A241239 Number of obtuse isosceles triangles, distinct up to congruence, on a centered hexagonal grid of size n.

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

Offset

1,3

Comments

A centered hexagonal grid of size n is a grid with A003215(n-1) 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 non-congruent obtuse isosceles triangles is the following:
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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