···
(1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6) ···
(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) ···
(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6) ···
(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6) ···
(5, 1) (5, 2) (5, 3) (5, 4) (5, 5) (5, 6) ···
(6, 1) (6, 2) (6, 3) (6, 4) (6, 5) (6, 6) ···
M
IN×IN
(1, 1)
a
1
= (1, 1), a
2
= (2, 1), a
3
= (1, 2), a
4
= (3, 1),
a
5
= (2, 2), a
6
= (1, 3), a
7
= (4, 1), . . .
···
.
.
.
IN × IN
(a, b) < (c, d) ⇐⇒ a + b < c + d (a + b = c + d b < d).
f
f(a, b) =
a + b − 1
2
+ b,