In linear algebra, the main diagonal (sometimes principal diagonal, primary diagonal, leading diagonal, or major diagonal) of a matrix
A
is the collection of entries
A
i
,
j
where
i
=
j
. All off-diagonal elements are zero in a diagonal matrix. The following three matrices have their main diagonals indicated by red 1's:
[
1
0
0
0
1
0
0
0
1
]
[
1
0
0
0
0
1
0
0
0
0
1
0
]
[
1
0
0
0
1
0
0
0
1
0
0
0
]
The antidiagonal (sometimes counterdiagonal, secondary diagonal, trailing diagonal or minor diagonal) of a dimension
N
square matrix,
B
, is the collection of entries
B
i
,
j
such that
i
+
j
=
N
+
1
. That is, it runs from the top right corner to the bottom left corner:
[
0
0
1
0
1
0
1
0
0
]
