Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%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