A241226 - OEIS

%I #12 May 31 2014 10:39:21

%S 0,12,480,4488,22278,78288,220374,531594,1143648,2254440,4145244,

%T 7202190,11940444,19029300,29318754,43872402,63999528,91288794,

%U 127642164,175326336,237000816,315772032

%N Number of obtuse triangles 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/ObtuseTriangle.html">Obtuse Triangle</a>.

%F a(n) = A241223(n) - A241224(n) - A241225(n).

%Y Cf. A190020.

%K nonn,more

%O 1,2

%A _Martin Renner_, Apr 17 2014

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

%E a(8)-a(22) from _Giovanni Resta_, May 31 2014