A333030 Number of edges in an equilateral triangle "frame" of size n (see Comments in A328526 for definition).

3, 21, 132, 432, 951, 1800, 3069, 4956, 7569, 11040, 15585, 21492, 28971, 38328, 49527, 62922, 79113, 98358, 120939, 147486, 177873, 212358, 251823, 296898, 347709, 404772, 468189, 538680, 617439, 704742, 801039, 907842, 1024629, 1151544, 1289319, 1438686

Offset: 1

Author

Scott R. Shannon and N. J. A. Sloane, Mar 05 2020

Keywords

• nonn

Comments

For n<=3 the terms equal A274586(n). See A328526 for images of the triangular frame.

Links

• Lars Blomberg, Table of n, a(n) for n = 1..65

Crossrefs

Cf. A328526 (regions), A333031 (vertices), A331776 (square frame), A274586 (filled triangle).

Extensions

a(12) and beyond from Lars Blomberg, May 01 2020