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%I #18 Apr 21 2018 10:36:18
%S 1,2,3,4,6,8,9,12,16,18,24,32,33,36,48,64,65,66,67,72,73,96,97,128,
%T 129,130,131,132,134,144,146,192,193,194,195,256,257,258,259,260,262,
%U 264,265,268,288,289,292,384,385,386,387,388,390,512,513,514,515,516,518
%N Integers n > 0 such that n XOR 20*n = 21*n.
%C n is in the sequence iff 2*n is in the sequence. - _Robert Israel_, Nov 11 2016
%H Ivan Neretin, <a href="/A115422/b115422.txt">Table of n, a(n) for n = 1..10203</a>
%F This sequence also seems to satisfy:
%F 5*a(n) XOR 16*a(n) = 21*a(n);
%F 5*a(n) XOR 17*a(n) = 20*a(n); etc.
%F a(A224809(n+4)) = 2^n. - _Gheorghe Coserea_, Nov 11 2016
%p select(n -> Bits:-Xor(n,20*n)=21*n, [$1..1000]); # _Robert Israel_, Nov 11 2016
%t Select[Range[600],BitXor[#,20#]==21#&] (* _Harvey P. Dale_, Apr 21 2018 *)
%o (Perl)
%o $cnt=0;
%o foreach(1..1_000){
%o print ++$cnt," $_\n" if ((20*$_)^$_)==21*$_;
%o }
%o # _Ivan Neretin_, Nov 10 2016
%o (PARI) isok(n) = bitxor(n, 20*n) == 21*n; \\ _Michel Marcus_, Nov 11 2016
%Y Cf. A003714 (Fibbinary numbers), A048715, A048718, A115423, A115424.
%K nonn
%O 1,2
%A _Paul D. Hanna_, Jan 22 2006
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