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 A263402 Define Z(1) = {1}, and Z(n+1) = Z(n) (+) { x+y, with x and y in Z(n) } for any n>0 (where (+) stands for the symmetric difference of two sets). Then a(n) gives the number of elements in Z(n). 1
 1, 2, 3, 7, 10, 22, 42, 87, 170, 342, 686, 1365, 2727, 5468, 10919, 21857, 43680, 87389, 174756, 349539, 699039, 1398115, 2796191, 5592422, 11184795, 22369639, 44739229, 89478503, 178956950, 357913967, 715827858, 1431655793, 2863311503, 5726623097, 11453246088 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) can also be interpreted as the number of ON cells at the n-th stage of the following automaton: - At first stage, we have only one ON cell at position 1, - An ON cell appears at position x+y if the cells at positions x and y are ON, - An ON cell dies at position x+y if the cells at positions x and y are ON. a(n) <= 2^(n-1) for any n>0. LINKS Paul Tek, Table of n, a(n) for n = 1..250 Paul Tek, PERL program for this sequence FORMULA a(n) = A000120(z(n)) for any n>0 where z(n) is a binary encoding of Z(n), defined as follows: - z(1) = 2^1, - z(n+1) = z(n) XOR A067398(z(n)) for any n>0 (where XOR stands for the binary XOR operator). EXAMPLE Z(1) = {1}; Z(2) = {1} (+) {2} = {1,2}; Z(3) = {1,2} (+) {2,3,4} = {1,3,4}; Z(4) = {1,3,4} (+) {2,4,5,6,7,8} = {1,2,3,5,6,7,8}; Hence: a(1) = 1, a(2) = 2, a(3) = 3 and a(4) = 7. PROG (Perl) See Links section. (PARI) lista(nn) = {zprec = Set([1]); print1(#zprec, ", "); for (n=2, nn, zs = setbinop((x, y)->x+y, zprec); zn = setminus(setunion(zprec, zs), setintersect(zprec, zs)); print1(#zn, ", "); zprec = zn; ); } \\ Michel Marcus, Oct 20 2015 CROSSREFS Cf. A067398. Sequence in context: A291241 A318406 A079380 * A062113 A130968 A007748 Adjacent sequences:  A263399 A263400 A263401 * A263403 A263404 A263405 KEYWORD nonn AUTHOR Paul Tek, Oct 17 2015 STATUS approved

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Last modified April 12 03:14 EDT 2021. Contains 342912 sequences. (Running on oeis4.)