OFFSET
1,3
COMMENTS
Here the "neighbors" of a(n) are defined to be the adjacent elements to a(n) in the same row, column or diagonals, that are present in the spiral when a(n) is the new element of the sequence in progress.
FORMULA
From Robert Israel, Nov 22 2016: (Start)
a(n) = 3 if n>=4 is in A002620.
a(n) = 2 if n>=2 is in A033638.
Otherwise, a(n) = 4 if n > 2. (End)
EXAMPLE
Illustration of initial terms as a spiral (n = 1..169):
.
. 2 - 3 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 2
. | |
. 4 2 - 3 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 2 3
. | | | |
. 4 4 2 - 3 - 4 - 4 - 4 - 4 - 4 - 4 - 2 3 4
. | | | | | |
. 4 4 4 2 - 3 - 4 - 4 - 4 - 4 - 2 3 4 4
. | | | | | | | |
. 4 4 4 4 2 - 3 - 4 - 4 - 2 3 4 4 4
. | | | | | | | | | |
. 4 4 4 4 4 2 - 3 - 2 3 4 4 4 4
. | | | | | | | | | | | |
. 4 4 4 4 4 3 0 - 1 4 4 4 4 4
. | | | | | | | | | | |
. 4 4 4 4 3 2 - 4 - 3 - 2 4 4 4 4
. | | | | | | | | |
. 4 4 4 3 2 - 4 - 4 - 4 - 3 - 2 4 4 4
. | | | | | | |
. 4 4 3 2 - 4 - 4 - 4 - 4 - 4 - 3 - 2 4 4
. | | | | |
. 4 3 2 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 3 - 2 4
. | | |
. 3 2 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 3 - 2
. |
. 2 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 4 - 3
.
MAPLE
0, 1, seq(op([2, 4$floor(i/2), 3]), i=0..30); # Robert Israel, Nov 22 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Nov 19 2016
STATUS
approved