OFFSET
3,9
COMMENTS
Property "(x3-x1)*(y4-y2) > 0" holds iff the diagonals (spokes) of the quadrilateral have unequal signs of their slope. In this case the spokes are tilted in the same direction (see example). The framed quadrilateral may be classified as "unisense" iff (x3-x1)*(y4-y2) > 0.
All quadrilaterals of A353532 are classified according to the sign of the product (x3-x1)*(y4-y2) as "all" = "unisense" (> 0) + "contrasense" (< 0) + "static" (= 0). The distinction is invariant under symmetry.
LINKS
Rainer Rosenthal, Rows n = 3..100, flattened
EXAMPLE
The triangle begins
\ m 3 4 5 6 7 8 9 10
n \-------------------------------------
3 | 0 | | | | | | |
4 | 0, 0 | | | | | |
5 | 0, 0, 0 | | | | |
6 | 0, 1, 2, 2 | | | |
7 | 0, 1, 8, 15, 12 | | |
8 | 0, 3, 16, 27, 49, 29 | |
9 | 0, 7, 21, 44, 71, 103, 66 |
10 | 0, 9, 30, 61, 106, 152, 216, 131
.
T(6,4) = 1 because of the third example for (6,4) in A353532:
.
4 | . . . C . .
3 | D . . . . . A = (x1,1) = (2,1), B = (6,y2) = (6,2)
2 | . . . . . B C = (x3,4) = (4,4), D = (1,y4) = (1,3)
1 | . A . . . .
y /------------ (x3-x1) * (y4-y2) = (4-2)*(3-2) > 0
x 1 2 3 4 5 6
.
Spokes AC and BD are tilted in the same direction, to the right. The signs of the slopes are unequal: AC has positive slope, and DB has negative slope.
CROSSREFS
KEYWORD
nonn,tabl
AUTHOR
Rainer Rosenthal, May 13 2022
STATUS
approved