

A257321


Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "2" arm.


24



2, 5, 7, 5, 3, 11, 5, 2, 7, 2, 3, 2, 5, 2, 5, 7, 3, 5, 7, 2, 5, 7, 11, 3, 7, 3, 5, 3, 5, 3, 5, 3, 7, 3, 2, 5, 2, 7, 2, 3, 5, 7, 2, 7, 2, 7, 2, 7, 2, 5, 11, 2, 5, 2, 3, 2, 5, 2, 5, 7, 3, 7, 3, 7, 3, 5, 3, 5, 3, 5, 7, 2, 5, 7, 3, 7, 3, 7, 3, 2, 5, 2, 7, 2, 3, 5
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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.
Repetitions in arms are permitted.
The four arms are A257321, A257322, A257323, A257324.


LINKS

Lars Blomberg, Table of n, a(n) for n = 1..10000
Popular Computing (Calabasas, CA), Problem 146: Gcd, Vol. 4 (No. 45, Dec 1976), page PC454.
N. J. A. Sloane, Spirals showing initial terms of A257321A257332


CROSSREFS

Cf. A064413, A257322A257340.
Sequence in context: A199275 A135007 A082880 * A011038 A079378 A066035
Adjacent sequences: A257318 A257319 A257320 * A257322 A257323 A257324


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Apr 21 2015


EXTENSIONS

More terms from Lars Blomberg, Apr 27 2015


STATUS

approved



