For example, 123456 (step is 1), 666 (step is 0), or 7531 (step is -2).
While there are infinite straight-line numbers, only 96 of them are not repdigits, the largest being 9876543210.
The first straight-line numbers are 111, 123, 135, 147, 159, 210, 222, 234, 246, 258, 321, 333, 345, 357, 369, 420, 432, 444, 456, 468, 531, 543, 555, 567, 579, 630, 642, 654, 666, 678 more terms