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 A257336 Construct spiral of numbers on square grid as in Comments; sequence gives terms along the "3" arm. 4
 3, 8, 11, 14, 25, 22, 21, 26, 33, 34, 45, 44, 59, 48, 67, 58, 69, 68, 79, 66, 89, 84, 97, 90, 103, 98, 113, 106, 115, 108, 127, 128, 133, 132, 145, 138, 151, 144, 161, 150, 169, 162, 175, 176, 183, 178, 189, 188, 195, 194, 209, 202, 219, 208, 227, 216, 235 (list; graph; refs; listen; history; text; internal format)
 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. Every term in the entire grid must be different. The four arms are A257335, A257336, A257337, A257338. Conjecture: every number > 1 appears in one of the four arms. 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 PC45-4. N. J. A. Sloane, Spiral showing initial terms of A257335-A257338 CROSSREFS Cf. A064413, A257321-A257340. Sequence in context: A003157 A310281 A287365 * A234431 A310282 A081858 Adjacent sequences:  A257333 A257334 A257335 * A257337 A257338 A257339 KEYWORD nonn AUTHOR N. J. A. Sloane, Apr 21 2015 EXTENSIONS Corrected a(5)-a(9) and more terms from Lars Blomberg, Apr 28 2015 STATUS approved

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Last modified May 12 10:17 EDT 2021. Contains 343821 sequences. (Running on oeis4.)