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A278180 Square spiral in which each new term is the sum of its two largest neighbors.

%I A278180 #41 Apr 12 2023 10:52:19
%S A278180 1,1,2,3,4,7,8,15,16,17,33,35,37,72,76,80,84,164,172,180,188,368,384,
%T A278180 401,418,435,853,888,925,962,999,1961,2037,2117,2201,2285,2369,4654,
%U A278180 4826,5006,5194,5382,5570,10952,11336,11737,12155,12590,13025,13460,26485,27373,28298,29260,30259,31258,32257,63515
%N A278180 Square spiral in which each new term is the sum of its two largest neighbors.
%C A278180 To evaluate a(n) consider the neighbors of a(n) that are present in the spiral when a(n) should be a new term in the spiral.
%C A278180 For the same idea but for a hexagonal spiral see A278619; and for a right triangle see A278645. It appears that the same idea for an isosceles triangle and also for a square array gives A030237. - _Omar E. Pol_, Dec 04 2016
%H A278180 Peter Kagey, <a href="/A278180/b278180.txt">Table of n, a(n) for n = 1..10000</a>
%H A278180 Peter Kagey, <a href="/A278180/a278180.png">Bitmap illustrating the parity of the first one million terms</a>. (Even and odd numbers are represented with black and white pixels respeectively.)
%e A278180 Illustration of initial terms as a square spiral:
%e A278180 .
%e A278180 .          84----80----76-----72----37
%e A278180 .           |                        |
%e A278180 .          164    4-----3-----2     35
%e A278180 .           |     |           |      |
%e A278180 .          172    7     1-----1     33
%e A278180 .           |     |                  |
%e A278180 .          180    8-----15----16----17
%e A278180 .           |
%e A278180 .          188---368---384---401---418
%e A278180 .
%e A278180 a(21) = 188 because the sum of its two largest neighbors is 180 + 8 = 188.
%e A278180 a(22) = 368 because the sum of its two largest neighbors is 180 + 188 = 368.
%e A278180 a(23) = 384 because the sum of its two largest neighbors is 368 + 16 = 384.
%e A278180 a(24) = 401 because the sum of its two largest neighbors is 384 + 17 = 401.
%e A278180 a(25) = 418 because the sum of its two largest neighbors is 401 + 17 = 418.
%e A278180 a(26) = 435 because the sum of its two largest neighbors is 418 + 17 = 435.
%Y A278180 Cf. A030237, A078510, A141481, A274917, A278181, A278619, A278645.
%K A278180 nonn
%O A278180 1,3
%A A278180 _Omar E. Pol_, Nov 14 2016