A190310 -id:A190310 - cp's OEIS frontend

A190309 Number of acute isosceles triangles, distinct up to congruence, on an n X n grid (or geoboard).

0, 0, 2, 5, 11, 19, 29, 40, 58, 74, 94, 113, 141, 168, 201, 227, 267, 304, 348, 390, 438, 483, 537, 590, 657, 709, 776, 837, 913, 979, 1057, 1130, 1225, 1299, 1396, 1472, 1576, 1663, 1768, 1863, 1974

Offset: 1

Martin Renner, May 08 2011

nonn

Eric Weisstein's World of Mathematics, Geoboard.
Eric Weisstein's World of Mathematics, Acute Triangle.
Eric Weisstein's World of Mathematics, Isosceles Triangle.

Cf. A028419, A108279, A189978, A190021, A190310.

a(n) = A189978(n) - A190310(n) - A108279(n).

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

Original entry on oeis.org

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

Offset: 1

Martin Renner, Apr 17 2014

A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice.

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

Eric Weisstein's World of Mathematics, Hex Number.
Eric Weisstein's World of Mathematics, Obtuse Triangle.
Eric Weisstein's World of Mathematics, Isosceles Triangle.

Cf. A190310, A241230.

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

a(7) from Martin Renner, May 31 2014

a(8)-a(21) from Giovanni Resta, May 31 2014

cp's OEIS Frontend

A190309 Number of acute isosceles triangles, distinct up to congruence, on an n X n grid (or geoboard).

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