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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified March 28 07:59 EDT 2020. Contains 333079 sequences. (Running on oeis4.)