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A257327
Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "5" arm.
4
5, 2, 11, 4, 7, 13, 8, 17, 19, 3, 23, 9, 29, 21, 31, 15, 14, 37, 27, 41, 43, 16, 33, 26, 25, 6, 47, 12, 53, 22, 49, 10, 39, 35, 51, 59, 57, 61, 63, 65, 67, 20, 71, 73, 69, 79, 45, 83, 85, 28, 89, 97, 77, 75, 91, 55, 81, 101, 87, 95, 32, 103, 38, 107, 18, 109
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
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 21 2015
EXTENSIONS
More terms from Lars Blomberg, Apr 27 2015
STATUS
approved