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%I #8 May 04 2021 01:06:52
%S 0,1,1,3,5,8,13,14,28,43,45,89,135,138,143,284,430,438,451,897,1356,
%T 1404,1446,2878,4352,4423,4511,4645,9245,13979,14203,14476,14757,
%U 15184,30225,45693,46407,47275,48164,49512,98573,148982,151235,153968,156749
%N Spiro-tribonacci numbers: a(n) = sum of three previous terms that are nearest when terms arranged in a spiral.
%e Terms are written in square boxes radiating spirally (cf. Ulam prime spiral). a(0) = 0, a(1) = 1 and a(2) = 1, so write 0, then 1 to its right, and another 1 below the first 1. The next unfilled box forms a square with the three filled boxes, so a(3) = a(0) + a(1) + a(2) = 0 + 1 + 1 = 2.
%e .
%e 8--13--14--28
%e |
%e 5 0---1
%e | |
%e 3---2---1
%e .
%e a(4) = 2 because a(0) + a(1) + a(2) = 0 + 1 + 1 = 2.
%Y Cf. A078510, A092369.
%K easy,nonn
%O 0,4
%A _Michael Joseph Halm_, Apr 02 2004; corrected Apr 05 2004
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