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%I #6 May 11 2014 22:50:54
%S 1,3,7,12,27,41,114,191,307,908,1479,2517,7218,11819,20079,57564,
%T 94035,233905,327970,954519,1356507,3827708,5462751,15712989,21207042,
%U 61631203,87045927,251438028,339057531,986402633,1392602162,4023051167
%N 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 a(3) = 7 since 3 XOR (3+1) = 3 XOR 4 = 7.
%e a(4) = 12 since 7 XOR (7+3+1) = 7 XOR 11 = 12.
%e a(5) = 27 since 12 XOR (12+7+3+1) = 12 XOR 23 = 27.
%e The binary expansions of a(n) form a triangle (listed with ones place in leftmost column):
%e 1,
%e 1,1,
%e 1,1,1,
%e 0,0,1,1,
%e 1,1,0,1,1,
%e 1,0,0,1,0,1,
%e 0,1,0,0,1,1,1,
%e 1,1,1,1,1,1,0,1,
%e 1,1,0,0,1,1,0,0,1,
%e 0,0,1,1,0,0,0,1,1,1,
%e 1,1,1,0,0,0,1,1,1,0,1,
%e 1,0,1,0,1,0,1,1,1,0,0,1,...
%o (PARI) a(n)=if(n==1,1,if(n==2,3,bitxor(a(n-1),sum(k=1,n-1,a(k)))))
%Y Cf. A099810.
%K nonn
%O 1,2
%A _Paul D. Hanna_, Oct 26 2004
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