

A241239


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


1



0, 1, 4, 10, 19, 30, 45, 61, 84, 106, 134, 165, 199, 234, 277, 321, 364, 412, 478, 523, 595
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OFFSET

1,3


COMMENTS

A centered hexagonal grid of size n is a grid with A003215(n1) points forming a hexagonal lattice.


LINKS

Table of n, a(n) for n=1..21.
Eric Weisstein's World of Mathematics, Hex Number.
Eric Weisstein's World of Mathematics, Obtuse Triangle.
Eric Weisstein's World of Mathematics, Isosceles Triangle.


FORMULA

a(n) = A241237(n)  A241238(n).


EXAMPLE

For n = 2 the only kind of noncongruent obtuse isosceles triangles is the following:
/* *
. . *
\. .


CROSSREFS

Cf. A190310, A241230.
Sequence in context: A267882 A238705 A022785 * A057312 A219965 A008038
Adjacent sequences: A241236 A241237 A241238 * A241240 A241241 A241242


KEYWORD

nonn,more


AUTHOR

Martin Renner, Apr 17 2014


EXTENSIONS

a(7) from Martin Renner, May 31 2014
a(8)a(21) from Giovanni Resta, May 31 2014


STATUS

approved



