%I A099811
%S A099811 1,3,7,12,27,41,114,191,307,908,1479,2517,7218,11819,20079,57564,94035,
%T A099811 233905,327970,954519,1356507,3827708,5462751,15712989,21207042,
%U A099811 61631203,87045927,251438028,339057531,986402633,1392602162,4023051167
%N A099811 a(n) = a(n-1) XOR Sum_{k=1..n-1} a(k), with a(1)=1, a(2)=3, where XOR 
               is the binary exclusive OR operation.
%e A099811 a(3) = 7 since 3 XOR (3+1) = 3 XOR 4 = 7.
%e A099811 a(4) = 12 since 7 XOR (7+3+1) = 7 XOR 11 = 12.
%e A099811 a(5) = 27 since 12 XOR (12+7+3+1) = 12 XOR 23 = 27.
%e A099811 The binary expansions of a(n) form a triangle
%e A099811 (listed with ones-place in left-most column):
%e A099811 1,
%e A099811 1,1,
%e A099811 1,1,1,
%e A099811 0,0,1,1,
%e A099811 1,1,0,1,1,
%e A099811 1,0,0,1,0,1,
%e A099811 0,1,0,0,1,1,1,
%e A099811 1,1,1,1,1,1,0,1,
%e A099811 1,1,0,0,1,1,0,0,1,
%e A099811 0,0,1,1,0,0,0,1,1,1,
%e A099811 1,1,1,0,0,0,1,1,1,0,1,
%e A099811 1,0,1,0,1,0,1,1,1,0,0,1,...
%o A099811 (PARI) a(n)=if(n==1,1,if(n==2,3,bitxor(a(n-1),sum(k=1,n-1,a(k)))))
%Y A099811 Cf. A099810.
%Y A099811 Sequence in context: A034434 A167491 A062325 this_sequence A063072 A007626 
               A047068
%Y A099811 Adjacent sequences: A099808 A099809 A099810 this_sequence A099812 A099813 
               A099814
%K A099811 nonn
%O A099811 1,2
%A A099811 Paul D. Hanna (pauldhanna(AT)juno.com), Oct 26 2004