%I #15 Oct 17 2022 15:36:28

%S 0,3,15,35,69,106,162,222,300,382,486,587,715,840,997,1147,1313,1491,

%T 1700,1890,2129,2341,2598,2842,3126,3394,3711,3995,4341,4641,5024,

%U 5349,5750,6128,6540,6959,7381,7772,8255,8722,9252,9688,10220,10698,11277,11806,12381,12905

%N Number of isosceles triangles, distinct up to congruence, on a centered hexagonal grid of size n.

%C A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/HexNumber.html">Hex Number</a>.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/IsoscelesTriangle.html">Isosceles Triangle</a>.

%F a(n) = A241231(n) - A241236(n).

%e For n = 2 the three kinds of non-congruent isosceles triangles are the following:

%e /. * * * * .

%e . * * . . * . . *

%e \. . . . * .

%Y Cf. A189978, A241228.

%K nonn

%O 1,2

%A _Martin Renner_, Apr 17 2014

%E a(7) from _Martin Renner_, May 31 2014

%E a(8)-a(21) from _Giovanni Resta_, May 31 2014

%E More terms from _Bert Dobbelaere_, Oct 17 2022