The structure of digits represents an acute angle. The vertex is an internal digit. In the graphic representation the points are connected by imaginary line segments from left to right. This sequence is finite. The final term has 14 digits: 98765432102468.
If a(n) does not end in 0, then A004086(a(n)) is a term; if a(n) does not start with 9, then A061601(a(n)) is a term. - Michael S. Branicky, Aug 02 2022
EXAMPLE
Illustration using the final term of this sequence:
9 . . . . . . . . . . . . .
. 8 . . . . . . . . . . . 8
. . 7 . . . . . . . . . . .
. . . 6 . . . . . . . . 6 .
. . . . 5 . . . . . . . . .
. . . . . 4 . . . . . 4 . .
. . . . . . 3 . . . . . . .
. . . . . . . 2 . . 2 . . .
. . . . . . . . 1 . . . . .
. . . . . . . . . 0 . . . .
PROG
(Python)
progressions = set(tuple(range(i, j+1, d)) for i in range(10) for d in range(1, 10-i) for j in range(i+d, 10))
s = set()
for left in progressions:
for right in progressions:
dl, dr = left[1] - left[0], right[1] - right[0]
if dl + dr > 2:
if left[-1] == right[-1]: s.add(left[:-1] + right[::-1])
if left[0] == right[0]: s.add(left[::-1] + right[1:])
afull = sorted(int("".join(map(str, t))) for t in s if t[0] != 0)
