 = 1*2 2 + 1*2 1 + 0*2 0 = 110 2 , which can be inverted and placed after the decimal point to give ...
The Lucas sequences for some values of P and Q have specific names: U n (1, −1) : Fibonacci numbers V n (1, −1) : Lucas numbers U n (2, −1) : Pell numbers V n (2, −1) : Pell–Lucas numbers (companion Pell numbers) U n (1, −2) : Jacobsthal numbers V n (1, −2) : Jacobsthal–Lucas numbers U n (3, 2) : Mersenne numbers 2 n − 1
Order 5 hexagons, where the "X" are placeholders for order 3 hexagons, which complete the number sequence. The left one contains the hexagon with the sum 38 (numbers 1 to 19) and the right one, one of the 26 hexagons with the sum 0 (numbers −9 to 9). For more information visit the German Wikipedia article.