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A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice.
Table of n, a(n) for n=1..21.
Eric Weisstein's World of Mathematics, Hex Number.
Eric Weisstein's World of Mathematics, Isosceles Triangle.
a(n) = A241231(n) - A241236(n).
For n = 2 the three kinds of non-congruent isosceles triangles are the following:
/. * * * * .
. * * . . * . . *
\. . . . * .
Cf. A189978, A241228.
Sequence in context: A236693 A317183 A000466 * A145949 A015809 A293995
Adjacent sequences: A241234 A241235 A241236 * A241238 A241239 A241240
Martin Renner, Apr 17 2014
a(7) from Martin Renner, May 31 2014
a(8)-a(21) from Giovanni Resta, May 31 2014