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A241237 Number of isosceles triangles, distinct up to congruence, on a centered hexagonal grid of size n. 4
0, 3, 15, 35, 69, 106, 162, 222, 300, 382, 486, 587, 715, 840, 997, 1147, 1313, 1491, 1700, 1890, 2129 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

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, Isosceles Triangle.

FORMULA

a(n) = A241231(n) - A241236(n).

EXAMPLE

For n = 2 the three kinds of non-congruent isosceles triangles are the following:

/. *     * *     * .

. * *   . . *   . . *

\. .     . .     * .

CROSSREFS

Cf. A189978, A241228.

Sequence in context: A236693 A317183 A000466 * A145949 A015809 A293995

Adjacent sequences:  A241234 A241235 A241236 * A241238 A241239 A241240

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 January 22 19:16 EST 2020. Contains 331153 sequences. (Running on oeis4.)