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Construct a square spiral: a(n) is the sum of all adjacent terms a(k) in the spiral for k < n; a(1) = 0, a(2) = 1.
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%I #17 Dec 06 2022 09:24:25

%S 0,1,1,2,2,4,4,9,10,11,23,25,26,54,59,63,65,134,144,152,156,321,344,

%T 374,395,406,835,894,968,1019,1045,2144,2283,2459,2646,2774,2839,5812,

%U 6155,6585,7037,7345,7501,15323,16144,17183,18296,19471,20272

%N Construct a square spiral: a(n) is the sum of all adjacent terms a(k) in the spiral for k < n; a(1) = 0, a(2) = 1.

%C The terms "adjacent" to a(n) are terms in any of the 8 cells of the matrix which surround the cell containing a(n). See Github link for code (Python 3) which produces the matrix and sequence, and a picture of the matrix containing the first 49 terms.

%H Abraham C Leventhal, <a href="https://github.com/Abraham-Leventhal/Mathstuff">Code and the 7 X 7 spiral</a>, Github.

%e The spiral begins:

%e .

%e 65--63--59--54--26

%e | |

%e 134 2---2---1 25

%e | | | |

%e ... 4 0---1 23

%e | |

%e 4---9--10--11

%e .

%e The last term shown is a(18) = 134 = 65 + 63 + 2 + 4, which is the sum of its adjacent earlier terms.

%Y Cf. A094767, A094769, A141481.

%K nonn

%O 1,4

%A _Abraham C Leventhal_, Nov 15 2022