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Arrange numbers in a single clockwise spiral so that each number is relatively prime to its eight neighbors.
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%I #41 Apr 29 2015 16:06:55

%S 1,2,3,5,4,7,6,11,9,13,17,19,8,23,29,21,25,31,37,41,35,43,47,10,49,12,

%T 53,14,15,59,27,55,39,16,61,22,51,67,18,65,24,71,26,33,32,57,73,79,83,

%U 77,85,89,97,101,28,95,34,91,38,103,107,45,109,113,20,121

%N Arrange numbers in a single clockwise spiral so that each number is relatively prime to its eight neighbors.

%C Start with 1; always choose smallest number which has not yet appeared.

%C It is conjectured that every number appears.

%H Jon E. Schoenfield and Reinhard Zumkeller, <a href="/A257339/b257339.txt">Table of n, a(n) for n = 1..100000</a> (First 10000 terms from Jon E. Schoenfield)

%H Jon E. Schoenfield, <a href="/A257339/a257339_1.png">Illustration of initial terms</a>

%H Jon E. Schoenfield, <a href="/A257339/a257339_2.png">Graph of first 18000 terms</a>

%Y Cf. A064413, A257321-A257340.

%Y Cf. A257111 (first differences), A257455 (putative inverse).

%Y Indices of primes and prime powers: A257457 and A257458.

%K nonn

%O 1,2

%A _N. J. A. Sloane_, Apr 21 2015

%E Corrected and extended by _Jon E. Schoenfield_, Apr 23 2015