A092360 Spiro-tribonacci numbers: a(n) = sum of three previous terms that are nearest when terms arranged in a spiral.

0, 1, 1, 3, 5, 8, 13, 14, 28, 43, 45, 89, 135, 138, 143, 284, 430, 438, 451, 897, 1356, 1404, 1446, 2878, 4352, 4423, 4511, 4645, 9245, 13979, 14203, 14476, 14757, 15184, 30225, 45693, 46407, 47275, 48164, 49512, 98573, 148982, 151235, 153968, 156749

Offset: 0

Author

Michael Joseph Halm, Apr 02 2004; corrected Apr 05 2004

Keywords

• easy
• nonn

Examples

			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.
.
   8--13--14--28
   |
   5   0---1
   |       |
   3---2---1
.
a(4) = 2 because a(0) + a(1) + a(2) = 0 + 1 + 1 = 2.
		

Crossrefs

Cf. A078510, A092369.