%I #12 May 31 2014 10:41:33

%S 0,1,4,10,19,30,45,61,84,106,134,165,199,234,277,321,364,412,478,523,

%T 595

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

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

%F a(n) = A241237(n) - A241238(n).

%e For n = 2 the only kind of non-congruent obtuse isosceles triangles is the following:

%e /* *

%e . . *

%e \. .

%Y Cf. A190310, A241230.

%K nonn,more

%O 1,3

%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