%I #34 Feb 24 2021 02:48:19
%S 0,1,2,4,7,10,14,21,29,37,43,53,61,71,83,103,123,139,151,165,173,183,
%T 195,215,235,253,271,295,317,345,385,441,493,531,559,581,589,599,611,
%U 631,651,669,687,711,733,761,801,857,909,949,983,1015,1037,1065,1105,1161
%N Toothpick sequence similar to A160406, but always staying outside the wedge, starting with a horizontal half-toothpick which protrudes from the vertex of the wedge.
%C The initial half-tookpick makes an angle of 90 degrees w.r.t. the wedge's direction. This breaks the symmetry and explains the changing parity of the terms. - _M. F. Hasler_, Jan 29 2013
%H David Applegate, Omar E. Pol and N. J. A. Sloane, <a href="/A000695/a000695_1.pdf">The Toothpick Sequence and Other Sequences from Cellular Automata</a>, Congressus Numerantium, Vol. 206 (2010), 157-191. [There is a typo in Theorem 6: (13) should read u(n) = 4.3^(wt(n-1)-1) for n >= 2.]
%H N. J. A. Sloane, <a href="/wiki/Catalog_of_Toothpick_and_CA_Sequences_in_OEIS">Catalog of Toothpick and Cellular Automata Sequences in the OEIS</a>
%e From _M. F. Hasler_, Jan 29 2013: (Start)
%e The first steps are illustrated as follows, where two vertical "|" or three horizontal "_" correspond to one single full toothpick:
%e : ___ ___ |___ ___|
%e : ___ |___| |___| | |___| |
%e : _ |_ |_ | |_| | |_| | | |_|
%e : /\ |/\ |/\ |/\ ¯¯¯|/\ |¯¯¯|/\
%e : / \ / \ / \ / \ / \ / \
%e :
%e : a(0)=0, a(1)=1, a(2)=2, a(3)=4, a(5)=7, a(6)=10, ... (End)
%o (PARI) A170890(n, print_all=0)={ my( cnt=n>0, ee=[[1,1]], p=Set(vector(2*n-cnt,k,k-n-abs(k-n)*I)), c, d); for(i=2, n, print_all & print1(cnt","); p=setunion(p, Set(Mat(ee~)[, 1])); my(ne=[]); for(k=1, #ee, setsearch(p, c=ee[k][1]+d=ee[k][2]*I) || ne=setunion(ne, Set([[c, d]])); setsearch(p, c-2*d) || ne=setunion(ne, Set([[c-2*d, -d]]))); forstep( k=#ee=eval(ne), 2, -1, ee[k][1]==ee[k-1][1] & k-- & ee=vecextract(ee, Str("^"k"..", k+1))); cnt+=#ee); cnt} \\ - _M. F. Hasler_, Jan 29 2013
%Y See A170891 for the first differences.
%Y Cf. A139250, A170886, A170888, A170892.
%K nonn
%O 0,3
%A _Omar E. Pol_, Jan 09 2010
%E a(9) corrected by _Omar E. Pol_, following an observation by _Kevin Ryde_, Jan 29 2013
%E Terms beyond a(9) from _M. F. Hasler_, Jan 29 2013