|
| |
|
|
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:
/* *
. . *
\. .
|
|
|
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
|
| |
|
|
