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 A278180 Square spiral in which each new term is the sum of its two largest neighbors. 8
 1, 1, 2, 3, 4, 7, 8, 15, 16, 17, 33, 35, 37, 72, 76, 80, 84, 164, 172, 180, 188, 368, 384, 401, 418, 435, 853, 888, 925, 962, 999, 1961, 2037, 2117, 2201, 2285, 2369, 4654, 4826, 5006, 5194, 5382, 5570, 10952, 11336, 11737, 12155, 12590, 13025, 13460, 26485, 27373, 28298, 29260, 30259, 31258, 32257, 63515 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS 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. 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 LINKS Peter Kagey, Table of n, a(n) for n = 1..10000 Peter Kagey, Bitmap illustrating the of parity of the first one million terms. (Even and odd numbers are represented with black and white pixels respeectively.) EXAMPLE Illustration of initial terms as a square spiral: . . 84----80----76-----72----37 . | | . 164 4-----3-----2 35 . | | | | . 172 7 1-----1 33 . | | | . 180 8-----15----16----17 . | . 188---368---384---401---418 . a(21) = 188 because the sum of its two largest neighbors is 180 + 8 = 188. a(22) = 368 because the sum of its two largest neighbors is 180 + 188 = 368. a(23) = 384 because the sum of its two largest neighbors is 368 + 16 = 384. a(24) = 401 because the sum of its two largest neighbors is 384 + 17 = 401. a(25) = 418 because the sum of its two largest neighbors is 401 + 17 = 418. a(26) = 435 because the sum of its two largest neighbors is 418 + 17 = 435. CROSSREFS Cf. A030237, A078510, A141481, A274917, A278181, A278619, A278645. Sequence in context: A240690 A339593 A113050 * A015927 A097110 A116961 Adjacent sequences: A278177 A278178 A278179 * A278181 A278182 A278183 KEYWORD nonn AUTHOR Omar E. Pol, Nov 14 2016 STATUS approved

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Last modified February 3 21:17 EST 2023. Contains 360045 sequences. (Running on oeis4.)