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A257326 Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "3" arm. 4
3, 2, 7, 9, 5, 13, 15, 17, 11, 4, 19, 23, 21, 25, 8, 29, 10, 31, 14, 37, 16, 41, 22, 43, 47, 27, 53, 55, 26, 35, 33, 49, 59, 61, 67, 71, 65, 73, 77, 79, 83, 39, 89, 97, 101, 32, 103, 20, 91, 34, 107, 40, 109, 38, 51, 113, 57, 28, 85, 44, 119, 46, 81, 121, 45 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Place numbers 2,3,5,7 clockwise around a grid point (see illustrations in links). Divide grid into four spiral arms.
Extend each arm one step at a time, in rotation: first the 2 arm, then the 3 arm, then the 5 arm, then the 7 arm, then the 2 arm, etc.
Rule for extending: next term in arm is smallest number such that each cell in the grid is relatively prime to its eight neighbors. Inside an arm, the terms must be distinct.
Repetitions between arms are permitted.
The four arms are A257325, A257326, A257327, A257328.
LINKS
Popular Computing (Calabasas, CA), Problem 146: Gcd, Vol. 4 (No. 45, Dec 1976), page PC45-4.
CROSSREFS
Sequence in context: A026186 A026210 A367822 * A118966 A351904 A351903
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 21 2015
EXTENSIONS
More terms from Lars Blomberg, Apr 27 2015
STATUS
approved

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)