OFFSET
1,1
COMMENTS
This is defined to be the lexicographically earliest quaternary sequence with the following property:
If a(n) = 0, underline a(n+1); if a(n) = 1, underline a(n+2); if a(n) = 2, underline a(n+3); if a(n) = 3, underline a(n+4). Now, the subsequence of (once or more) underlined terms must be equal to the original sequence (copy #1), and the subsequence of non-underlined terms must also reproduce the original sequence (copy #2).
LINKS
Carole Dubois, Table of n, a(n) for n = 1..2004
EXAMPLE
The sequence starts (3, 2, 1, 0, 3, 3, 3, 3, 2, 1, 0, 3, 3, 2, 1, 3, 3, 0,...)
Instead of underlining terms, we will put parentheses around the terms we want to emphasize:
a(1) = 3 produces parentheses around a(1 + 4 = 5):
3, 2, 1, 0, (3,) 3, 3, 3, 2, 1, 0, 3, 3, 2, 1, 3, 3, 0,...
a(2) = 2 produces parentheses around a(2 + 3 = 5), which is now already done. Then,
a(3) = 1 produces parentheses around a(3 + 2 = 5), which is already done. Then,
a(4) = 0 produces parentheses around a(4 + 1 = 5), which is already done. Now,
a(5) = 3 produces parentheses around a(5 + 4 = 9):
3, 2, 1, 0, (3,) 3, 3, 3, (2), 1, 0, 3, 3, 2, 1, 3, 3, 0,...
a(6) = 3 produces parentheses around a(6 + 4 = 10):
3, 2, 1, 0, (3,) 3, 3, 3, (2), (1), 0, 3, 3, 2, 1, 3, 3, 0,...
a(7) = 3 produces parentheses around a(7 + 4 = 11):
3, 2, 1, 0, (3,) 3, 3, 3, (2), (1), (0), 3, 3, 2, 1, 3, 3, 0,...
a(8) = 3 produces parentheses around a(8 + 4 = 12):
3, 2, 1, 0, (3,) 3, 3, 3, (2), (1), (0), (3), 3, 2, 1, 3, 3, 0,...
a(9) = 2 produces parentheses around a(9 + 3 = 12) - already done. Then,
a(10) = 1 produces parentheses around a(10 + 2 = 12) - already done. Then,
a(11) = 0 produces parentheses around a(11 + 1 = 12) - already done. Then,
a(12) = 3 produces parentheses around a(12 + 4 = 16):
3, 2, 1, 0, (3,) 3, 3, 3, (2), (1), (0), (3), 3, 2, 1, (3), 3, 0,...
a(13) = 3 produces parentheses around a(13 + 4 = 17):
3, 2, 1, 0, (3,) 3, 3, 3, (2), (1), (0), (3), 3, 2, 1, (3), (3), 0,...
a(14) = 2 produces parentheses around a(14 + 3 = 17) - already done. Then,
a(15) = 1 produces parentheses around a(15 + 2 = 17) - already done. Etc.
We see in this small example that the parenthesized terms reproduce the initial sequence:
(3),(2),(1),(0),(3),(3),(3),...
The same is true for the subsequence of non-parenthesized terms:
3, 2, 1, 0, 3, 3, 3, 3, 2, 1, 0,...
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Eric Angelini and Carole Dubois, Apr 02 2019
STATUS
approved