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A257333
Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "2" arm.
2
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 35, 34, 33, 38, 37, 36, 41, 39, 43, 40, 47, 44, 45, 46, 49, 50, 53, 55, 42, 59, 51, 52, 57, 56, 61, 48, 65, 54, 67, 58, 63, 62, 71, 60, 73, 64
OFFSET
1,1
COMMENTS
Place numbers 2,3,4,5 clockwise around a grid point (see illustration in "Spiral" link). 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 4 arm, then the 5 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 four (N,S,E,W) neighbors. Inside an arm, the terms must be distinct.
Repetitions between arms are permitted.
The "2" arm is the present sequence (A257333), the "3" arm is A257334. The "4" arm begins 4,3,2,5, and thereafter agrees with the "2" arm. The "5" arm begins 5,2,3,4, and thereafter agrees with the "3" arm.
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 28 2015
STATUS
approved