OFFSET
0,2
COMMENTS
a(n) is also the number of grid points that are covered on the semi-infinite square grid after n-th stage in a D-toothpick structure in which the toothpicks placed in horizontal direction are prohibited, starting with a(0) = 1.
Note that both the structure mentioned in the above comment and the structure mentioned in the definition are essentially equivalents. Both structures are related to the Sierpinski triangle (see A047999). The number of edges in the structure after n-th stage is A006046(n). The number of edges added at n-th stage is A001316(n-1), for n >= 1 (see example).
LINKS
EXAMPLE
Illustration of initial terms:
---------------------------------------------------------
---------------------------------------------------------
. 0
. * 1 1
. 1 1 |
. * 1 2
. 2 3 / \
. * * 2 4
. 2 5 | |
. * * 2 6
. 4 9 / \ / \
. * * * 3 9
. 2 11 | |
. * * 2 11
. 4 15 / \ / \
. * * * * 4 15
. 4 19 | | | |
. * * * * 4 19
. 8 27 / \ / \ / \ / \
. * * * * * 5 24
. 2 29 | |
. * * 2 26
. 4 33 / \ / \
. * * * * 4 30
.
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Dec 25 2013
STATUS
approved